Federal Motor Vehicle Safety Standards Rear Impact Guards; Rear Impact Protection

SUMMARY: This final rule establishes two Federal Motor Vehicle Safety

Standards (FMVSS) which will operate together to reduce the number of

injuries and fatalities resulting from the collision of passenger

vehicles with the rear end of heavy trailers and semitrailers. The

first standard (FMVSS No. 223, Rear Impact Guards, or the ``equipment

standard'') specifies performance requirements that rear impact guards

(guards) must meet before they can be installed on new trailers and

semitrailers. It specifies strength requirements, as well as test

procedures that NHTSA will use to determine compliance with the

standard. The guard may be tested for compliance while mounted to a

non-vehicle ``test fixture'' or a complete vehicle. The equipment

standard also requires the guard manufacturer to provide instructions

on the proper installation of the guard. The final rule also specifies

requirements to ensure energy absorption by the guards.

The second standard (FMVSS No. 224, Rear Impact Protection, or the

``vehicle standard'') requires that most new trailers and semitrailers

with a Gross Vehicle Weight Rating of 4,536 kilograms (kg) (10,000

pounds (lbs)) or more be equipped with a rear impact guard meeting the

equipment standard. Requirements for the location of the guard relative

to the rear end of the trailer are also specified in the vehicle

standard. The vehicle standard further requires that the guard be

mounted on the trailer or semitrailer in accordance with the

instructions of the guard manufacturer.

DATES: This rule will become effective on January 26, 1998. Petitions

for reconsideration of this rule must be received no later than March

11, 1996.

ADDRESSES: Petitions for reconsideration should refer to the docket

number and notice number and be submitted in writing to: Docket

Section, National Highway Traffic Safety Administration, Room 5109, 400

Seventh Street, SW, Washington DC 20590. Telephone: (202) 366-5267.

FOR FURTHER INFORMATION CONTACT: Dr. Leon DeLarm, Dr. George

Mouchahoir, or Mr. Sam Daniel, in the Office of Vehicle Safety

Standards (Telephone: 202-366-4919), or Mr. Paul Atelsek, in the Office

of the Chief Counsel (202-366-2992), National Highway Traffic Safety

Administration, 400 Seventh Street, SW, Washington, DC 20590.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. The Safety Problem

II. Existing Regulations

III. Past Proposals

IV. Summary of the 1981 NPRM

V. Summary of 1981 NPRM Comments

VI. Summary of the 1992 SNPRM

VII. Summary of 1992 SNPRM Comments

VIII. Recent Testing by NHTSA

IX. Overview of the Final Rule

X. Summary of Changes From the 1992 SNPRM

XI. Analysis and Response to Comments on the 1992 SNPRM

A. Separate Equipment and Vehicle Standards

B. Standard for Equipment

1. Relationship of Strength, Energy Absorption, and PCI

2. Guard Strength

3. Guard Energy Absorption

4. Vertical Cross-sectional Height of Horizontal Cross-member

5. Shape of the Horizontal Cross-member

6. Guard Attachment

7. Compliance Test Requirements and Procedures

a. Dynamic Versus Static Testing

b. Test Sites

c. Labeling and Certification

C. Standard for Vehicles

1. Configuration Issues

a. Maximum Guard Ground Clearance

b. Guard Width

c. Specification of the Rear Extremity

d. Distance Between the Guard Rear Surface and the Vehicle Rear

Extremity

2. Exclusions

a. Single Unit (Straight Body) Trucks

b. Special Purpose Vehicles

c. Wheels Back Vehicle

D. Costs

E. Benefits

F. Lead Time

G. Miscellaneous Issues

1. Metric System Units

2. Federal Highway Administration Rulemaking on Underride Guards

XII. Rulemaking Analyses and Notices

A. Executive Order 12866 (Federal Regulation) and Regulatory

Policies and Procedures

B. Regulatory Flexibility Act

C. Executive Order 12612 (Federalism)

D. Preemptive Effect and Judicial Review

E. Paperwork Reduction Act

I. The Safety Problem

This rule addresses the problem of rear underride crashes, in which

a passenger car, light truck, or multipurpose vehicle with a Gross

Vehicle Weight Rating (GVWR) of 4,563 kg (10,000 lbs) or less (referred

to collectively in this rule as passenger vehicles) collides with the

rear end of a trailer or semitrailer (trailers and semitrailers are

referred to collectively in this rule as trailers) and the front end of

the passenger vehicle slides under (i.e., underrides) the rear end of

the trailer. Underride occurs to some extent in most collisions in

which a passenger vehicle crashes into the rear end of a large trailer

because most trailer beds are higher than the hoods of passenger

vehicles. In the worst cases, referred to as passenger compartment

intrusion (PCI) or ``excessive underride'' crashes, the passenger

vehicle underrides so far that the rear end of the trailer strikes and

enters its passenger compartment. PCI collisions generally result in

passenger vehicle occupant injuries and fatalities caused by occupant

contact with the rear end of the trailer.

The solution to PCI is upgrading underride guards to make them

stronger, but this introduces another concern. Even if guards succeed

in preventing PCI, overly rigid guards may stop the passenger vehicle

too suddenly, resulting in excessive occupant compartment deceleration

forces and killing or injuring passenger vehicle occupants.

The agency estimates that about 11,551 rear-end crashes with

trucks, trailers, and semitrailers occur annually. These crashes result

in approximately 423 passenger vehicle occupant fatali-ties and about

5,030 non-fatal injuries.

II. Existing Regulations

The initial Federal regulation addressing the issue of heavy

vehicle rear underride was issued in 1953 by the Bureau of Motor

Carriers of the Interstate Commerce Commission (presently the Office of

Motor Carriers of the Federal Highway Administration, DOT). This

regulation (49 CFR 393.86), which is still in effect, requires heavy

trucks, trailers, and semitrailers to be equipped with a rear-end

device designed to help prevent underride. The rule requires that the

ground clearance of the underride guard not exceed 760 mm (30 inches

(in)) when the vehicle is empty. The rule also requires that the device

be located not more than 610 mm (24 in) forward of the rear of the

vehicle and that it extend laterally to within 460 mm (18 in) of each

side. The regulation further requires that the ``[guards] shall be

substantially constructed and firmly attached.''

The Research and Special Programs Administration (RSPA) of DOT has

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specified configuration requirements for guards on tankers that carry

hazardous materials (49 Part 178.345-8). The bottom of the guard must

be at least 100 mm (4 in) below the lower surface of any part of the

rear of the vehicle, and not more than 1,520 mm (60 in) from the ground

when the tanker is empty. The guard must be very strong. It must

deflect 150 mm (6 in) forward when subjected to a 20 m/s\2\ (2 G)

impact while loaded, without contacting the cargo tank. These

requirements are designed primarily to protect the tank and piping, not

the colliding vehicle, in the event of a rear end collision.

III. Past Proposals

From time to time, NHTSA has assessed the requirements of the

Federal Highway Administration's (FHWA) regulation and considered

whether NHTSA should issue a Federal Motor Vehicle Safety Standard

(FMVSS) requiring heavy vehicles to be equipped with rear underride

protection. The issues of particular concern have been the requirements

for rear end guard ground clearance, guard strength, and the injury and

fatality benefits of such a standard. The most recent of several NHTSA

notices was a Supplemental Notice of Proposed Rulemaking (SNPRM) issued

in 1992 (57 FR 252; January 3, 1992). Prior to the 1992 SNPRM, the

agency issued a Notice of Proposed Rulemaking (NPRM) in 1981 (46 FR

2136; January 8, 1981.) The notices of proposed rulemaking issued by

NHTSA and FHWA prior to the 1981 NPRM are cited and discussed in the

1981 NPRM (Docket 1-11; Notice 8).

IV. Summary of the 1981 NPRM

The 1981 NPRM proposed to adopt a FMVSS for all new trucks and

trailers with a GVWR of 4536 kg (10,000 lbs) or more. This NPRM was

issued after research and computer modeling studies indicated that it

was feasible to manufacture light-weight guards that could prevent

excessive underride and absorb crash energy. Guard energy absorption is

important because overly rigid guards could result in passenger

compartment forces that would increase the risk of occupant injuries

even in the absence of underride.

The 1981 NPRM proposed that heavy trailers, semitrailers, and

single unit (i.e., unarticulated) trucks be equipped with an underride

guard that met certain requirements for strength and configuration. The

NPRM proposed exclusions from this requirement for trailers with

chassis that are low enough to the ground to meet the configuration

requirements for the underride guard (low chassis vehicle), trailers

that have the rear tires set back to within 305 mm (12 in) of the rear

(wheels back vehicle), and trailers that have work-performing equipment

in the lower rear whose function would be impaired by a guard (special

purpose vehicle).

NHTSA tentatively concluded that the proposed standard was superior

to the FHWA regulation in three major ways. First, NHTSA specified

objective requirements for guard strength (FHWA requires that the guard

be ``substantially constructed and firmly attached''). Second, the NPRM

proposed a guard configuration that permitted less ground clearance 560

mm (22 in), less longitudinal distance between the guard and the

trailer rear extremity 305 mm (12 in), and less lateral distance

between the guard and the vehicle side extremities 100 mm (4 in), than

the FHWA regulation. Third, the NPRM specified detailed procedures for

testing the guards as installed on the vehicle for which they were

intended by applying a specific force at certain points on the guard.

V. Summary of 1981 NPRM Comments

The agency received over 100 comments on the NPRM. Many of the

comments were from vehicle manufacturers and operators who believed

their vehicles should be excluded from the requirements because they

were special purpose vehicles. Some commenters objected to the proposed

requirements and suggested alternative means of reducing the injuries

and deaths caused by rear underride crashes. The alternative approach

most often cited involved reducing the incidence of underride crashes

through improved heavy vehicle conspicuity.

The agency agreed that conspicuity was an important issue. The

Fatal Accident Reporting System (FARS, a database containing a census

of all vehicle fatalities in the U.S.) statistics had indicated that

about 65 percent of the fatalities resulting from passenger vehicle

collisions with the rear end of heavy vehicles occurred under non-

daylight conditions. NHTSA conducted a fleet study between 1980 and

1985 of the effectiveness of improved conspicuity. As a result of this

study, the agency determined that conspicuity improvement could reduce

the incidence of the accidents by about 15 percent. Consequently, the

agency published a NPRM on improved heavy vehicle conspicuity in

December 1991, (56 FR 63474) and a final rule on conspicuity

improvement in December 1992 (57 FR 58406).

The agency believes, however, that improved rear impact guards

could mitigate some of the rear impact fatalities and serious injuries

not addressed by the improved conspicuity rule. The rear impact guard

is especially important in cases in which the passenger vehicle

driver's abilities are impaired by alcohol or drowsiness. Accident data

indicate that alcohol is a factor for passenger vehicle drivers in

about 30-40 percent of fatal rear underride accidents.

Commenters on the 1981 NPRM also expressed concern that the

proposed requirements would be a substantial financial burden on some

truck and trailer manufacturers. Several commenters argued that the

agency's cost estimate for rear underride guards was well below the

actual cost of equipping the wide variety of single unit trucks with

compliant guards. As to the trailer manufacturing industry, its members

were said to be predominantly small firms that lack the engineering

capabilities to meet the requirements of the proposed rule. In response

to the comments and statistical data, the agency sought to determine if

it could revise the proposed rule to reduce the financial burden on the

manufacturers.

VI. Summary of the 1992 SNPRM

The 1992 SNPRM contained requirements that are similar to those in

the 1981 NPRM in terms of the guard's strength and configuration.

However, the SNPRM differed substantially from the NPRM in terms of its

impact on the industry. In place of the 1981 proposal of a single

vehicle standard specifying the testing of guards on a completed

vehicle, the SNPRM proposed two standards: (1) An equipment standard

providing for the testing of guards on a test fixture, and (2) a

vehicle standard requiring installation of guards complying with the

equipment standard.

The equipment standard proposed strength requirements and an

objective test for determining compliance with these requirements. The

guard manufacturer would conduct a test involving quasi-static loading

of the guard with the guard mounted on a rigid test fixture rather than

installed on a completed vehicle. Guards certified as passing the test

could then be marketed to vehicle manufacturers for installation in

accordance with the configuration requirements of the vehicle standard.

Testing in this manner would relieve vehicle manufacturers, especially

small ones, of the burden associated with compliance testing.

The other major difference from the NPRM is that the SNPRM proposed

to exclude single unit trucks from the rulemaking. NHTSA added this

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exclusion to those in the NPRM because single unit trucks are far less

likely to be involved in fatal accidents than combination trucks (i.e.,

trailers and semitrailers). FARS and GES accident statistics indicate

that only about 27 percent of the 423 average annual rear end

fatalities and 18 percent of the 5,030 injuries involve single unit

trucks, even though these vehicles represent 72 percent of the

registered heavy vehicles. Thus, single unit trucks are significantly

under-represented in rear end crashes. On the other hand, trailers are

highly over-represented in rear end crashes, as they represent only 28

percent of the registered heavy vehicles, but account for 73 percent of

the occupant fatalities and 82 percent of the injuries. Therefore, the

agency believed that excluding single unit trucks from the proposed

rule would result in a better rule in terms of the ratio of benefits to

costs.

VII. Summary of 1992 SNPRM Comments

The agency received approximately 2,250 individual comments on the

SNPRM. Industry-related comments were generally supportive of the

proposal, while consumer interest organizations, local and State

governments, and private citizens were generally critical.

Representing the industry were comments from automobile and truck

manufacturers, trade associations, manufacturers of trailers and

semitrailers, and manufacturers of specialized usage heavy duty

vehicles. Most of these commenters supported Federal rulemaking in this

area. The trade associations and manufacturers of trucks and trailers

were generally in agreement with the proposed requirements.

Manufacturers and operators of specialized vehicles suggested that the

proposed rule be modified to better define the types of vehicles that

would be excluded from the standard.

The vast majority of the critical comments were post cards or

letters with multiple signatures from private citizens. These post

cards and letters, as well as more detailed submittals from consumer

interest organizations, expressed concern that the agency's proposal

had three deficiencies. First, the commenters recommended that the

rulemaking apply to single unit trucks as well as trailers and

semitrailers. Second, the commenters recommended that the proposed

maximum ground clearance, 560 mm (22 in), be reduced to a 405 to 455 mm

(16 to 18 in) range. Third, these commenters expressed the opinion that

the agency should mandate ``energy absorbing'' rear impact guards for

heavy vehicles, i.e., guards with hydraulic pistons or shock absorbers

designed to deflect or deform in a controlled manner upon impact and

thereby lessen the deceleration experienced by passenger vehicles

colliding with them. Several consumer interest organizations and

private citizens also suggested that the proposed minimum guard

strength requirements were insufficient.

The consumer interest organizations and some private citizens also

expressed concern that the proposed equipment standard for the rear

impact guard did not require guards to be tested while mounted on a

vehicle. As a result, guards complying with the proposed strength

requirements could be installed on vehicles in such a location or in a

manner that the guard/vehicle combination would be ineffective. In

addition, some of these commenters stated that the crash tests NHTSA

relied on in formulating the SNPRM were inadequate because they were

not conducted under representative conditions of guard height, car

bumper height, and car speed. Specifically, they stated that car bumper

height would be depressed if the driver were braking to avoid colliding

with the trailer, thus increasing the likelihood that the car hood

would underride a 560 mm (22 in) high guard without engaging any

substantial body structure. The consumer interest organizations also

questioned the validity of the FARS accident data that NHTSA used to

determine the benefits of the SNPRM, contending that the agency had

underestimated the benefits of the rule. The state and local

governments that commented expressed concerns similar to those raised

by private citizens and consumer interest organizations.

A summary of comments has been prepared and is available for

inspection in Docket No. 1-11. Significant SNPRM issues raised by the

commenters and NHTSA's response to the comments are discussed below. In

response to the comments, the final rule includes several modifications

to the rule proposed in the SNPRM, including clarified definitions,

improved compliance test procedures, and a minimum guard energy

absorption requirement.

VIII. Recent Testing by NHTSA

In responding to comments to the SNPRM and a congressional request

for a report on several heavy truck underride issues, NHTSA conducted a

research project on Heavy Truck Rear Underride Protection at the

Vehicle Research and Testing Center (VRTC) between September 1992 and

June 1993 to evaluate the effectiveness of an underride guard meeting

the requirements of the SNPRM. A copy of the test report (VRTC-82-0267)

was placed in the public docket (No. 01-11-N09-54. See also Publication

No. DOT-HS-808-081).

