Federal Motor Vehicle Safety Standards; Electronic Stability Control Systems for Heavy Vehicles

Federal RegisterJun 23, 2015

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DEPARTMENT OF TRANSPORTATION

National Highway Traffic Safety Administration

49 CFR Part 571

[Docket No. NHTSA-2015-0056]

RIN 2127-AK97

Federal Motor Vehicle Safety Standards; Electronic Stability Control Systems for Heavy Vehicles

AGENCY:

National Highway Traffic Safety Administration (NHTSA), Department of Transportation (DOT).

ACTION:

Final rule.

SUMMARY:

This document establishes a new Federal Motor Vehicle Safety Standard No. 136 to require electronic stability control (ESC) systems on truck tractors and certain buses with a gross vehicle weight rating of greater than 11,793 kilograms (26,000 pounds). ESC systems in truck tractors and large buses are designed to reduce untripped rollovers and mitigate severe understeer or oversteer conditions that lead to loss of control by using automatic computer-controlled braking and reducing engine torque output.

In 2018, we expect that, without this rule, about 34 percent of new truck tractors and 80 percent of new buses affected by this final rule would be equipped with ESC systems. We believe that, by requiring that ESC systems be installed on the rest of truck tractors and large buses, this final rule will prevent 40 to 56 percent of untripped rollover crashes and 14 percent of loss-of-control crashes. As a result, we expect that this final rule will prevent 1,424 to 1,759 crashes, 505 to 649 injuries, and 40 to 49 fatalities at $0.1 to $0.6 million net cost per equivalent life saved, while generating positive net benefits.

DATES:

The effective date of this rule is August 24, 2015. The incorporation by reference of certain publications listed in the rule is approved by the Director of the Federal Register as of August 24, 2015.

Petitions for reconsideration:

Petitions for reconsideration of this final rule must be received not later than August 7, 2015.

ADDRESSES:

Petitions for reconsideration of this final rule must refer to the docket and notice number set forth above and be submitted to the Administrator, National Highway Traffic Safety Administration, 1200 New Jersey Avenue SE., Washington, DC 20590.

FOR FURTHER INFORMATION CONTACT:

For technical issues, you may contact Patrick Hallan, Office of Crash Avoidance Standards, by telephone at (202) 366-9146, and by fax at (202) 493-2990. For legal issues, you may contact David Jasinski, Office of the Chief Counsel, by telephone at (202) 366-2992, and by fax at (202) 366-3820. You may send mail to both of these officials at the National Highway Traffic Safety Administration, 1200 New Jersey Avenue SE., Washington, DC 20590.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. Executive Summary

II. Statutory Authority

III. Background

IV. Safety Need

A. Heavy Vehicle Crash Problem

B. Contributing Factors in Rollover and Loss-of-Control Crashes

C. NTSB Safety Recommendations

D. Motorcoach Safety Plan

E. International Regulation

V. Summary of the May 2012 NPRM

VI. Overview of the Comments

VII. Key Differences Between the Final Rule and the NRPM