For the purposes of the evaluation, NHTSA took the conservative

approach of modifying the most common conventional guard design and

developed a rear impact guard that was only slightly (10 percent)

stronger than the minimum requirements of the SNPRM when tested at the

vertical supports, which is the most significant location along the

width of the guard's horizontal member. NHTSA arrived at this

``minimally complying'' design through an iterative process of

fabrication and testing in accordance with the proposed compliance test

procedures.

These minimally compliant guards were then evaluated in two series

of full scale crash tests. The guards provided the proposed maximum

ground clearance 560 mm (22 in). For the initial series of crash tests,

the guards were mounted to a test fixture simulating the geometry of

the rear end of heavy trailers. The guards were mounted on a late model

production trailer for the other series. A total of seven crash tests

were conducted with the minimally complying guard design. The tests

were conducted at an impact speed of 48 kph (30 miles per hour (mph))

with late model compact and subcompact cars with mass between 1135 and

1590 kg (or weight between 2500-3200 lbs). In each category, vehicles

were selected which had low hood profiles, and were therefore most

likely to underride the 560 mm (22 in) guard height.

Four of the seven crash tests resulted in no PCI when the minimally

compliant guard was mounted flush with the rear extremity of the

trailer and simulated trailer. See Tables 6, 8, and 10 of the VRTC test

report. The hood of one passenger car was driven through the windshield

during one of these tests (Corsica 1, VRTC test report, page 26). The

magnitude of the passenger compartment intrusion by the hood was

marginal, however, and the test dummies were not contacted by the hood

during the collision. Two cases of PCI were caused by guard system

failure, one in a simulated trailer test and one in a production

trailer test (respectively, Saturn 1 in Table 8 and Corsica (trailer)

in Table 10 of VRTC test report). The guard system failure in the

simulated trailer test was due to attachment hardware failure. The

failure in the production trailer test was the

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result of trailer structural failure at the guard attachment locations.

In each case, the guard attachment hardware and the trailer structure

were upgraded with simple, inexpensive materials for subsequent tests.

Retests with the modified hardware and trailer frame showed adequate

guard system performance.

All these crash tests included Hybrid III test dummies positioned

in the driver and outboard front passenger seating locations for each

crash test. The procedures used for frontal barrier crash test

preparation under FMVSS No. 208, Occupant Crash Protection, were

followed with respect to dummy positioning, restraint usage, and dummy

instrumentation. Dummy instrumentation indicated very low potential for

serious or fatal injury in all seven of the crash tests with the

minimally compliant guard, even those in which there was PCI.

The VRTC research project also performed a crash test using a very

strong, i.e., ``rigid,'' guard, to compare the amount of underride and

deceleration forces generated with those generated by the minimally

compliant guard. The 48 kph (30 mph) impact generated a peak force of

about 415 kN (93,000 lbs) and the guard sustained an insignificant

amount of permanent deformation. Although underride in this crash test

was minimal, occupant compartment forces generated during the crash

were significant, with on-board dummy readings indicating a potential

for serious driver chest injuries (dummy chest acceleration was 61 G,

slightly higher than the 60 G permitted in FMVSS No. 208, Occupant

Crash Protection). A similar crash test with the minimally compliant

guard was conducted with the same make and model passenger vehicle. The

minimally compliant guard, with a force resistance capability of about

200 kN (45,000 lbs), allowed more underride than the rigid guard and

marginal PCI. However, at 48 kph (30 mph), the minimally compliant

guard test generated occupant compartment forces low enough that they

posed essentially no potential for life-threatening occupant injuries.

This test further demonstrated the adequacy of the proposed guard

ground clearance requirement of 560 millimeters (mm) (22 in).

IX. Overview of the Final Rule

This rule establishes two Federal Motor Vehicle Safety Standards.

The two standards are being announced in this single notice because

they are complementary and because their substantive requirements both

derive from a single standard proposed in an earlier NPRM (Docket No.

1-11, notice 8). The first standard will be referred to as the

``equipment standard'' because it sets forth requirements that a rear

impact guard must meet as an item of motor vehicle equipment. The

second standard will be referred to as the ``vehicle standard'' because

it requires a new trailer or semitrailer to be equipped with a guard

that meets the equipment standard.

The equipment standard specifies the procedures that the agency

will use when testing a guard. The guard is first mounted to a rigid

test fixture or a secured trailer, in accordance with the installation

instructions which the guard manufacturer is required to provide. The

standard describes how to select three test locations across the width

of the guard. At these three locations, the testing procedure provides

that force be slowly applied until the guard has been deflected by 125

mm (5 in). The standard specifies procedures for determining whether

the tested guard has met the minimum requirements for strength and

energy absorption. Guards that can pass the strength and energy

absorption tests may be certified and labeled as complying with the

equipment standard and sold to vehicle manufacturers if accompanied by

the necessary attachment hardware and mounting instructions.

The guard mounting instructions are a crucial interface between the

equipment standard and the vehicle standard. NHTSA has modified the

equipment standard proposed in the SNPRM to require the guard

manufacturer's instructions to include (1) a description of the types

of structures to which attachment must be made, and (2) the manner in

which attachment must be made, in order for the guard to perform in its

designed fashion.

The vehicle standard requires that most new trailers and

semitrailers be equipped with a rear impact guard certified to the

equipment standard. The vehicle manufacturer can manufacture and

certify the guards according to the equipment standard, or simply

purchase and install certified guards from a guard manufacturer. The

vehicle standard requires that the guards extend laterally to within

100 mm (4 in) of the sides of the trailer, that the guard have a ground

clearance of no more than 560 mm (22 in), and that the guard be placed

as close to the rear of the vehicle as possible. To ensure that the

guard will perform properly, the vehicle standard further requires that

the guard be mounted on the trailer or semitrailer in accordance with

installation instructions provided by the guard manufacturer.

The vehicle standard lists and defines certain types of vehicles

that are excluded from the requirement to have rear impact guards.

Single unit (unarticulated) trucks, truck tractors, pole trailers, low

chassis vehicles, special purpose vehicles, and wheels back vehicles do

not have to have rear impact guards.

X. Summary of Changes From the 1992 SNPRM

The greatest change from the SNPRM is the addition to the equipment

standard of a requirement for energy absorption. The SNPRM would have

permitted fairly rigid guards because it did not require the guard to

yield in response to force. Rigid guards may stop the passenger vehicle

too quickly, causing occupant deaths and injuries from sudden

deceleration. To ensure that the guards will yield, this rule adds a

requirement that the guards absorb a certain amount of energy during

the strength test. The new requirement does not necessitate the use of

any additional new test equipment or the following of any additional

test procedures. It does require more frequent measurements of the load

during the strength test, and a few extra calculations after the test.

The test procedures in the equipment standard have been modified to

allow velocity-sensitive rear impact guards. Velocity-sensitive guards

would have failed the quasi-static strength test procedure proposed in

the SNPRM because these guards are designed to provide resistance that

is proportional to the displacement rate, and the test procedure

displaces the guard very slowly. The final rule provides for modifying

the guards to deactivate the energy absorbing components prior to the

strength test. Because velocity sensitive guards typically have

excellent energy absorption characteristics and because quasi-static

testing does not test their energy absorbing capabilities, velocity-

sensitive guards do not have to be tested for energy absorption. The

only type of velocity-sensitive guards that the agency is aware of use

hydraulic fluid properties to deform in a controlled manner. Therefore,

these ``hydraulic guards'' are the only ones excluded from the energy

absorption test.

The final rule requires greater specificity in statements regarding

trailer structure in the installation instructions provided by the

guard manufacturer. The SNPRM said only that the instructions had to

specify the types of vehicles for which the guard was intended, state

the necessity for

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attaching the guard to the vehicle chassis, and explain how the

attachment hardware was to be used. The regulatory text of the final

rule makes it clear that the installation instructions must specify all

aspects of the trailer that are necessary to the proper functioning of

the guard. The test procedure has been modified to indirectly test the

adequacy of the attachment.

NHTSA has changed some of the guard configuration requirements in

the vehicle standard. The SNPRM proposed to require that the horizontal

member of the guard extend to within 100 mm (4 in) of the side

extremities of the vehicle and to within 305 mm (12 in) of the rear

extremities. These requirements have been modified to allow rounded

guard ends. The final rule allows an extra six inches in these

dimensions only for the portion of a guard that is curved. Using

rounded guard ends will diminish the hooking potential of the guards

when the trailer is turning sharply. Guard ends that are rounded upward

and attached to the vehicle may add strength to the horizontal member

near the side extremity of the vehicle.

To account for high, overhanging rear protrusions on trailers,

NHTSA changed the definition of the vertical zone to be considered when

determining the trailer's ``rear extremity.'' Determination of the

``rear extremity'' is important because the location of the guard is

based on the location of the rear extremity. The SNPRM defined ``rear

extremity'' as the rearmost point above 560 mm (22 in) from the ground.

Since high overhangs pose no risk to colliding passenger vehicles,

NHTSA has set a maximum height of 1905 mm (75 in) from the ground on

the zone. Higher protrusions will not be considered as the rear

extremity.

Another change in the configuration requirements is that the final

rule requires the guard to be mounted as close to the rear extremity as

practical within the 305 mm (12 in) zone forward of the rear extremity.

The SNPRM did not regulate where in the zone the guard had to be

mounted.

XI. Analysis and Response to Comments on the 1992 SNPRM

A. Separate Equipment and Vehicle Standards

Companies such as Waltco and industry groups such as the National

Truck Equipment Association supported the separate equipment and

vehicle standards as a method to prevent undue testing burdens.

One of the concerns raised by consumer interest organizations is

that allowing the guards to be tested on a ``non-vehicle'' rigid test

fixture posed a problem if it is done in the expectation that the

guards would necessarily perform in a similar manner once they are

installed on vehicles. The Institute for Injury Reduction (IIR)

commented that neither the equipment standard nor the vehicle standard

specifies or regulates the interface between the guard and the vehicle.

Therefore, IIR was concerned that there are no ``real-world'' tests

performed on the guards as installed on the vehicle and suggested that

it is unclear whether a failure of such a test would represent

noncompliance by the guard manufacturer, the vehicle manufacturer,

both, or neither.

NHTSA agrees that an underlying assumption of this regulatory

scheme is that the guards would perform in the real world in a manner

similar to the way they do in the tests. This assumption is supported

by the results of the VRTC research project, which show that the

maximum force measured in quasi-static tests is similar to the maximum

force generated in dynamic crash tests. Moreover, this regulatory

scheme has worked well for tires, which also have separate equipment

(49 CFR 571.109) and vehicle (Sec. 571.110) standards.

NHTSA disagrees with IIR's argument that separate guard and vehicle

standards leave the guard/vehicle interface unregulated. The vehicle

standard specifies that the guard be attached in accordance with the

guard manufacturer's installation instructions, the same instructions

used to attach the guard to the test fixture during agency compliance

testing under the equipment standard.

When writing installation instructions, the guard manufacturer must

take into account the possibility of inadequate trailer structure to

support the guard. Depending on the guard design, the guard

manufacturer may want to specify in the instructions that the guard

cannot be attached to certain structures (e.g., floorboards) and that

it must be attached to other surfaces, for example, frame rails with a

horizontal surface and specified wall thickness of a certain material

(e.g., hardened steel). The guard manufacturer may have to specify

local reinforcement if the trailer chassis is inadequate to pass the

compliance test with the chassis surface mounted on the rigid test

fixture.

The installation instructions must be appropriate to the trailer

design, so that the vehicle manufacturer knows which guard to purchase

and does not have to deviate from the instructions to install the

guard. To help assure this, the regulatory text has been modified to

make it clear that the guard manufacturer must either list appropriate

trailers or specify in the installation instructions all attributes

that make a trailer suitable for the proper installation and

functioning of the guard. These include the types of trailer

structures, design types with dimensions, materials thickness and tire

track widths that are appropriate as an installation location.

NHTSA will install the guards during compliance testing based on

these instructions. Therefore, it is essential that the attachment site

and attachment method be adequately specified. This is especially

important to avoid failure of the attachment itself during the test.

In a VRTC test of the minimally complying guard mounted on a

typical trailer, the trailer frame rails worked with the guard by

bending/deforming to absorb the colliding vehicle's crash energy.

However, the attachment site on the frame rails had to be strengthened

with an inexpensive local reinforcement.

IIR's argument that failure during compliance testing would leave

the identity of the non-complying party in doubt is incorrect. The only

testing procedures in NHTSA's rule are the compliance tests in the

equipment standard. Therefore, the only party that can be responsible

for a testing failure is the guard manufacturer. Noncompliance by the

vehicle manufacturer may be established by inspecting the vehicle and

observing improperly installed guards, such as during an FHWA heavy

truck inspection. If the vehicle manufacturer manufactures the guard

which it uses, as NHTSA believes will usually be the case, there will

be no ambiguity as to the party responsible for testing failure or

improper installation.

B. Standard for Equipment

1. Relationship of Strength, Energy Absorption, and PCI

In specifying performance standards for rear impact guards, the

agency must balance various performance attributes. The vast majority

of the commenters, including virtually all of the consumer safety

groups, asserted that underride guards should be strong, yet energy

absorbing. NHTSA agrees that these are both desirable properties in an

underride guard, but emphasizes that an increase in strength may result

in a decrease in the capability of the guard to absorb energy, and vice

versa. An impact guard strong enough to restrain a large car travelling

at high speeds would impart high deceleration forces to a small car

crashing into it at the

[[Page 2009]]

same speed. Conversely, an impact guard that is optimized to restrain a

small car without excessive deceleration forces might fail (i.e.,

deform so much that it allows PCI) if a large car crashes into it, or

if a small car crashes into it at higher speeds.

Energy absorption must also be balanced against PCI prevention.

Energy absorption may be maximized by allowing the guard to yield for a

greater distance before bringing the passenger car to a stop. However,

the more the guard yields, the farther the colliding vehicle travels

and the greater the likelihood of PCI. This rulemaking has focussed on

balancing the need for PCI-prevention against minimizing crash

injuries. FARS data show a strong correlation between PCI and

fatalities or serious injuries. Preventing PCI demands a guard that is

strong enough to prevent the passenger vehicle from advancing very far

after contact with the guard.

Compounding the difficulty of balancing the guard's performance

attributes is the wide range of colliding passenger vehicle weight,

speed, and size. The combination of weight and speed determines the

level of kinetic energy to which the guard will be subjected. Passenger

vehicle weight generally correlates with the hood height and length,

which determines how far the vehicle can proceed after contact with the

guard before PCI occurs. Fortunately, these factors offset one another

for large cars (i.e., the greater weight promotes greater amounts of

underride, while the higher hood profile results in better guard

engagement and the longer hood allows for more underride before

experiencing PCI).

Small pickups and vans have relatively high profiles but a

relatively short distance from the front of the vehicle to the occupant

compartment. A guard would have to yield only slightly, or have high

strength to prevent minivans and some pickups (which typically have a

mass more than 1810 kg or weigh more than 4,000 lbs and have short

hoods) from experiencing PCI. Because the passenger compartment is so

close to the front of a heavy standard van, no underride guard is

likely to be very effective in preventing PCI for these vehicles.

Nevertheless, some reduction in fatalities and non-fatal injuries can

be expected due to the initial energy absorption of the guard.

Fortunately, vans have only been involved in 0.5 percent of all

underride fatalities from 1982 to 1992. Pickups have been involved in

about 18 percent of the fatalities during this period.

It should be recognized, therefore, that impact guards cannot be

optimized for all situations. The requirements in this rule should

reduce the incidence of PCI, fatalities, and injuries for all passenger

vehicles, but some more than others. A minimally compliant guard should

protect all passenger vehicles from PCI and excessive deceleration

forces up to some speed in the 40 kph (25 mph) to 56 kph (35 mph)

range, although that speed will vary on a sliding scale depending on

the vehicle weight and front end profile. For example, NHTSA

analytically estimates that mid and full size cars and light trucks and

vans with a mass greater than 1590 kg (3,500 lbs) will experience PCI

at approximately 43 kph (27 mph), while mini-compacts of less than 1135

kg (2,500 lbs) will be able to collide with the required guard at about

61 kph (38 mph) without PCI. This estimate is obtained by equating the

energy absorbed by a 48 kph (30 mph) collision of a 1590 kg (3,500 lb)

vehicle rigid barrier crash to the energy absorbed by a different

weight vehicle). For example, for a 907 kg (2000 lb) vehicle, the

calculated impact speed without PCI is: (square root of (1,590 kg/907

kg)) x 48 kph=63.5 kph, or (square root of (3,500 lb/2,000 lb)) x 30

mph=39.7 mph.

2. Guard Strength

Several consumer interest organizations and private citizens

criticized the 1992 SNPRM's proposed guard strength requirements. These

commenters' objections are either that guards meeting the requirements

would be too weak to prevent underride or that they would be so strong

that the passenger vehicle would be subjected to excessive deceleration

forces. As explained above, the issues of strength and energy

absorption are closely related. However, issues relating primarily to

energy absorption will be addressed in the next section.

The SNPRM, which was premised upon underride protection being

provided by a horizontal member, proposed to require that the

horizontal member resist a force of 50 kilonewtons (kN) (11,240 lbs)

applied at the center (site P2) and near the outboard ends (sites P1),

and a force of 100 kN (22,480 lbs) at an intermediate position (sites

P3), in separate quasi-static strength tests. For these tests, guard

resistance at the specified force level would have to occur at less

than or equal to a 125 mm (5 in) displacement of the guard's horizontal

member.

Several commenters stated that overly ``rigid'' or non-yielding

guards would be permitted by the proposed rule. They expressed concern

that those guards would be too stiff, citing the results of full-scale,

heavy truck rear underride crash tests conducted in the late 1970's and

early 1980's by the Texas Transportation Institute (TTI), Dynamic

Sciences, Inc., and the Insurance Institute for Highway Safety (IIHS).

These crash tests indicated occupant compartment forces generated in

collisions with rigid guards at impact speeds above 48 kph (30 mph)

could produce potentially fatal driver and front passenger head and

chest injuries.

Advocates for Highway and Auto Safety (Advocates) stated that the

proposed guard would not perform as well as the agency expects, and

would be excessively deformed or fail in impacts not much above 40 to

48 kph (25 to 30 mph). Advocates further stated that NHTSA directed its

contracted researcher in 1982 to reduce the impact speed of a dynamic

crash test on a Chevrolet Impala from 48 kph (30 mph) to 40 kph (25

mph), specifically to ensure that excessive underride did not occur.

The actual speed of the tested 1,840 kg (4,060 lb) Chevrolet Impala was

38.5 kph (23.9 mph). Advocates contends that the agency admitted in a

memorandum from Mr. Tomassoni (who worked for NHTSA at the time) that

the test would have resulted in PCI at 48 kph (30 mph). IIHS also

included these criticisms in its comment.

Some commenters recommended that NHTSA require specific levels of

strength higher than those proposed in the SNPRM. Advocates attached a

1991 technical paper by Mr. G. Rechnitzer, of Monash University in

Australia, which reviewed European truck underride data. The example

with the widest application, the Economic Commission for Europe's (ECE)

Regulation No. 58 for heavy truck rear underride guards, currently

requires a guard force resistance of 100 kN (22,480 lbs) at the point

on the guard corresponding with this rule's P3 test point, 50 kN

(11,240 lbs) at the center, and up to 25 kN (5,620 lbs) at the outboard

test position corresponding with this rule's P1 position. Mr.

Rechnitzer recommended that the rear impact guard strength requirements

be upgraded to 150 kN (33,370 lbs) at the P3 location and 100 kN

(22,480 lbs) at the center and P1 locations. Mr. Byron Bloch, of Auto

Safety Design, suggested an even stronger guard. He thought the rule

should require that the guard resist 222 kN (50,000 lbs) at the P3 test

location, where the SNPRM requires that the guard resist a force of 100

kN (22,480 lbs).

The VRTC tests indicate that the strength of the 1992 SNPRM guard

is adequate for preventing underride with

[[Page 2010]]

PCI in a collision with an impact speed of up to 48 kph (30 mph) for

vehicles with a mass of about 1,450 kg (3,200 lbs). PCI resistance

would be expected at higher impact speeds for lighter vehicles and

lower impact speeds for heavier vehicles. The test data also indicate

that rear impact guards having somewhat more strength than the proposed

level of strength could resist PCI at higher impact speeds without

generating life-threatening passenger compartment force levels.

Although stronger guard strengths may be desirable, the agency cannot

quantify the increased benefits that might be obtained without further

testing.

Based on the VRTC tests, the agency believes that the guard

strength requirements proposed in the 1992 SNPRM are of sufficient

magnitude to prevent PCI for most late model passenger vehicles at

impact speeds of about 45 kph (28 mph). This rule has an additional

requirement that guards yield enough to maintain survivable levels of

occupant compartment deceleration when impacted by passenger vehicles.

Therefore, the agency has decided to retain the strength requirements

of the SNPRM in the final rule.

The IIHS advocated a specific guard design, which it said was

preferable for strength purposes. That organization believes a diagonal

strut from the horizontal member of the guard to the trailer chassis

could augment guard strength without a large increase in guard weight.

NHTSA agrees with the IIHS that this type of design is quite efficient

with respect to weight and strength, though not necessarily with

respect to energy absorption. However, the agency does not believe that

it is necessary or desirable to mandate a specific design, since

similar crash performance may be achieved with other designs.

3. Guard Energy Absorption

Although all non-rigid guards absorb some of the kinetic energy of

the striking vehicle, there was considerable concern that the SNPRM did

not require energy absorbing guards. The consumer interest

organizations and about 2,200 private citizens urged NHTSA to mandate

``energy absorbing'' guards. By deforming, rear impact guard structures

absorb some of the kinetic energy of the striking vehicle. The more

energy the guard absorbs, the less energy must be absorbed by

deformation of the striking vehicle before it stops. Commenters were

concerned that the SNPRM would have permitted rigid guard designs that

would impart high levels of crash forces to the striking vehicle's

occupants.

As used by the consumer interest groups, the term ``energy

absorbing guards'' generally refers to guards whose vertical support

members are designed to pivot about their attachment braces at the

vehicle chassis. These guards absorb energy by means such as

cylindrical, telescoping hydraulic or plastic struts, which are also

attached to the guard's horizontal member and the vehicle chassis. When

impacted, these energy absorbing units respond by compressing without

substantial deformation until the units have reached their maximum

deflection, or ``bottomed out.'' On the other hand, the primary energy

absorbing mechanism of a fixed guard, such as the design used in the

VRTC tests, is the flexing and bending of the guard's vertical

supports. Keeping this in mind, the agency uses the term ``energy

absorbing guards'' below in the same sense as used by the commenters,

as a shorthand way of referring to guard designs with special energy

absorbing design features.

Advocates recommended that guards be required to be energy

absorbing so that 64 kph (40 mph) impacts of small cars with the rear

of heavy vehicles are survivable through the combined energy absorption

of the car and the guard. The National Association of Independent

Insurers (NAII) suggested that the proposed rule be modified to require

a more flexible, energy absorbing guard. Citizens for Reliable and Safe

Highways (CRASH) stated that the agency fails to acknowledge the need

for and potential benefits from improved, slightly more expensive,

energy absorbing guards that are in use in Europe.

To ensure that the guard will provide the combination of strength

and energy absorption necessary to prevent underride with PCI at a

specified impact speed, as recommended by Advocates, a full-scale

dynamic compliance test including a passenger vehicle would be

necessary. VRTC conducted full-scale crash tests with guards that were

also tested in accordance with the SNPRM compliance procedures. These

tests demonstrated that the proposed quasi-static compliance test is

adequate for determining guard strength. The peak forces generated by

the guard in the quasi-static compliance tests and the full-scale crash

tests were approximately the same. Guard strength or peak force

capability is the primary factor in underride prevention. Guard energy

absorption characteristics determine the guard's ability to maintain

impact forces at survivable levels in the striking vehicle, as well as

the guard's resistance to structural failure.

The agency has decided to retain the quasi-static compliance test

for guard strength due to the greater complexity and cost of a dynamic

compliance test procedure. Although the guard's ability to resist PCI

at a specific impact speed will not be tested directly, the VRTC tests

show that dynamic guard performance can be accurately estimated from

the quasi-static compliance test results. Therefore, it is not

necessary to conduct expensive full-scale dynamic tests to attain most

of the benefits of dynamic testing.

Advocates also stated that British researchers assess the potential

fatality reduction effectiveness of stronger, energy absorbing guards

at 25 to 35 percent. This is about twice the current guard

effectiveness in Europe, according to the document cited by Advocates,

an opinion paper by P.F. Gloyns, et al., of Vehicle Safety Consultants,

Ltd., entitled ``Legislative Implications of Accident Experience in the

UK of Rear Under-Run Guards.'' The Gloyns paper does not quantify the

increase in guard strength or the magnitude of guard energy absorption

required to achieve the estimated increase in guard effectiveness. The

agency acknowledges that various combinations of guard strength and

energy absorption capability could increase the effectiveness of rear

impact guards. However, without more quantitative information, NHTSA

cannot address the guard effectiveness claims of Gloynes, et al.

It may be that energy absorbing rear underride guards, which were

referred to by CRASH and which are currently in use on one to two

percent of vehicles in Europe, are superior to a moderate strength,

fixed guard meeting the minimum performance requirements specified in

the rulemaking proposal. The agency notes that these European guards,

or guards with similar energy absorbing characteristics and design

features, would not be prohibited by NHTSA's proposed rule and will no

doubt be considered by the industry as a possible means of compliance,

just as they were in Europe.

The agency has tested one guard, the Quinton-Hazel rear impact

guard, which utilized pivoting vertical support members along with

telescoping hydraulic struts and coil springs. The guard demonstrated

excellent overall performance in a crash test conducted in 1979 by the

Texas Transportation Institute. The striking crash test vehicle was a

1,810 kg (4,000 lb) Chevrolet and the impact speed was 56 kph (35 mph).

The collision did not result in PCI, and all measured occupant

responses indicated that the potential for driver

[[Page 2011]]

and front passenger serious injuries was low. It is estimated that

similar guards would weigh about 1.33 to 3 times more and cost 3 times

more than a fixed, moderate strength guard designed to meet the

requirements of the SNPRM. In other words, it would cost $300-$350 and

have a mass of 136 kg (300 lbs) to 181 kg (400 lbs). Further, hydraulic

energy absorbing guards would be considerably more complex than fixed

guards that comply minimally with this rulemaking, and would require

periodic maintenance. It is NHTSA's understanding that there are

currently no guards in production in this country or in Europe that

utilize hydraulic or plastic energy absorbing, telescoping units. A

letter from one of the former manufacturers, Quinton-Hazel, indicates

that the market probably rejected them as too costly.

Nevertheless, in response to the comments recommending energy

absorbing guards, the agency has added a performance requirement for

guard energy absorption to the rule. The requirement does not include

design specifications such as pivoting vertical supports or telescoping

energy absorbing units. The agency is requiring that each guard absorb

a minimum amount of energy based on the forces and displacements

specified in the 1992 SNPRM. The same quasi-static compliance test

procedure proposed for strength testing will be used to determine

compliance with this new specification. The test for guard energy

absorption will be conducted only at the P3 location used for guard

strength testing. The minimum magnitude of guard energy absorption will

be 5,650 joules (4,170 foot-pounds), which is based on the force

required to comply with the strength test at the P3 test location and

the maximum displacement allowed for the guard to generate the force

(125 mm, or 5 in). The energy absorption test will require that the

guard's horizontal member undergo 125 mm (5 in) of displacement while

the force generated by the guard is recorded at least ten times per 25

mm. The magnitude of guard energy absorption at the P3 location is

sufficient to absorb about 12 percent of the total kinetic energy of a

48 kph (30 mph) centric collision with a 1,135 kg (2500 lb) vehicle.

This magnitude of guard energy absorption capability is also similar to

the amount recommended in several British research papers provided by

Advocates.

Several commenters, including consumer interest organizations and

trailer manufacturers, stated that the proposed rule would permit

overly ``rigid'' or non-yielding guards that would absorb little or no

crash energy. The commenters expressed concern that those guards would

be too stiff and would result in fatal driver and front vehicle

passenger head and chest injuries.

The agency has drafted the energy absorption requirement to address

these concerns. NHTSA recognizes the potential trade-off between

designs of underride guards that minimize occupant injury criteria

responses and those that provide the most protection from PCI. The

agency also recognizes that an increase in the level of rigidity from

the minimally compliant guard used in the VRTC tests is desirable, but

this should not be at the expense of energy absorption. On the other

hand, the agency does not want to restrict or dictate guard design by

specifying the rigidity of the guard. Therefore, to discourage overly

rigid guards, this rule requires that a minimum amount of the energy be

absorbed during the energy absorption test from permanent yielding, or

plastic deformation, of the guard. After the guard has reached the full

125 mm (5 in) of deformation, the load is reduced and any elastic

``rebound'' of the guard is measured until the load is zero. The

elastic component of the energy that is returned by the guard is not

included in the calculation of total energy absorbed by the guard. This

method gives guard designers flexibility to select guard material

properties and frame member spatial configuration.

Some commenters observed that the test procedures proposed in the

SNPRM precluded the use of hydraulic energy absorbing guards. Mr. John

Tomassoni stated that the 125 mm (5 in) displacement maximum allowed in

the strength test would allow only passive structures such as steel

struts designed to bend on impact. This is because active energy

absorbing struts that are hydraulic (analogous to a vehicle shock

absorber) are velocity sensitive. With the slow application of force

during the quasi-static test, the hydraulic fluid units would develop

almost no resistance. He recommended adding a ``bottoming'' provision

to allow static testing after hydraulic systems have reached full

stroke.

NHTSA agrees that quasi-static test procedures are inappropriate

for hydraulic guards, or any other type of velocity sensitive guard

(although NHTSA is unaware of any non-hydraulic guards that are

velocity sensitive). A dynamic test would be required to assess their

energy-absorbing capabilities by supplying the sudden onset of force

their energy absorbing units require to generate resistance. Because

the agency does not want to discourage the use of these advanced guard

designs by requiring expensive dynamic tests, and because these guards

typically have excellent energy absorbing capabilities, the final rule

excludes these guards from the energy absorption requirements.

There are also problems with subjecting velocity sensitive guards

to the strength requirement. However, complete exclusion of those

guards from the performance requirements would be inappropriate.

Accordingly, the agency has modified the test procedures to allow

velocity sensitive guards to be tested for compliance with the strength

requirement. The agency is concerned that, if the hydraulic energy

absorbing units do not operate properly, the guard will not generate

significant resistance and energy absorption. NHTSA wants to assure

that the guard has enough residual strength, even without the energy

absorbing units, to meet the same strength requirements as other

guards. Therefore, velocity sensitive energy absorbing guards will be

tested by slowly compressing the energy absorbing units to the full

extent of their designed travel or 610 mm (24 in), whichever occurs

first. This will allow the frame of the guard itself to generate

resistance, rather than having the piston simply compress the hydraulic

shock absorbers.

4. Vertical Cross-sectional Height of Horizontal Cross-member

The SNPRM proposed a minimum vertical cross sectional height of 100

mm (4 in) across the entire width of the guard's horizontal cross-

member. Advocates stated in its comment that the guard must be at least

205 mm (8 in), and preferably 305 mm (12 in), high to better manage the

loading impact forces and assure full engagement of the vehicle front

end. In contrast, the Truck Trailer Manufacturers Association (TTMA)

suggested reducing the requirement, urging that the guard be only 50 mm

(2 in) high because that is all that is required for adequate strength.

It asserted that requiring greater vertical cross section height just

adds unnecessary weight and cost to the guards.

NHTSA agrees with Advocates' position that a higher vertical cross

section has the potential to better distribute the impact forces, but

this does not mean that the proposed 100 mm (4 in) height is

insufficient. The 100 mm (4 in) height would be inferior if it sheared

or ``cut'' through the front of the striking vehicle, thus allowing

forward vehicle motion without much energy absorption due to the low

magnitude of

[[Page 2012]]

force generated by the guard. A guard should cause the vehicle to

absorb energy by crushing, rather than shearing through, frontal

vehicle structural components. Shearing through did not occur in the

agency's testing with a 100 mm (4 in) high guard horizontal member.

None of the crash tests conducted pursuant to this rulemaking resulted

in significant shearing of the passenger vehicle's frontal structure

(above the 560 mm (22 in) high guard). The crash tests show that the

100 mm (4 in) profile of the guard horizontal member resulted in

adequate engagement of the car's front end and is harmonized with the

guard specified in ECE Regulation 58. Moreover, a 205 mm (8 in) high

profile may require heavier and more expensive guards. Finally, the

agency notes that 100 mm (4 in) is only a minimum height, so guard

manufacturers are free to manufacture the guards that Advocates

recommends. Accordingly, the agency concludes that a higher vertical

cross sectional height requirement is unnecessary.

NHTSA also disagrees with TTMA's position that a 50 mm (2 in)

vertical cross sectional height would be appropriate. The TTMA did not

provide any data to support its assertion that the strength should be

adequate. Even if the 50 mm (2 in) height were sufficient for strength

purposes, it would have a greater tendency to shear into the front of

the passenger vehicle instead of crushing it. This would result in a

reduction of energy absorption by the guard and an increase of the

striking vehicle damage in low speed crashes of 16 to 24 kph (10 to 15

mph). Accordingly, the agency has decided to retain the 100 mm (4 in)

cross sectional vertical height requirement in the final rule.

5. Shape of the Horizontal Cross-member

Some commenters stated that NHTSA should require the guards to have

blunted or rounded ends. The Florida Department of Transportation,

based on visual evaluations of the installed guard, stated that the

requirement that the guard extend to within 100 mm (4 in) of the side

of the vehicle would make it a dangerous ``hook'' for adjacent

vehicles, especially during sharp turns of the trailer. It suggested

requiring a ``U'' shaped guard, similar to one used by some carriers

which is attached at either end to the underside or rear of the

vehicle. It thought that the ends on these guards could be located

further inboard. The TTMA had a similar suggestion, proposing that

NHTSA allow (but not require) guards with rounded corners, to lessen

the hooking potential when the sliding tandem is positioned forward.

The TTMA suggested that the rule be modified to allow such guards to

begin curving at a point 255 mm (10 in) inboard of the edges of the

vehicle, while retaining the 100 mm (4 in) requirement for straight

guards.

NHTSA agrees that there is some potential for hooking the guard on

the fenders and wheel wells of adjacent passenger vehicles when the

rear end of the trailer swings out laterally during a sharp turn. This

phenomenon would be accentuated when the rear wheels on a sliding

tandem are positioned forward. The rear wheels are generally positioned

forward to give the trailer greater maneuverability, so it is likely

that trailers in this configuration will be making sharp turns.

On the other hand, rounded or U-shaped guards would be more

expensive to manufacture and would weigh more. Moreover, rounded

corners offer very limited potential added value on roadways where

sharp turns are infrequent, such as on the interstate highways, which

are heavily traveled by trailers. Therefore, while the agency wants to

allow guards with rounded ends for operations where they are desired,

NHTSA does not think it is necessary or even appropriate to require

them.

The commenters referred to rounded guard ends that curve upward,

but a rounded end that curves forward could also be useful. It would

serve the purpose of making hooking less likely because the guard end

would sweep through a smaller arc and present a less pointed profile to

adjacent passenger vehicles. Moreover, forward-curving guards could

slightly enhance guard effectiveness if a passenger vehicle strikes the

trailer in the rear corner at an angle. However, forward-curving guard

ends might interfere with the rear wheels if a sliding tandem were

moved to the rearmost position.

NHTSA notes that the SNPRM would not prohibit guards with rounded

ends, but its configuration requirements would have restricted their

curves to a 100 mm (4 in) radius of curvature. To minimize hooking

potential and property damage in some applications, the final rule

adopts the TTMA's suggestion and allows a guard with rounded ends to

begin curving 255 mm (10 in) inboard of the side extremity of the

trailer. This will allow a radius of curvature of 150 mm (6 in), or 255

mm (10 in) if the guard end extends all the way to the side

extremities. To make the same allowances for forward-curving guards,

should guard manufacturers want to produce them, NHTSA is allowing

those guards to begin curving forward 255 mm (10 in) inboard of the

side extremities, even if the guards are already mounted as far forward

as possible--305 mm (12 in) forward of the rear extremity.

6. Guard Attachment

The SNPRM did not specify a particular guard attachment method. To

assure an adequate interface between the guard and the trailer, the

SNPRM proposed to require that the guard be attached to the trailer

chassis in accordance with the instructions provided by the guard

manufacturer.

Several commenters thought the SNPRM inadequately addressed the

issue of guard attachment and discussed the merits of certain guard

designs. Citing a study by Vehicle Safety Consultants (VSC) Ltd.,

Advocates stated that attaching the horizontal member of the guard to

the vehicle with vertical members is not ideal because the guard tends

to pivot forward and up if it is struck from the rear by a passenger

vehicle and fails. It said that the vertical members then form an

inverse ramp, thus aggravating any underride tendency by pushing the

passenger vehicle down and the trailer up. To solve this problem,

Advocates appears to recommend either guards with diagonal hydraulic

struts or the use of hinged, pivoting energy absorbing guards that can

fold up for rail or other intermodal transportation. IIHS also believed

a diagonal strut would improve guard strength without adding weight and

would make it more likely that the guard will move downward as it

deforms, thus helping to stop the passenger vehicle.

The agency agrees with IIHS and Advocates that designs employing

diagonal struts are strong yet light, but believes it would be

inappropriate to require such designs. There is no evidence that only

designs with diagonal struts perform adequately. To the contrary, the

design used in the VRTC tests did not have diagonal struts and

performed acceptably. Diagonal struts may also be impracticable in some

cases, due to trailer construction and use.

Likewise, while the pivoting, fold-away design that Advocates

recommended has obvious practical advantages in some circumstances, the

agency does not believe that there is any necessity for mandating that

all guards incorporate that design. Such designs would be unneeded by

many trailer operators since most trailers do not travel by ship or

train. If trailer operators need fold-away guards for intermodal

transportation or other

[[Page 2013]]

operational environments, they may specify such guards when ordering

new trailers.

NHTSA believes that specifying a particular attachment

configuration, as suggested by Advocates and IIHS, would unnecessarily

restrict design flexibility on the part of guard manufacturers.

Adequate performance may be achieved by a variety of attachment

methods. Moreover, it is impracticable for NHTSA to attempt to

anticipate all the factors that may go into the choice of attachment

method, given the variety of possible guard and trailer configurations.

The agency's decision not to specify a particular attachment method

leaves the guard manufacturers free to choose an appropriate design.

Some commenters had conflicting impressions that the SNPRM required

a particular attachment method. Transamerica Leasing interprets the

SNPRM's reference to ``attachment hardware'' as meaning that the

proposed rule contemplates only bolt-on guards. It thinks that guards

that are welded on should also be allowed. In contrast, Advocates

suggested that the SNPRM requires guards with vertical supports for the

horizontal member and welded steel construction.

No specific attachment method was proposed in the SNPRM. Nothing in

the SNPRM nor in this final rule requires vertical supports or welded

construction. Similarly, the agency did not intend its references to

attachment hardware in the SNPRM to imply that only bolt-on guards are

permitted. The agency's intent was to require that any necessary

attachment hardware be included with the guard when a guard

manufacturer sells the guard to a trailer manufacturer if the guard

manufacturer's method of attachment involves attachment hardware, as in

the case of bolt-on guards. Weld-on guards are also permitted. However,

if the guard manufacturer's installation instructions do not adequately

specify the welding procedures, welds of poor quality could break in

NHTSA's compliance testing. Weld strength could probably be assured

through incorporating by reference welding industry standard practices.

Some commenters believed that the guard-trailer interface was

inadequately addressed by the SNPRM. IIHS noted that the SNPRM proposed

no minimum strength for the chassis or the attachment method, and

concluded that the attachment may fail before the guard. It stated that

NHTSA's static tests showed that the trailer frame rails failed without

a doubler plate and that, even with a doubler plate, the flange welds

failed in dynamic tests. It also believed that NHTSA should require

installation instructions that are specific to each make and model of

trailer. IIHS reiterated these comments in a September 16, 1994 letter

that pointed to failures of the guard attachment hardware and trailer

structures resulting in PCI in two of the VRTC crash tests. IIHS urged

NHTSA to either require minimum strength levels for the guard

attachment hardware and frame rail or require that the guard be tested

together with the type of trailer frame rail to which it would be

attached.

Mr. John Tomassoni suggested that the preamble to this rule should

encourage manufacturers to install guards with due care so that the

attachment is as good as the guard. He said that the trailer frame is

the ``weak link'' in crashes today, and that adding ``doubler plates''

to trailer frame members helps to maintain the integrity of the

attachment in a crash.

NHTSA's test results show the importance of considering the

strength of the attachment point when designing a guard. The agency

does not at this time believe that it is necessary to define strength

requirements for the chassis or the attachment hardware because the

necessary strength is dependent on the design of the guard. For

example, a guard that is attached to the rear of the frame rail with

two vertical supports (i.e., the commonly used cantilever design used

in the VRTC tests and on most trailers) would require a stronger

attachment site and attachment hardware than a guard with many

attachment points or with diagonal struts. Therefore, without knowing

the design of the guard, NHTSA cannot readily specify minimum strengths

for the trailer frame or the attachment hardware, as suggested by IIHS.

However, the guard manufacturer must consider frame and hardware

strength in order to have a basis for certifying the guard for use on

the types of vehicles specified in the installation instructions. NHTSA

agrees with Mr. Tomassoni that, if a cantilever design is used, guard

manufacturers should consider doubler plates or other appropriate frame

reinforcement to prevent frame failure. NHTSA does not want to require

such features, however, because a different attachment design or a

sturdier trailer frame may eliminate the need for reinforcement. It is

not a requirement of this rule that guard manufacturers specify frame

strength or reinforcement procedures in the installation instructions.

However, as a practical matter, to have a basis for certification, they

must consider frame strength using testing, engineering analysis, or

both, to be assured that the guard attachment is appropriate for the

types of vehicles specified in those instructions.

The VRTC test experience illustrates why guard manufacturers should

appropriately design the strength of the attachment. In one case,

attachment bolts which were marginally weaker than those used in the

quasi-static test sheared under the sudden onset of force in the

dynamic test. In another case, the proximity of the guard to the rear

edge of the frame rail resulted in tearing of the trailer frame rail

webbing. In each case, the guard itself was not really exercised

because the attachment failed. In each case, simple modifications

solved the problem. The importance of careful attachment hardware

material selection and attachment design cannot be overemphasized.

Although guard manufacturers are free to issue separate

instructions for each specific make and model of trailer, as IIHS

recommends, it is not necessary for NHTSA to require such instructions.

An efficient way to specify trailer type would be to list specific

make/model combinations. However, as long as the instructions are

adequate to identify which vehicles are appropriate for the

installation of the guard, specification of the make and model of the

trailer may not be necessary. One reasonable alternative for a guard

manufacturer with a very adaptable guard design is to show in its

instructions the types of trailer, types of chassis configurations, and

frame strengths that are necessary to the functioning of that

particular guard. For example, the guard manufacturer might specify

that any flatbed or van trailer with longitudinal frame rails extending

to within 305 mm (12 in) of the rear, spaced between 760 mm and 1,270

mm (30 and 50 in) apart, and with the bottom of the frame rails

configured as a horizontal surface at least 100 mm (4 in) wide,

composed of steel that is at least 6 mm (1/4 of an inch) thick, would

be an appropriate trailer for mounting the guard.

Some commenters believed that defining ``chassis'' as the ``load

supporting structure of a motor vehicle'' was too restrictive or

otherwise inadequate. NSWMA asked NHTSA to modify S5.3.2 of the vehicle

standard to allow vehicle manufacturers with ``unique design

considerations'' to attach the underride guard ``to a load supporting

structure of the vehicle or body, or through other means that provide

equivalent protection.'' It believed that this change is necessary to

take into account body designs that do not use a conventional chassis

frame.

[[Page 2014]]

Mr. John Tomassoni also suggested that NHTSA further define the term

``load supporting structure'' because the longitudinal frame members

don't extend all the way to the rear end of some trailers.

Although NSWMA did not provide any specifics on its vehicles, NHTSA

agrees that there may be some trailers that do not have adequate

chassis structure, in terms of a frame structure, to support a

conventionally designed rear impact guard. However, no change to the

requirements is necessary. Although the frame components are the

obvious attachment point in the case of most trailers, attachment to

this chassis member is not required by this rule. In certain cases, an

unconventional guard design that is attached to other parts of the

chassis may be necessary. In rare cases, custom-designed guards or even

extension of the trailer chassis may be necessary to mount the guard.

The TTMA suggested changing the installation requirements in S5.3

to apply to ``guards that are produced or modified and installed by a

vehicle manufacturer * * *,'' so that a trailer manufacturer can modify

stock guards to fit its particular trailers. It assumes that the guard

manufacturer is unlikely to provide installation instructions for the

wide variety of trailer configurations. It reasons that, since the

trailer manufacturer has to certify that the trailer is in compliance

with all Federal motor vehicle safety standards anyway, why not let it

modify the guard?

Vehicle manufacturers are allowed to modify purchased guards to

suit their own trailers. There may be minor modifications to widely

available guard designs that will make them suitable for trailers for

which they were not designed. However, if a vehicle manufacturer

modifies the guard in a way not contemplated by the instructions

provided by the guard manufacturer, that vehicle manufacturer becomes a

guard manufacturer. The vehicle manufacturer may no longer rely on the

certification of the original guard manufacturer, because the original

manufacturer presumably did not intend its guards to be so modified. As

a guard manufacturer, the vehicle manufacturer would have to certify

that the guard, as modified, complies with the equipment standard.

Also, the vehicle manufacturer would have to affix its own

certification label and prepare modified installation procedures. The

installation procedures are necessary both to ensure that the guards

are modified and installed the same way each time, and to allow NHTSA

to duplicate the modification when conducting compliance testing.

The original guard manufacturer's installation instructions may

provide for some flexibility in the installation. For example, they may

specify that a certain kind of spacer may be used to achieve a proper

fit, or that a doubler plate be installed if the thickness of the

chassis is below a certain amount. However, NHTSA may employ any of the

installation options provided to the vehicle manufacturer when

subjecting a guard to compliance testing. Any test failure of a

properly installed guard will represent noncompliance by the guard

manufacturer.

7. Compliance Test Requirements and Procedures

a. Dynamic Versus Static Testing. Several commenters, including

Advocates, urged that NHTSA require that the guards be tested

dynamically, that is, by crashing cars into the rear of trailers

equipped with the rear impact guard. The agency agrees that dynamic

testing more closely simulates the conditions in which underride

crashes occur in the real world than the quasi-static testing does.

However, dynamic testing is also far more expensive. To test one guard/

trailer combination with a dynamic test for strength and energy

absorption would entail total test costs of approximately $30,000.

Dynamic tests would be so expensive that specifying such testing of

trailers could raise practicability concerns regarding those trailer

manufacturers that are small businesses. A requirement based on such

tests would place these small manufacturers, which are numerous, at a

competitive disadvantage, relative to larger companies, and would

represent a significant financial burden.

Quasi-static tests provide similar information far more

economically than dynamic tests. The VRTC research project demonstrated

that quasi-static testing generates similar forces to those generated

in an actual crash test, albeit at a slower rate. The project also

demonstrated that guards only ten percent stronger than the minimum

level of strength necessary to pass quasi-static test requirements

performed adequately in dynamic tests. The quasi-static compliance test

for a single guard at VRTC cost only about $3,500. Based on the

foregoing and the discussion in the section above on separate equipment

and vehicle standards, the agency believes that dynamic testing of

underride guards is unnecessary and overly expensive. NHTSA further

believes that quasi-static testing is adequate to ensure the

manufacture of safe and effective rear impact guards and that it will

do so at a far lower cost. Therefore, the quasi-static testing

procedure has been retained in the final rule.

Some commenters commented on the definition of ``rigid test

fixture.'' The TTMA assumes that a trailer can be used as a rigid test

fixture, and other commenters urged that testing be permitted on

trailers. The Institute for Injury Reduction commented that the terms

``sufficiently large,'' ``appropriately configured,'' and ``no

significant amount of energy'' in the definition of rigid test fixture

are vague, imprecise, ambiguous and in no way ``stated in objective

terms.''

NHTSA notes that a trailer may meet the equipment standard's

definition of a rigid test fixture, but because of slight flexing of

the vehicle structure, in other cases, they may not meet this

definition. NHTSA is persuaded that the benefits of testing on trailers

outweigh the possible effect on testing repeatability and does not want

to discourage testing on trailers by conducting its compliance testing

only on a rigid test fixture. The TTMA comment indicates that, although

it is not required, some vehicle manufacturers will conduct quasi-

static guard testing on trailers or trailer portions. NHTSA sees no

reason why this should not serve as a basis for manufacturer

certification even if the trailer is not a rigid test fixture. The use

of a trailer would be desirable because there is nothing more

``appropriately configured'' for guard mounting than the actual trailer

the guard will be installed on and because the structural integrity of

the trailer chassis will also be tested. However, caution must be

exercised to assure that the trailer is secured so that it does not

move during the test. If the guard is mounted to a trailer, the trailer

chassis will be secured so that there is no rotation or translation of

the trailer tires during the tests for guard strength and energy

absorption.

When conducting compliance testing, the agency will give the guard

manufacturer the option of designating testing on a rigid test fixture

or on a trailer. NHTSA notes that it may test on any trailer described

as appropriate in the guard manufacturer's installation instructions,

even if the guard manufacturer based its certification for that trailer

not on actual testing but on engineering analysis.

NHTSA agrees with the Institute for Injury Reduction that the

definition of ``rigid test fixture'' needs a slight modification. The

reference to size has been eliminated because size is not really as

important as rigidity. However, it is not necessary to define the

amount

[[Page 2015]]

of energy the fixture can absorb, because, like the ``fixed collision

barrier'' defined in 49 CFR 571.3, the guards will be expected to pass

the test no matter how little energy is absorbed by the fixture. Also,

the term ``appropriately configured'' has been clarified. There is no

way to precisely define how the test fixture will have to be configured

because that will depend on the design of the guard being tested. There

may be a number of appropriate configurations. As long as the guard can

be attached to the test fixture in the same way that the guard

manufacturer's instructions specifies the guard is to be attached to

the vehicle, without either modifying the guard or adding adaptive

parts to obtain a better fit between the guard and the fixture in a way

that is inconsistent with the instructions, the test fixture is

appropriately configured.

The agency had modified the strength test procedures to promote

ease of testing. Paragraph (b) of S6.5 now requires the application of

the force to the loading device to achieve a constant deflection rate,

rather than a constant increase in force, as proposed in the SNPRM. In

other words, rather than increasing the force at a constant rate, the

deflection rate is required to be held constant and the force will vary

depending on the resistance offered by the guard. Specification of a

deflection rate procedure is consistent with existing agency practice.

For example, the quasi-static compliance tests in S4(d)-(e) of Standard

No. 214, Side Impact Protection and S6.3 of Standard No. 216, Roof

Crush Resistance utilize this technique for force application.

b. Test Sites. Several commenters recommended changes in the

language 'specifying the test sites to be used during the compliance

tests. Mr. John Tomassoni recommended defining the P1 test site such

that the ``3/8 L'' lateral dimension (see Figure 1) is defined relative

to the side extremities of the trailer, as opposed to the center of the

guard. He suggested that this change would account for newer 2,600 mm

(102 in) wide trailers which have a 1,270 mm (50 in) longitudinal frame

rail span, or for any other width trailer. This approach, however, is

inconsistent with a separate equipment standard because the exact width

of the trailer may not be known at the time of testing. Moreover, the

requirement that guards extend to within 100 mm (4 in) of the side of

the trailer should assure that the P1 site will be sufficiently

outboard on the trailer, because wider guards will be required for

wider trailers, and the P1 location is dependent upon guard width.

Mr. Tomassoni also suggested that S5.2.2 and Figure 1 should be

modified to specify that the vertical center of force should not be

more than 560 mm (22 in) from the ground, rather than at ``the

horizontal plane that passes through the vertical center of the

horizontal member,'' as proposed in the SNPRM. Mr. Tomassoni indicates

that a guard with a horizontal member of cross sectional vertical

height greater than 100 mm (4 in) would result in higher test points.

Higher test points would yield test results that are not indicative of

the guard's effective impact strength near the bottom edge, where force

is likely to be concentrated in real world crashes. Although it is not

possible to define the test points relative to the ground because the

guard is not required to be mounted on the vehicle during testing,

NHTSA has modified the rule to define the test points relative to the

bottom of the guard itself. This should assure adequate strength and

energy absorption at the level of likely impact force.

Mr. John Kourik pointed out that the P1 test site was defined

incorrectly in the SNPRM, although it was correctly portrayed in Figure

1. The text of S5.2.2(a) (redesignated S6.4(a) in this rule) read ``3/8

of the transverse horizontal distance * * * between the * * * vertical

centerline of the guard [and] and the outermost edge * * * of the

guard.'' The P1 definition has been corrected to reflect that the point

is located 3/8 of the total guard width outboard of the centerline. Mr.

Kourik also suggested that the four asterisks showing the P3 test sites

in Figure 1 be reduced to two asterisks. NHTSA has modified the figure

to make it clearer that there is only one P3 test site on each side of

the guard, but that the location of the site is within a range from the

centerline.

The TTMA and other commenters suggested broadening the range of

locations of the P3 test site to allow it to be ``any point selected by

the manufacturer * * * between 14 and 25 [rather than 20] inches

outboard'' of the guard centerline. Most new trailers are wider than in

the past with a frame rail span of 127 cm (50 in), and the frame rail

is a likely chassis structure for guard attachment. TTMA wanted NHTSA

to conduct the more demanding 100 kN (22,480 lb) P3 test near the

attachment point of the guard's supports. This was NHTSA's general

objective in specifying the P3 test location, and this objective is

furthered by accommodating TTMA's request in part. The rule has been

modified to provide that P3 is located 355 to 635 mm (14 to 25 in) from

the guard centerline. However, NHTSA will select any point within the

range for compliance testing, rather than permit a manufacturer to

specify a single test site within the 355 to 635 mm (14 to 25 in)

range.

c. Labeling and Certification. The TTMA suggested that affixing a

certification label is redundant in those instances in which the guard

is manufactured by the vehicle manufacturer because the vehicle

manufacturer has to certify compliance with all the safety standards

anyway. Although this is true, allowing some guard manufacturers to

omit the label would be impractical from an enforcement standpoint,

because vehicle inspectors would not be able to tell whether the guard

was certified by the guard/vehicle manufacturer as part of the vehicle

or whether the vehicle manufacturer installed a guard purchased from a

guard manufacturer who neglected to make a required certification.

Moreover, NHTSA does not believe that affixing the label is a

significant burden. Therefore, the final rule retains the requirement

of a separate guard certification for all guards.

The TTMA also recommended that the label be affixed to the roadside

vertical supporting member of the guard, instead of the center of the

horizontal guard member, to prevent damage and abuse. NHTSA believes

that docking and other routine operations could damage the label if

affixed in the proposed location. Therefore, the rule has been modified

to require the label to be affixed in a less vulnerable location. The

rule now requires the certification label to be placed on the

forwardmost surface of the horizontal member of the guard at an offset

location 305 mm (12 in) inboard of the right side end of the guard.

The TTMA also suggested changes in the label format. Specifically,

it recommended that the letters and numbers should be 2.5 mm (\3/32\ of

an inch) high, which is the same as the trailer certification label,

rather than 13 mm (\1/2\ inch) high as proposed in the SNPRM. TTMA also

asked that NHTSA require that the label be furnished to the vehicle

manufacturer with a protective cover that can be removed after

painting.

The agency believes that the smaller letters suggested by TTMA are

sufficiently legible for inspection purposes, and has changed the rule

to adopt this suggestion. However, market forces should determine

whether protective covers are provided. Vehicle manufacturers will

probably cover the labels themselves when painting to avoid having

their guard confused with a noncomplying guard.

[[Page 2016]]

C. Standard for Vehicles

1. Configuration Issues

a. Maximum Guard Ground Clearance. One of the major issues

addressed by nearly all the commenters was the maximum ground clearance

of the horizontal member of the rear impact guard. The SNPRM proposed a

maximum guard height of 560 mm (22 in). Consumer safety groups and

private citizens generally favored lowering the guard to within 405 or

460 mm (16 or 18 in) of the ground in the belief that doing so would

provide more complete protection for low profile vehicles such as sub-

compact and mini-compact passenger cars. Since the real issue is not

ground clearance, but guard height relative to the front structure of

colliding passenger vehicles, some of these commenters addressed

related issues such as the height of the engine block, hood, and cowl

(windshield base) of those vehicles. Except for the consumer safety

groups and a few private citizens, few provided a rationale or any data

to support a lower guard height. The organizations and private

companies related to the trucking industry generally supported a 560 mm

(22 in) height, but offered a variety of reasons not to lower the guard

further. Most of their concerns related to operational difficulties

that would be caused by lower guard heights.

The consumer safety groups focussed their comments on guard

effectiveness. Advocates advanced several reasons for reducing the

guard height in order to achieve better engagement between the guard

and the engine block, bumper, and tires of colliding passenger

vehicles. Advocates stated that lower engine block heights on modern

automobiles, combined with the lowering of the passenger vehicle's

front end due to suspension compression during severe braking, will

result in the rear impact guard passing over the engine and engaging

only the hood and fenders of most cars. In addition to the front end

lowering caused by braking, Advocates claim that additional frontal

lowering will occur on downgrades due to forward weight transfer.

Citing a random survey it made of subcompact cars and urging NHTSA to

conduct a more thorough survey, it said that no engine block is higher

than 560 mm (22 in) above the ground and bumpers are in the 430 to 535

mm (17 to 21 in) range. It stated that earlier NHTSA data using the

average hood height above the ground was misleading because its

``casual'' survey of subcompact hood front edges showed none higher

than 635 mm (25 in). It interprets these data to mean that only fender

top and hood sheet metal would be engaged, and concluded that air bag

sensors probably will not be triggered. Advocates also maintains that,

even if the top of the engine were engaged, the underride guard will

cause the blocks of transversely-mounted engines used in most

subcompacts to rotate (roll) rearward, crushing the car occupant's

legs. Based on British research, Advocates recommends a guard height of

no more than 405 mm (16 in), and ideally 305 mm (12 in). Both Advocates

and Mr. Byron Bloch, of Auto Safety Design, cited the 1980 study by

Dynamic Science which concluded that the guard height should not exceed

510 mm (20 in). Mr. Bloch recommended a height of 405 to 460 mm (16 to

18 in). CRASH solicited many private citizens to send in petitions,

letters, and pre-printed cards stating that the guard height should be

set at 405 mm (16 in), but none provided supporting technical

information.

The IIHS, citing the same studies as Advocates, urged NHTSA to

adopt a maximum ground clearance of 460 mm (18 in). IIHS is primarily

concerned that a 560 mm (22 in) high guard will override car bumpers,

thus bypassing much of the potential front end energy absorption. Other

concerns expressed by IIHS were late air bag activation, braking-

induced bumper depression of two to 100 mm (4 in) or more, and possible

lifting of the rear end of the trailer as the car wedges under the

guard. IIHS implied that a 460 mm (18 in) requirement is practical,

noting that one U.S. freight carrier reportedly sets its guards at 495

mm (19.5 in).

IIHS believes NHTSA's estimate that trailers probably sit 50 to 75

mm (2 to 3 in) lower when loaded is wrong. IIHS tests on 11 trailers

showed the most heavily loaded trailers showed only 38 to 57 mm (1.5 to

2.25 in) of depression with an average of 28 mm (1.1 in). Four of the

trailers even raised in the rear, indicating that load distribution is

probably a factor in determining rear extremity compression height.

IIHS believes that modern air suspensions compensate for loading

depression. Even if loaded trailers are depressed, it believes that

passenger vehicles should be protected from partially loaded or empty

trailers, which it says are involved in 29 percent of fatal crashes.

Therefore, IIHS urges NHTSA to assume no depression of the trailer bed

due to loading.

Mr. John Tomassoni commented that a lower guard would be better

because engine block resistance to a rigid guard doesn't start until

460 to 610 mm (18 to 24 in) behind the bumper. However, Mr. Tomassoni

concluded that a 560 mm (22 in) requirement is a significant

improvement over the existing 760 mm (30 in) height, and one that can

be implemented with little or no difficulty. He notes that trailers 16

meters (m) (53 feet (ft)) or longer are currently being equipped with

560 mm (22 in) high guards.

Some municipalities sent comments in favor of lower guard heights.

For example, the City of Durham, North Carolina sent an unsigned

resolution that the height be set at no more than 460 mm (18 in). Its

Transportation Advisory Committee submitted a similar comment. About

2,300 private citizens recommended a guard height of 405 mm (16 in).

The industry groups focussed their comments relating to guard

height on operational restrictions that would result from the reduced

``angle of departure'' that lower ground clearance would cause. The

angle of departure is basically the acute angle formed by the ground

and a line connecting the point where the rear tires meet the ground

with the bottom of the guard. The lower the guard, and the further

forward the rear wheels are positioned relative to the guard, the

smaller the departure angle is, and therefore the more likely the guard

is to scrape or ``hang'' on the ground when the trailer mounts a steep

incline. The problem is exacerbated for the longer 16 m (53 ft)

trailers being used today, because they have correspondingly greater

rear overhangs, and thus smaller departure angles. Many trailers have

their rear wheels mounted on sliding tandems, or bogeys, that can be

moved forward or rearward on the trailer's frame, depending on the load

and the need for maneuverability. The further forward the wheels are,

the more maneuverable the trailer is and the more the rear end of the

trailer ``swings out'' in turns.

Changes in the industry since 1981 seem to have relieved the

concerns of the rail industry that the proposed ground clearance of 560

mm (22 in) would interfere with rail car loading and unloading

operations, in which trailers are driven up steep ``circus ramps'' onto

flat cars. The Association of American Railroads (AAR) and TTX Company,

a trailer-on-flat-car operator, opposed the 1981 NPRM, but now support

the 560 mm (22 in) requirement because there are few ``circus'' ramps

still operating. However, they caution that a significantly lower

height would interfere with intermodal flatcar operations. TTX asserted

that such a reduction in guard clearance could interfere with lift-on

and lift-off operations for one type of railroad car (TTAX ``spin

cars'') handling 16 m (53

[[Page 2017]]

ft) trailers. It added that there must be extra guard clearance to

account for loading depression and bouncing. To illustrate the

potential economic impact of lower guard clearance, TTX stated that

there are 2,300 such cars costing $340 million, which are only 1.8

years old on average. TTX estimates that lowering the guard clearance

could eliminate 75 percent of the capacity for 14 railroads.

In contrast, the 560 mm (22 in) guard height is still considered

low by the portion of the industry that transports trailers in ships.

Transamerica Leasing, Inc. recommends that NHTSA conduct further study

before issuing this rule because a 560 mm (22 in) high guard would

scrape loading ramps during roll-on/roll-off ship loading when the

wheels are positioned forward to provide the maneuverability necessary

in ships. The American Trucking Associations (ATA) supports the 560 mm

(22 in) proposed ground clearance, but stated that any lower clearance

would be unacceptable. It calculates that a loaded trailer driven onto

a barge or vessel, which it says have departure angles as high as 15

degrees, would drag the guard if the rear axle is 190 cm (74.5 in) or

more forward of the guard. It said that many states have restrictions

on trailer kingpin-to-rear- axle distances that result in a 245 to 275

cm (96 to 108 in) rear-wheel-to-guard distance on 16 m (53 ft)

trailers. It concludes that these trailers' guards would hang on such

vessel loading ramps or on any 20 percent grade. It finds the 560 mm

(22 in) clearance acceptable only because 16 m (53 ft) trailers are

rarely used on vessels, and because 20 percent grades are rare. The

Truck Maintenance Council of the ATA recommends a guard clearance of

560 mm (22 in) for general freight equipment. According to Mr. Robert

Crail, a trailer designer and manufacturer, the proposed 560 mm (22 in)

height is acceptable because, although many trailers are still driven

into ships rather than being crane loaded, vessel owners can adjust

their ramps, and because it is compatible with the dimensions

established by the trucking industry and loading dock restraint device

manufacturers. Ford Motor Company had no specific data, but is

concerned that 560 mm (22 in) may be inadequate ground clearance for

loading and unloading of long trailers in trains or ships. Ford also

noted that some single unit trucks are equipped with kneel-down air

suspensions to facilitate loading and unloading, which Ford says are

incompatible with a 560 mm (22 in) high guard.

Even outside the context of intermodal loading and unloading

operations, some commenters were concerned about the reduced departure

angle that a 560 mm (22 in) high guard would create. The National Solid

Waste Management Association (NSWMA) emphasized the importance of

maneuverability for sanitation trucks in negotiating driveways and

backing into tight places. It estimated that a 560 mm (22 in) guard

mounted flush with the rear extremity of a sanitation truck would have

a departure angle of only 9 degrees, which it says is typical of many

driveway entrances. Although it appears that many of the trucks NSWMA

is concerned with are single unit trucks that are excluded from the

rule, NSWMA is also concerned about the guards getting hung up on the

ground when the trailers are taken off-road onto the soft, unpaved,

uneven roads at landfills and construction sites.

One additional industry concern is engagement of the guard with

``dock locks.'' When trailers back up to loading docks, these devices

engage the underride guard to keep the trailer from moving away from

the loading docks as forklifts repeatedly travel across the rear door

sill. Transamerica Leasing believes that the 560 mm (22 in) high guards

may interfere with ``dock lock'' engagement arms. Yellow Freight System

states that thousands of dock locks have been installed according to

the 560 mm (22 in) guard height recommended by the Maintenance Council

of the ATA, and urges NHTSA not to change now. However, Rite Hite

Corporation, a manufacturer of dock locks, submitted information

indicating that dock locks can accommodate guard heights between 355

and 760 mm (14 and 30 in).

One industry group endorsed a lower guard height. The AFL-CIO

Teamsters Union suggested that NHTSA could require a ground clearance

lower than 560 mm (22 in) because auto carriers and UPS trailer fleets

have reported no problems with lower guard heights. It also observed

that 16 m (53 ft) trailers in many states have no problem using 560 mm

(22 in) guards.

The question of proper guard ground clearance involves a balancing

of the effectiveness of the guard in providing protection against PCI

against the cost and operational restrictions that lower guard heights

could impose on the industry.

The effectiveness of the guards is a primary consideration.

Regarding Advocates' survey of bumper and hood heights on compact and

subcompact cars, NHTSA conducted a similar survey of engine block

height and front end profile of a sample of 40 vehicles. The results of

this survey were summarized in the agency's Truck Underride Report to

Congress, dated November, 1993. The NHTSA survey showed that the height

of the top of the engine block was between 660 and 790 mm (26 and 31

in), with an average height of 840 mm (28 in). The hood leading edge in

NHTSA's survey averaged about 685 mm (27 in) and the lower edge of the

windshield frame averaged about 840 mm (33 in). The agency is not aware

of the basis upon which Advocates selected the cars for its survey, but

NHTSA's survey was targeted preferentially at cars with the lowest

front end profile. Since NHTSA's average heights were higher than those

obtained by Advocates, NHTSA has no explanation for the discrepancy,

unless the survey methodologies were different. Hood heights have been

getting lower over the past few years, but that trend may have stopped

in the last two years. NHTSA believes that the average hood heights in

its survey are representative of the anticipated dimensions for new

passenger vehicles 5 to 10 years in the future. NHTSA concludes from

the VRTC test results that a 255 to 305 mm (10 to 12 in) overlap

between the guard bottom and the lower edge of the windshield will

ensure adequate structural engagement with the guard for the vast

majority of compact and subcompact cars.

NHTSA agrees with IIHS that a guard 560 mm (22 in) high will

override most bumpers, but disagrees that bypassing the bumper

sacrifices much of the potential front end energy absorption

capability. The bumper is designed to prevent cosmetic damage in low

speed crashes (less than 16 kph, or 10 mph) and provides only a small

portion of the energy absorption by a car crashing at higher speeds.

The bumper is mounted to the frontal crash energy management components

which extend rearward and upward to the rearmost section of the engine

compartment. These components will be adequately engaged by the rear

impact guard during a collision. Regarding IIHS's contention that NHTSA

should assume no loading-induced depression of the trailer bed, NHTSA

has not made such an assumption. The final rule regulates the guard

height only when the trailers are unloaded, and the 560 mm (22 in)

guard height was adequate in NHTSA's VRTC tests.

The agency conducted seven full scale crash tests with the proposed

guard in the course of the recent research project, using two types of

subcompact and two types of compact cars. These vehicles were

representative of average hood and engine heights for cars in those

size classes. The minimally compliant rear

[[Page 2018]]

impact guard was set 560 mm (22 in) above the ground. During these

tests, the cars had their front ends depressed to simulate the lowering

that would be experienced during heavy braking, but the guard was not

depressed to a level below the minimum clearance, as it might be if the

trailer were loaded. In some sense, therefore, these tests represented

a ``worst case scenario'' with regard to guard height. In each test,

the air bags were fully deployed before dummy contact and the

deceleration readings were much better than the minimum requirements in

Standard No. 208, Occupant Crash Protection. When there was no guard

attachment failure, they adequately engaged the structure of each car

and prevented PCI. There was little movement of the engine and no

contact between the engine and fire wall. The transversely mounted

engines did not rotate substantially, and none of the dummies legs were

crushed. Therefore, based on the docket comments, the recently

completed crash tests, and the assessment of late model passenger

vehicle frontal structure characteristics, NHTSA concludes that the 560

mm (22 in) maximum guard ground clearance is adequate to engage the

frontal crash energy management structure of most subcompact and

compact cars.

Although some small sectors of the industry may be affected, NHTSA

does not believe that there will be any insurmountable problems with a

560 mm (22 in) guard height. Several states have required 560 mm (22

in) maximum guard ground clearances in conjunction with the passage of

laws allowing 16 m (53 ft) trailers. NHTSA contacted several

distributorships/dealerships that sell heavy trailers in excess of 15 m

(50 ft) in length to the trucking industry and was unable to obtain

information documenting substantial operational problems due to guard

ground clearances of 560 mm (22 in) or less. The AFL-CIO Teamsters

Union did not give NHTSA enough information about the operating

environment of Carolina Freight Carriers Corporation, the trucking

company that sets its guards at 495 mm (19.5 in), to determine why they

have not experienced the problems that the other commenters expect with

guards lower than 560 mm (22 in).

NHTSA does not believe that the number of trailers involved in ship

roll-on/roll-off and trailer-on-flat-car circus ramp operations is

significant. TTMA data indicate that less than 5 percent of trailers in

the U.S. are ever transported by ship or barge, and that between one

and less than ten percent of new trailers are produced for trailer-on-

flat-car use. Modifications of may solve these problems. Most of the

vehicles in the waste services fleet mentioned by NSWMA are single unit

trucks excluded from the rule. However, in those few cases where there

are still problems, movable or adjustable guards may be needed.

There is adequate evidence in the comments to conclude that

requiring a guard height lower than 560 mm (22 in) would cause an undue

burden on the industry. Of particular concern are the comments of ATA,

TTX, AAR, and Transamerica Leasing, indicating that any height below

560 mm (22 in) will cause interference in intermodal operations.

Moreover, a lower height will increase the probability that the guard

will scrape or snag during normal vehicle operations and be damaged as

a result. Therefore, because the 560 mm (22 in) maximum ground

clearance proposed in the SNPRM appears to be the lowest height that

provides adequate effectiveness without imposing an undue burden, it

has been retained in the final rule. The agency notes that guards may

be mounted with less than the maximum allowable ground clearance.

b. Guard Width. The SNPRM proposed that the horizontal member of

the guard be required to extend across the width of the trailer to

within 100 mm (4 in) of the side extremities, but not outboard of the

side extremities. Advocates commented that the 100 mm (4 in) allowance

appeared arbitrary, based on the rulemaking record, but did not

actually suggest that the guard should extend fully to the side

extremities of the trailer. The AFL-CIO Teamsters Union indicated that

it fully supports the SNPRM's 100 mm (4 in) allowance, while noting

much anecdotal information from drivers about the importance of a

``full width'' guard, especially for crashes that occur at an angle to

the rear of the trailer.

NHTSA notes that there is no requirement of a 100 mm (4 in) inset.

Vehicle manufacturers are permitted to install guards extending the

full width of the trailer. However, the 100 mm (4 in) allowance gives

trailer and guard manufacturers some flexibility in choosing and

providing guards, without sacrificing safety or effectiveness. From the

perspective of guard effectiveness, it is doubtful that the extra

lateral coverage would significantly increase the strength of the guard

at its extremities or its ability to protect passengers in an offset

collision.

In fact, a 100 mm (4 in) inset would decrease the previously

mentioned ``hooking'' potential during sharp turns of the trailer and

provide more clearance in certain passing situations. The Florida

Department of Transportation and the TTMA recommended allowing rounded

guard ends to alleviate this potential problem, but NHTSA notes that a

100 mm (4 in) inset on an unrounded guard will partially accomplish the

same goal. As discussed above in the section on shape of the horizontal

cross member, pursuant to the TTMA's suggestion NHTSA has modified the

rule to allow rounded corners on guards to begin curving at a point 255

mm (10 in) inboard of the edges of the vehicle, while retaining the 100

mm (4 in) requirement for straight guards. Curved guards still have to

meet the other requirements of the vehicle standard (i.e., extend to

within 100 mm, or 4 in, of the side extremity). This modification

merely removes for the curved portion of the guard the requirement that

the bottom of the horizontal member be within 560 mm (22 in) of the

ground, in the case of upward curving guards, and the requirement that

the rear surface of the horizontal member be within 305 mm (12 in) of

the vehicle rear extremity, in the case of forward curving guards.

c. Specification of the Rear Extremity. Some commenters requested

that NHTSA modify the proposed definition of ``rear extremity'' to take

into account vehicles with high protrusions in the rear. The SNPRM

defined the rear extremity as the rearmost point of the vehicle that is

located 560 mm (22 in) or more above the ground. The specification of

the rear extremity is important because the SNPRM also requires that

the rear impact guard be located no more than 305 mm (12 in) forward of

the rear extremity of the vehicle. Some trailers and semitrailers, such

as hopper trailers with V-shaped bins and trailers with liftgates or

refrigerator units in the upper rear, are shaped such that the rear

extremity of the vehicle is located well above the road surface. These

protrusions do not present a danger of PCI because they are located

well above the roof line of most passenger vehicles. Yet, applying the

rear extremity definition in the SNPRM, a rear impact guard would have

to be mounted such that it extends rearward from the base of the

trailer to a position within 305 mm (12 in) of the back of the high

protrusion. Such an extended guard might pose a safety hazard as well

as operational difficulties.

Several manufacturers of vehicles with high rear end overhang

recommended alternative definitions of ``rear extremity'' that excluded

portions of the trailer rear that were high enough to clear the roofs

of passenger vehicles. The TTMA and the ATA recommended that vehicle

structure with a ground

[[Page 2019]]

clearance of 1,680 mm (66 in) or more be excluded from the definition

of rear extremity. NSWMA recommended excluding that portion of the rear

of the vehicle located 1,520 mm (60 in) or more above the ground.

The agency acknowledges the potential problem with the proposed

specifications and believes that redefining the rear extremity to

accommodate these vehicles is possible without reducing rear impact

guard effectiveness or creating new safety hazards. NHTSA contacted

officials from TTMA and ATA to obtain more information about the

current number and future production plans for vehicles of this type.

According to TTMA, these are mostly highly specialized vehicles and the

high overhang often consists of equipment such as cranes in addition to

``bubble door'' type container trailers. TTMA estimates that these

vehicles constitute less than one percent of the annual trailer and

semitrailer production and there is no trend toward increasing the

numbers substantially. ATA also estimated that the number of vehicles

produced annually with high rear overhanging structure represents less

than 5 percent of the total annual production of trailers and

semitrailers. ATA did not provide information on the future trend of

production of these vehicles, but indicated that the number has been

fairly constant in the recent past with new vehicles brought into

service primarily to replace vehicles going out of service.

The NSWMA recommended that the rule specifically state that, for

roll-off/hoist type trailers, the containers on the hoist frame be

considered as part of the load and not as part of the vehicle for

purposes of rear extremity specification. It suggests that the rearmost

part of the hoist frame should be considered the rear extremity.

Containers extend up to 1.5 m (5 ft) rearward from the end of the hoist

frame.

The agency has decided to revise the SNPRM's definition of ``rear

extremity'' to limit its ambit to the portion of the vehicle's rear

located between a lower and upper height limit. The lower limit

specification remains unchanged at 560 mm (22 in) (that is, guard

ground clearance). An upper limit for the area in which the rear

extremity is located has been specified at 1,900 mm (75 in) above the

ground surface for purposes of the vehicle standard. The portion of the

rear of the trailer that is located in the same horizontal planes as a

passenger vehicle windshield is the critical area for rear underride

protection. This is between 760 mm and 1,900 mm (30 and 75 in) above

the ground for almost all passenger cars, vans, and light trucks.

With regard to roll-on/hoist type trailers, the agency agrees with

NSWMA that there would be numerous regulatory problems involved in

considering the containers to be part of the vehicle, rather than part

of the load. Although the containers may extend beyond the end of the

vehicle and are capable of causing PCI just like the rear end of a

trailer, they are not part of a new vehicle as manufactured. Further,

the boxes, tanks, and other specialty containers are manufactured,

maintained, and in many cases owned separately from the vehicle. NHTSA

has no authority to regulate vehicle loads under 49 U.S.C. Chapter 301.

While NHTSA cannot require guards on the container on roll-on/hoist

type trailers, it can require guards on the rear of the trailer that

carries it. If the vehicle is designed to carry containers that do not

extend appreciably beyond the rear of the vehicles, the agency sees no

basis for excluding it. Casual observations indicate that the

containers do not usually extend beyond the rear of the vehicle, so

these trailers are required to have guards. The rear extremity will be

determined without the container.

d. Distance between the Guard Rear Surface and the Vehicle Rear

Extremity. Several commenters urged NHTSA to change the requirement

proposed in the SNPRM that the guard's horizontal member be mounted not

more than 305 mm (12 in) forward of the rear extremity of the trailer

and not rearward of the rear extremity. The distance between the guard

and the trailer rear extremity is significant because the sooner the

passenger vehicle engages the underride guard, the farther its occupant

compartment will be from the rear of the trailer when the guard is

engaged, and the better the chance that the passenger vehicle will stop

short of PCI.

Some commenters thought that NHTSA should allow the guard's

horizontal member to extend rearward of the rear extremity. Mr. John

Tomassoni stated that he saw no good safety reason for restricting rear

extension, since it is beneficial for preventing PCI. The TTMA also saw

no reason why the guard should not be located rearward of the rear

extremity. It also suggested a change in the language of S5.1.3 that

makes it clear that, even above 560 mm (22 in) the guard cannot be more

than 305 mm (12 in) from the rear extremity of the vehicle.

The Rite Hite Corporation stated that, for dock locks to function,

there must be no more than 230 mm (9 in) between the rear extremity and

the guard. It is concerned that the 305 mm (12 in) allowance will

render the dock locks useless.

NHTSA notes that the 305 mm (12 in) allowance is not a minimum, but

a maximum requirement. Casual observations by the agency indicate that

nearly all trailers currently have their guards mounted flush with the

rear extremity of the trailers. This practice is also specified as the

recommended practice in the ATA Maintenance Council guidance (RP 707).

It is also the configuration most compatible with dock locking

mechanisms. Based upon the TTMA's comment relating to mounting rearward

of the rear extremity, the industry appears to be in favor of mounting

as far rearward as possible. Therefore, NHTSA believes that trailer

manufacturers will continue to mount guards flush with the rear

extremity of the vehicle.

The main incentive to change the prevailing practice relates to the

smaller departure angle that will be created by lowering the maximum

guard ground clearance from 760 mm to 560 mm (30 to 22 in). Moving the

guard 305 mm (12 in) forward will slightly increase the departure

angle. However, nothing in this rule increases that existing incentive.

Therefore, the agency does not expect that a 305 mm (12 in) allowance

would have any effect on prevailing practice. Further, NHTSA does not

believe the benefit of moving the guard forward would be very

significant. Nevertheless, the agency had modified the requirement in

section 5.1.3. for guard rear surface location, or off-set, to state

that the guard should be mounted as close as practical to the rear

extremity of the vehicle. This will prevent vehicle manufacturers from

mounting the guard with up to 305 mm (12 in) of forward off-set from

the rear extremity of the vehicle unless the off-set is necessary and

not merely convenient. It should be noted that the requirement to mount

the guard as close to the rear extremity as practical is identical to

the requirements of ECE Regulation 58.

NHTSA agrees that having the horizontal member of the guard

positioned rearward of the rear extremity would be beneficial for

preventing PCI in the event of a crash. Some meritorious guard designs,

such as the Quinton-Hazel hydraulic energy absorbing guard and the Hope

rearguard underrun device, utilize horizontal members that are hinged

so that they are angled down and slightly rearward from the rear of the

trailer. This rearward positioning enables the guard to engage a

striking vehicle at a greater distance from the rear extremity and

gives the guard a greater distance to swing

[[Page 2020]]

forward and ``ride down'' the energy of the striking vehicle before PCI

occurs. If vehicle manufacturers want to provide this extra measure of

safety, this agency will not discourage it, as long as vehicle

manufacturers consider State laws governing overall combination truck

length. However, NHTSA does not want to require rearward positioning

because this configuration exacerbates the previously mentioned

potential for ``hooking'' adjacent vehicles during sharp trailer turns

and in other situations. Therefore, NHTSA has removed the SNPRM's

prohibition on positioning the horizontal member rearward of the rear

extremity. The new requirement that the member be as close to the rear

extremity as practical is limited so that it does not prohibit mounting

rearward of the rear extremity.

Advocates stated that NHTSA has no data to support the 305 mm (12

in) allowance because all crash tests were done with guards positioned

at the very rear of the trailer, thus implying that testing in the

forward-mounted position is required to support the allowance.

Even though the crash tests conducted by NHTSA had the rear impact

guards mounted in the usual position, flush with the rear of the

trailer, NHTSA has used a simple mathematical calculation to determine

whether, and to what extent, PCI would have occurred if the guard had

been mounted 305 mm (12 in) forward of the rear extremity (see VRTC

report ``Heavy Truck Rear Underride Protection,'' June, 1993. DOT-HS-

808-081). The agency assumed that if the guard had been mounted 305 mm

(12 in) farther forward, the car's occupant compartment would have come

to a rest 305 mm (12 in) closer to the rear of the trailer after the

crash. There is no reason to expect that the guards would have

performed more poorly if mounted further forward in the 305 mm (12 in)

zone at the rear of the trailer. Therefore, there is no need, as

Advocates suggests, to mount the guards at the ``worst case''

forwardmost point for testing purposes. In any case, the new

requirement to mount the guards as close to the rear extremity as

possible minimizes the number of trailers with guards mounted forward

of the rear extremity.

Mr. Byron Bloch recommended that the guard should be located no

more than 150 mm (6 in) forward of the rear extremity instead of 305 mm

(12 in). He said that the 150 mm (6 in) gained could be used to make

the guard more effective, by permitting the guard to absorb more energy

by utilizing a 255 mm (10 in) stroke rather than the proposed 125 mm (5

in) stroke. He stated that this would allow the manufacturers greater

flexibility in choosing an energy absorbing type of guard.

While it might be desirable to have guards that absorb an

equivalent amount of energy over a greater distance, Mr. Bloch's

suggestion could make PCI more likely. NHTSA does not want to reduce

the vehicle manufacturer's flexibility to offset the guard up to 305 mm

(12 in) forward of the rear of the trailer. If the agency permitted a

greater stroke for guards designed to be mounted closer to the rear

extremity, it would be difficult to control where these guards are

actually mounted. If mounted too far forward within the permitted

offset, they would allow excessive penetration under the trailer. NHTSA

is also concerned that guards with a greater amount of stroke will

pivot at the vehicle chassis, causing the horizontal member of the

guard to rotate up until it no longer engages substantial striking

vehicle structure of lower profile vehicles. This also would make PCI

more likely.

2. Exclusions

The SNPRM excluded certain categories of vehicles from the

requirement for rear impact guards. These categories were: Single unit

trucks (also referred to as ``straight body'' because they are

unarticulated); truck tractors; pole trailers; low chassis trailers;

special purpose vehicles; and wheels-back vehicles.

Almost every comment addressed one or more of these exclusions. The

consumer safety groups and most of the comments from the general public

were especially opposed to the exclusion for single unit trucks. The

consumer groups were also opposed to the exclusion for wheels back

vehicles. There was little opposition from the consumer safety groups

or the public to the exclusion for special purpose vehicles. Industry

groups generally supported all the exclusions. Many industry groups and

equipment manufacturers requested that their vehicles be explicitly

included in the special purpose vehicle category. Industry groups also

commented on the wheels back vehicle definition, generally requesting

that it be expanded to cover more vehicles.

The comments on the excluded vehicles are discussed in more detail

below. Since there was no substantive comment on the exclusions for

pole trailers, low chassis trailers, and truck tractors, these

exclusions are not discussed.

a. Single Unit (Straight body) Trucks. NHTSA expressly solicited

comment on the issue of applicability of the proposed rule to single

unit trucks. The majority of docket submissions, including comments

from trade associations, safety and consumer interest groups, and

private citizens, expressed the opinion that the proposed rule should

apply to single unit trucks. Many of these commenters stated that the

exclusion did not make sense because the underriding passenger vehicle

would not be any less at risk in striking the rear end of a single unit

truck than striking the rear of a trailer. Advocates said single unit

trucks account for about 300,000 of the 500,000 heavy vehicles produced

each year. IIHS and Advocates stated that medium and heavy duty single

unit trucks account for 36 percent of all the vehicle miles traveled by

heavy vehicles and 68 percent of all non-fatal (AIS 1-5) injuries

associated with passenger vehicle impacts with the rear of heavy

vehicles. CRASH's analysis indicated that the number of fatal accidents

in which passenger vehicles collide with the rear of trailers has been

increasing at a rate of about 6 percent per year. According to CRASH,

rear impacts involving single unit trucks have been increasing at a

rate of 11 percent annually in the recent past.

Mr. Robert Crail and Transamerica Leasing opposed the exclusion

because single unit truck manufacturers would be able to obtain guards

from the same places as trailer manufacturers. Mr. Byron Bloch

recommended that single-unit trucks should be excluded only by

exemption petition from individual manufacturers, and that if petitions

are granted, NHTSA should require a warning sign on the truck. The

State of New York Attorney General expressed the opinion that NHTSA is

required by the 49 U.S.C. Chapter 301 requirement that a safety

standard must meet the need for motor vehicle safety to include single

unit trucks in this rule, based on the ``very modest costs involved.''

Mr. John Kourik could find no definition anywhere in NHTSA's

regulations for the term ``single unit truck.''

Additional organizations recommending that the rule apply to single

unit trucks include the International Brotherhood of Teamsters, the

Owner-Operator Independent Drivers' Association, the Specialized

Carriers and Rigging Association, and the American Insurance Services

Group. About 2,200 private citizens also recommended that the rule

apply to single unit trucks as well as trailers and semitrailers.

Mr. John Tomassoni commented that including vehicles with a gross

vehicle weight rating (GVWR) of greater than 14,536 kg (10,000 lbs) in

the statistical

[[Page 2021]]

cost benefit analysis made the trailers appear unfairly dangerous

because most single unit trucks are in the low end of this weight

range, yet the larger trucks can still cause underride fatalities. He

suggested that cost effectiveness be reassessed on the basis of

requiring guards on trucks and trailers weighing greater than 11,790 kg

(26,000 lbs). He further recommended that even if the single unit

exclusion were retained in the final rule, the rule should at least

``encourage'' manufacturers of single unit trucks above 9,070 kg

(20,000 lbs) GVWR to install ``upgraded'' guards.

Manufacturers, owners, and operators of single unit trucks

supported the agency proposal to exclude those vehicles from the

rulemaking. Single unit trucks have many different configurations,

according to Ford Motor Company (Ford), some of which would make

installation of the rear impact protection guard impracticable. For

example, school buses with a 3,810 mm (150 in) distance from the rear

axle to the rear extremity of the vehicle would have their angle of

departure severely limited by the proposed rear impact protection

guard. Ford also indicated that there would be many questions

concerning guard installation responsibility because many units are

sold without bodies to secondary manufacturers.

The National Truck Equipment Association (NTEA) supported NHTSA's

proposal to exclude single unit trucks from the guard requirements,

citing the low rate of rear end impacts for trucks as compared to

trailers. NTEA also stated that single unit truck rear impact guard

installation cost would be considerably more (up to $3,000 where

custom-made guards are required) than the installation cost for

trailers because of the high number of special purpose single unit

trucks. It also said that single unit trucks are often farm vehicles,

dump trucks, and delivery trucks that travel short distances, at lower

speeds, generally in the daytime.

NSWMA says the single unit truck exclusion is important because the

safety benefits to passenger vehicles would be offset by the increased

risk to the truck operator and waste service personnel resulting from

the design restrictions that would be imposed by requiring guards on

single unit trucks.

Agency accident data indicate that approximately 27 percent of the

striking vehicle occupant fatalities and 15.8 percent the serious

injuries (AIS 3-5) in rear end collisions with heavy vehicles involve

single unit trucks, while 73 percent of striking vehicle occupant

fatalities and 84.2 percent of serious injuries involve trailers and

semitrailers. This relatively low involvement of single unit trucks

contrasts sharply with their contrasts sharply with their predominance

among heavy vehicles. Single unit trucks represent 72 percent of

registered heavy vehicles. Also, there are 1.6 times as many single

unit trucks produced as there are trailers and semitrailers that would

be candidates (i.e., assuming they do not qualify for some other

exclusion) for underride protection guards. Therefore, this rule covers

about 28 percent of the total vehicles and would achieve about 73

percent of the fatality reduction benefits. The SNPRM estimated that

collisions with single unit trucks account for approximately 68 percent

of the total injuries based on 1986 NASS data. Based on a reevaluation

of the data from the newer General Estimate System (GES) data set,

NHTSA has revised this estimate to about 18 percent.

According to FARS data from 1982 through 1992, fatalities resulting

from passenger vehicle collisions with the rear of single unit trucks

have remained fairly constant, with a slight increasing trend. This

shows that single unit trucks are not an increasing problem, as

suggested by CRASH.

NHTSA has concluded that this category of vehicles should not be

covered by the rule at this time. It may be desirable to cover at least

some single unit trucks. However, the agency lacks sufficient

information at this time to deal with single unit trucks as it has with

trailers, i.e., by excluding from the larger group of single unit

trucks those subgroups with special problems. The agency is concerned

that the variety, complexity, and relatively low weight and chassis

strength of many single unit trucks could require guards that are

substantially more costly than the guards for trailers and

semitrailers. This would prevent the industry from benefiting from the

economies of scale that the separate equipment and vehicle standards

were intended to promote. NHTSA is currently conducting a study of the

single unit truck production to see if there are groups of single unit

trucks that, like trailers, could be fitted with rear impact guards

without excessive costs.

The vast majority of heavy truck striking vehicle occupant

fatalities (73 percent) and injuries (84.2 percent) involve collisions

with the rear ends of trailers and semitrailers. Therefore, NHTSA can

capture most of the benefits from rear underride guards by requiring

them at the outset for trailers and semitrailers. The agency may

supplement this action by initiating a separate rulemaking action to

consider rear impact guards for single unit trucks after completion of

its study.

The agency does not see any merit in Mr. Tomassoni's suggestion.

There would be little benefit in requiring or encouraging manufacturers

to install guards on single unit trucks with a GVWR greater than 11,790

kg (26,000 lbs), because only 10 percent of single unit trucks are

between 4,536 and 11,790 kg (10,000 and 26,000 lbs).

In response to Mr. Kourik's observation that there was no

definition in the SNPRM or elsewhere for ``single unit truck,'' the

regulatory text of the final rule does not use that term, thus such a

definition is not necessary there. Single unit truck refers to trucks

that do not have an articulated chassis.

b. Special Purpose Vehicles. Several manufacturers and operators of

specialty vehicles such as vehicles with rear mounted liftgates, dump

trailers, auto transporters, farm equipment, and recreational vehicles

recommended that their vehicles be explicitly excluded from the rule.

They recommended that the definition of ``special purpose vehicle'' in

the 1992 SNPRM be revised to include these vehicles.

A number of liftgate manufacturers submitted comments. Thieman

Tailgates, Waltco Truck Equipment Company (Waltco), and Leyman

Manufacturing Company all recommended explicit exclusion of trailers

equipped with liftgates. Most liftgates are installed after the trailer

leaves the manufacturer. They also stated that it would be very

burdensome on small businesses to design liftgates around the guard

configuration requirements.

Waltco estimated that several thousand new vehicles are equipped

with liftgates annually. If required, guards for trailers equipped with

liftgates would be more expensive than NHTSA's cost estimate, according

to Waltco. Some guards would have to be movable and compliance testing

would be more complicated since some configurations would necessitate

that the guard be mounted to the liftgate itself. Waltco provided

diagrams to show that all of its liftgate designs are incompatible

because they must either swing through the guard area or create

dangerous shear/pinch zones between gate and guard.

Anthony Liftgates (Anthony) estimated that each year 3,000 new

trailers and semitrailers are equipped with rear mounted liftgates, 500

of the liftgates being manufactured by Anthony. Anthony stated that

rail-type liftgates are the most commonly used and their rail-type

models would be compatible with the proposed guard.

[[Page 2022]]

Anthony requested that NHTSA give special consideration to vehicles

equipped with liftgates since certain restrictions would be highly

detrimental to the industry.

NTEA stated that vehicles equipped with liftgates comprise the

largest group of special purpose vehicles. NTEA estimated that 2,500 of

the 150,000 trailers built each year are equipped with liftgates at the

rear, comprising only 1.7 percent of the market. The NTEA assured NHTSA

that no trailer manufacturer would use the special purpose vehicle

exclusion to evade the guard requirement because liftgates cost

($6,000) so much more than guards.

The Leyman Manufacturing Company stated that positioning the guard

as specified in the proposal would eliminate the installation of

liftgates. Leyman also pointed out that vehicles equipped with

liftgates were excluded from the January 8, 1981 NPRM.

The agency concurs with the observations made by liftgate

manufacturers regarding the complexities associated with the

installation of rear impact protection guards on these vehicles. NHTSA

acknowledges that vehicles equipped with liftgates were cited in the

January 8, 1981 NPRM as vehicles that would fall within the special

purpose vehicle exclusion. The agency also agrees that the rear impact

protection guard would interfere with the operation of some rear

liftgates. However, NHTSA does not think it is necessary to exclude all

liftgate-equipped trailers explicitly. Instead, the agency has modified

the definition of special purpose vehicle to make it clear that

vehicles with rear mounted liftgates that operate by swinging through

the area that is designated for the rear impact guard are excluded.

Consequently, vehicles equipped with the rail type liftgates that

Anthony Liftgates said would be compatible with a guard are not

excluded, while vehicles equipped with tuckunder and other types of

incompatible liftgates are excluded.

The Manufactured Housing Institute (MHI) stated that manufactured

homes are generally moved once or twice over their lifetime on an

integral, temporary chassis under strict oversize permits. MHI

recommended that NHTSA exclude these trailers from the proposed rule,

stating that the standard should not apply to manufactured homes,

modular structures, and mobile homes. According to MHI, there are about

300,000 units transported annually in the United States, being hauled

as trailers for an average distance of 160 to 200 kilometers (km) (100

to 125 miles (mi)). MHI also noted that mobile homes in transport have

305 to 560 mm (12 to 22 in) of ground clearance.

Mobile homes are not covered by the FMVSS. NHTSA has long

interpreted the Mobile Home Construction and Safety Standards Act of

1974 (Pub.L. 93-383) as withdrawing NHTSA's authority to regulate

mobile homes as motor vehicles and vesting this authority in the

Department of Housing and Urban Development. Therefore, mobile homes

are not covered by this rule. This conclusion does not, however, apply

to motor homes.

The Recreational Vehicle Industry Association (RVIA) recommended

that recreational trailers be excluded from the proposed regulation as

special purpose vehicles. According to RVIA, recreational vehicles are

probably involved in a small percentage of the rear end collisions due

primarily to low mileage and little nighttime highway exposure. RV

trailers often require high ground clearance for off-road use,

according to RVIA.

The agency does not believe that recreational vehicles should be

included in the definition of special purpose vehicle because they do

not have work performing equipment in their lower rear extremity.

However, NHTSA has concluded that certain recreational vehicles should

be explicitly excluded under the applicability section of the rule.

Most of these vehicles are believed to be low chassis vehicles, and

even if they are not, their chassis will generally be too weak to

support a guard. Therefore, vehicles with ``temporary living

quarters,'' as defined in 49 CFR 523.2, are excluded from the rule.

The Specialized Carriers and Rigging Association (SC&RA) suggested

that two types of heavy hauler trailers be listed in the final rule as

examples of ``special purpose vehicles''. The vehicles cited have rear

end configurations that vary based on use. According to SC&RA, rear

underride guards would interfere with the function of these types of

vehicles. The SC&RA asserts that design considerations prevent

compliance with the proposed rule.

If SC&RA is correct in asserting that design considerations prevent

the two vehicle types from having rear impact guards, these vehicles

would clearly meet the special purpose vehicle definition. The

illustrations provided indicate that they have work performing

equipment or would qualify for the low chassis vehicle exclusion.

Therefore, the agency sees no need to explicitly list these vehicles as

examples of special purpose vehicles.

NSWMA recommended that the ``special purpose vehicle'' definition

be modified to include vehicles with special equipment mounted at the

rear that is not directly affected in an adverse manner by the rear

impact protection guard. NSWMA believes that this exclusion is

necessary because of the potential impairment of function in waste

industry specialized hauling vehicles from factors such as reduced

departure angle and off-road use.

NHTSA does not believe that the special purpose vehicle definition

should be modified in response to NSWMA's recommendation. Vehicles with

work performing equipment at the rear whose operation would not be

adversely affected by the rear impact guard should be equipped with

guards. All trailer users will have to deal with a reduced angle of

departure. Further, exclusions of vehicles need to be made on the basis

of physical attributes instead of anticipated functional restrictions.

NSWMA has not alleged that these trailers are physically different from

any other trailers, only that they are used in a demanding operational

environment.

NHTSA believes that the use of adjustable guards will alleviate

most operational restrictions where the work performing equipment does

not qualify the vehicle for the special purpose vehicle exclusion, such

as trailers that travel on uneven surfaces or that have beds that raise

and lower at their rear ends. NSWMA acknowledged that most of the

vehicles it refers to are excluded as single unit trucks.

The National Potato Council recommended that vehicles used

primarily for harvesting be excluded from the rule. The Potato Council

stated that rear-unload semitrailers have rear conveyors whose function

would be significantly impaired if rear impact guards were required. It

also requested that eighteen wheelers that travel no more than 240 km

(150 mi) from their base farm should be excluded from the proposed

rule. These vehicles are on road for very short periods, according to

the Potato Council--one to two months in the spring to haul seeds, and

a similar period in the fall to bring the crop to market.

Assuming the Potato Council is correct that underride guards would

substantially impair the function of the rear-unload semitrailers,

these vehicles would qualify as special purpose vehicles. A specific

mention of them in the rule is therefore unnecessary. Regarding the

eighteen wheelers, sporadic road use and short travel distances have

been considered in the past as factors in determining whether vehicles

are ``motor vehicles'' that are subject to NHTSA's safety standards.

However, the fact that the vehicles are

[[Page 2023]]

used on the public roads only two to four months a year does not

disqualify them as motor vehicles. The same may be true for many pickup

trucks used on farms. Merely because a given trailer happens to be used

on the farm most of the year does not mean it was not manufactured

primarily for use on the public streets. Similarly, the shortness of

the trips the vehicle takes is not dispositive, unless it is used only

to cross from field to field or to travel between job sites. It appears

that the trailers the Potato Council refers to are used primarily for

transportation during the spring and fall. Therefore, the definition of

special purpose vehicles has not been modified as recommended by the

Potato Council.

Mr. John Kourik suggested that the application section be expanded

to show whether or not the rule covers the following kinds of vehicles:

boat trailer, fire fighting vehicle (some have trailers), trailer

converter dolly, agricultural commodity truck, auto transporter (a

combination vehicle), container chassis trailer, pulpwood trailer,

heavy hauler trailer, and straddle trailer. In the alternative, he

suggests that some method for obtaining interpretations of

configurations is needed, other than tedious petitions for exemptions.

NHTSA is not providing interpretations for each of the vehicles

listed by Mr. Kourik. Applicability is based on the configuration of

the vehicle, rather than vehicle function, as Mr. Kourik's list

suggests. The agency is unsure about the physical attributes of some of

the listed vehicles. In the absence of more detailed information, NHTSA

cannot give definitive interpretations for the listed vehicles. NHTSA

believes that the rule adequately defines those vehicles that are

included and those that are excluded. NHTSA believes further that the

applicability will be obvious in almost all cases to persons

sufficiently familiar with details of the physical attributes of the

vehicles in question. Given his knowledge about these vehicles, Mr.

Kourik should be able to determine whether they fall within the

agency's exclusions. The agency notes that the public is not required

to petition for an exemption to obtain an interpretation of the rule's

applicability to a particular vehicle configuration. The Office of the

Chief Counsel issues such interpretations in response to letters of

inquiry which provide sufficient background information.

FHWA initially indicated that the definition of a special purpose

vehicle should include certain dimensions for the work performing

equipment. The maximum ground clearance, minimum width, or maximum

distance between any work performing equipment and the side of the

vehicle were cited by FHWA as dimensions that should be included in the

definition. According to FHWA, adding language to the rule that further

defines the location of work performing equipment would provide better

guidance to vehicle manufacturers and reduce potential enforcement

problems for NHTSA and FHWA.

NHTSA believes that the relationship of the work performing

equipment to the location in which the rear impact guard would have to

be installed, and not the mere presence of the equipment, should be the

criterion for determining exclusion. If the equipment needs to move

through the area that could be occupied by the horizontal member of the

guard, as defined in S5.1.1 through 5.1.3 of the vehicle standard, the

presence of a guard would impair or eliminate the usefulness of the

equipment. NHTSA has decided that it would be both impracticable and an

undue burden to require rear impact guards on such vehicles. However,

if the equipment is detached or stows out of the guard area while in

the vehicle is in transit, a guard would not be an impediment to the

equipment, and a guard is required. Although it is not required, NHTSA

encourages vehicle manufacturers to move the guard within the limits of

S5.1.1 through S5.1.3 to accommodate the work performing equipment.

It is neither practical nor necessary to specify location or

dimensions for the work performing equipment. The ground clearance,

width, and distance from the work performing equipment to the side of

the vehicle are not relevant because the work performing equipment is

not required to perform as a guard. NHTSA does not want to restrain

innovation by giving direction to vehicle manufacturers on the

configurations of their work performing equipment. Defining the

dimensions or location of the work performing equipment is not

necessary for an enforceable rule. All that is required to confirm the

applicability of the exclusion is a demonstration that the work

performing equipment, while the vehicle is in transit, resides in the

area defined by S5.1.1 through S5.1.3 as the guard's horizontal member

or passes through that area to perform its function. Therefore, the

definition of special purpose vehicle in the rule has been revised to

reflect that the foundation of the special purpose vehicle exclusion is

the presence of work-performing equipment that resides in or, to

perform its function, moves through the area designated for the

underride guard while the vehicle is in transit.

The definition of special purpose vehicle has been modified to

explicitly recognize the piping of hazardous materials tankers as work

performing equipment. RSPA's rule for underride guards on hazardous

materials tankers (49 CFR 178.345-8) is generally compatible with this

rule, and this rule applies to hazardous materials tankers. However, to

prevent any confusion as to the relationship between RSPA's rule and

NHTSA's rule, this rule explicitly recognizes that piping that carries

hazardous materials while in transit needs the special protection that

is provided by RSPA's rule. Therefore, hazardous materials tankers with

piping in front of the guard are excluded from the requirements of this

rule.

c. Wheels Back Vehicle. A ``wheels back vehicle'' was defined in

the SNPRM's vehicle standard as a vehicle which has a permanently fixed

rear axle with tires whose rearmost surface is located not more than

305 mm (12 in) forward of a vertical transverse plane tangent to the

rear extremity of the vehicle. Several commenters recommended that the

wheels back vehicle definition be changed to include vehicles with rear

tires located as much as 610 mm (24 in) from the rear extremity of the

vehicle. Other commenters expressed concern that impacting the rear

tires of a trailer or semitrailer is similar to impacting a rigid

barrier and the agency should delete this category of exclusion.

Industry groups and some other commenters favored an expansion of

the wheels back definition by allowing the wheels to be positioned more

than 305 mm (12 in) forward of the rear extremity. The ATA and the TTMA

noted that the proposed rule allowed guards to be mounted up to 305 mm

(12 in) forward of the rear extremity while allowing an additional 125

mm (5 in) to meet the strength requirements of the 1992 SNPRM. TTMA

recommended, therefore, that the distance between the rear tires and

the rear extremity of the vehicle be increased from 305 to 430 mm (12

to 17 in). According to ATA, the spirit of the ``wheels back vehicle''

exclusion would not be violated by allowing the tires to be located as

much as 560 mm (22 in) forward of the rear extremity. ATA reasons that

guards mounted 305 mm (12 in) forward of the rear of the vehicle will

allow some vehicles to underride more than 305 mm (12 in) prior to

contact with the guard since the forward most area of the car may not

be contacted.

TTMA's recommendation to add the 125 mm (5 in) of permitted test

[[Page 2024]]

deflection to the 305 mm (12 in) of permitted setback, resulting in 430

mm (17 in) of permitted setback, is not practical. It does not account

for the fact that, in a crash, a portion of the impacting vehicle's

initial energy and velocity will be absorbed after the guard has

undergone 125 mm (5 in) of deflection or deformation. This is a very

different situation from one in which the initial impact contact

between the passenger car and the underride guard takes place 430 mm

(17 in) forward of the trailer's rear extremity. With a 430 mm (17 in)

setback, even if the rear impact guard were completely rigid, the

striking vehicle would still advance closer to the rear of the trailer

(and potential PCI) before coming to rest because the vehicle would be

forced to absorb more energy (thus increasing the likelihood of

occupant injury).

While some passenger vehicles may underride the impact protection

guard prior to contact, as stated by ATA, this non-contact underride is

not likely to be more than a few inches. If anything, this fact

mitigates in favor of requiring the guards to be positioned farther to

the rear. This final rule adds the requirement that the underride guard

be positioned as far to the rear of the vehicle as practical.

Some commenters recommended allowing the wheels to be positioned

even farther forward if there were a guard in between the rear wheels.

The ATA encouraged NHTSA to allow vehicles to use the ``wheels back''

exclusion vehicles with tires up to 610 mm (24 in) forward of the rear

extremity if a ``center'' guard were provided. This partial guard would

be located no more than 305 mm (12 in) forward of the rear extremity

and no more than 150 mm (6 in) inboard of the inside sidewalls of the

tires. The center guard's placement between the wheels would complement

the tires in resisting underride. Mr. Robert Crail suggested that a

partial underride protection guard be specified for double trailers

with the rear tires mounted between 430 and 610 mm (17 and 24 in)

forward of the rear extremity, because the trailer wheels are as

effective as a guard at full deflection. He said that the partial rear

underride protection guard should extend to within 205 mm (8 in) of the

inboard sidewalls of the rear tires.

The agency believes that the specification of a partial rear impact

guard would not enhance safety because it is unlikely that a passenger

vehicle would pass between the rear tires of the trailer. The spacing

between the inside surfaces of the rear tires on a 2,600 mm (102 in)

wide trailer was measured by the agency as 1,310 mm (51.5 in). There

are almost no passenger vehicles produced with widths of less than 1600

mm (63 in). Therefore, even a centric collision between the widest

trailers and the narrowest cars would probably result in considerable

engagement of the tires with the frontal vehicle structure.

Other commenters, in addition to the ATA and Mr. Crail, believe

that the 305 mm (12 in) maximum offset makes the exclusion too

restrictive. Yellow Freight System suggested that the wheels back

definition be changed to allow the wheels to be 560 mm (22 in) forward

of the rear extremity. It states that most trailers cannot position the

wheels closer than 460 to 560 mm (18 to 22 in) from the rear extremity

because the combined effect of shorter distances and the Federal Bridge

formula would be to restrict the weight of the load that can be

carried. Strick Trailers stated that operators routinely position the

rear axle at 915 and 1,065 mm (36 and 42 in) forward of the rear

extremity of the vehicle, which would exclude them from the wheels back

vehicle category.

The rationale for all these suggestions appears to be that most

trailers with the axle in the rearmost position have the rear tire

within a range of 405 to 610 mm (16 to 24 in) forward of the rear, and

an expanded wheels-back definition would lower costs by allowing more

trailers to qualify as wheels back. The agency notes that many of the

commenters mentioned ``positioning'' of the rear wheels, which implies

that they are referring to the vehicles that do not have fixed axles.

Therefore, these vehicles would not be eligible for the wheels back

exclusion anyway. NHTSA does not believe that carriers would change the

wheel positioning of their fleets merely to avoid the small one-time

incremental cost of installing an upgraded guard, as Yellow Freight

suggests. Moreover, while NHTSA is concerned with the costs of the

rule, the ultimate goal is to prevent PCI without imparting

unacceptable deceleration forces to the impacting vehicle. Allowing

vehicles to have their wheels farther forward would increase the

likelihood of PCI. The agency does not believe, based on available

information, that the definition of wheels back vehicle should be

modified to increase the allowable distance between the rear extremity

of the vehicle and the rear tires.

Advocates appeared to favor eliminating the wheels back exclusion

altogether. Advocates stated that the agency has no test data on

``wheels back vehicles'' which support the conclusion that they should

be excluded from the proposed rulemaking. Advocates further stated that

the agency has contradicted the argument that impacting the rear wheels

of trailers results in acceptable crash forces, because the Preliminary

Regulatory Evaluation (PRE) likens a collision with the wheels to

striking a ``rigid wall.'' Trailer tires will not provide an acceptable

level of rear impact protection, according to Advocates. Advocates

acknowledged that two crash tests with wheels back vehicles were

conducted by the Texas Transportation Institute (TTI), but referenced a

paper co-authored by John Tomassoni, a former NHTSA engineer, as

evidence that the collision forces would be ``relatively high.''

Advocates also stated that the rule should define ``permanent''

settings for sliding bogeys by requiring that they be welded or bolted

in place.

Vehicles meeting the wheels back requirements should be capable of

preventing the trailer structure from penetrating a passenger vehicle

occupant compartment during a rear end collision. Two full-scale crash

tests involving ``wheels back vehicles'' were conducted by the TTI in

1979. For these wheels back vehicle tests, the rear tires were located

about 100 to 205 mm (4 to 8 in) forward of the rear extremity of the

trailer. In each test, in an offset crash in which a Chevrolet Impala

struck the tires and in a centric crash in which a VW Rabbit struck the

axle and other components between the tires, PCI was prevented at about

56 kph (35 mph). In the test with the VW Rabbit, post-crash photos

indicate that, when dynamic underride reached the maximum, the body of

the trailer was 305 to 355 mm (12 to 14 in) from the A-pillar and

windshield area of the passenger vehicle. These crash tests indicate

that a fixed rear axle with the tires mounted within 305 mm (12 in) of

the vehicle's rear extremity constitutes an adequate substitute for a

rear impact protection guard from the standpoint of preventing PCI.

The rear wheels of a trailer are adequate for managing the energy

of an underride crash. The on-board dummy instrumentation during both

crashes indicated a relatively low potential for serious injuries. In

fact, the wheels back vehicle performed better in the offset crash than

all other guards tested in the TTI research project except the Quinton-

Hazel guard. Although the maximum vehicle deceleration of a VW Rabbit

that was driven centrically into a wheels-back trailer at 33 mph was

similar to the deceleration of the same make/model vehicle driven into

a rigid wall (35 mph), partial guards for the sole purpose of energy

absorption in centric crashes are not warranted from a cost-benefit

standpoint.

[[Page 2025]]

NHTSA has decided to retain the wheels back exclusion for vehicles

with the rear wheels within 305 mm (12 in) of the rear extremity of the

vehicle. Vehicles with wheels set farther forward than that will have

sufficient room between the guard and the trailer rear tires for the

guard to deflect and absorb some of the passenger vehicle's energy

before the guard contacts the rear wheels of the trailer. Vehicles with

rear wheels within 305 mm (12 in) of the rear extremity will not have

sufficient room for the guard to do much good before it contacts the

wheels.

The wheels back vehicle exclusion is intended to apply exclusively

to vehicles with the rear tires permanently located close to the rear

extremity of the vehicle. The concept of ``permanent'' is clear enough

and does not require elaboration, as Advocates suggests. The rear

wheels must be either welded in place or designed so that they can

occupy only one position. Vehicles with moveable bogeys cannot be

wheels back vehicles even if their wheels are set in a wheels back

position, as suggested by the comments of Yellow Freight and Strick

Trailers.

D. Costs

Many of the commenters addressed the question of cost of the guard.

The consumer safety groups thought that the agency's estimate of the

cost of energy absorbing guards was too high. Conversely, the industry

commenters generally thought the agency's estimate was either low or

about right. Most of the private citizens who commented on guard cost

said that energy absorbing guards were worth the price, without giving

specifics.

Advocates stated that NHTSA had not taken into account the fact

that economies of scale would lower the cost of hydraulic energy-

absorbing guards to nearly that of the proposed guard. It said that the

hydraulic guards are within the price range of the proposed guard.

Advocates also commented that NHTSA provides no guidance information to

carriers on effectiveness, cost/benefit ratio, mounting heights, or

crashworthiness that would allow them to choose a superior (i.e.,

energy absorbing) guard.

The American Automobile Association (AAA), the New York Attorney

General, and many private citizens expressed the view that the

additional cost for energy absorbing guards (variously described by

them as approximately $200 additional, or ``modest'') is reasonable.

These comm

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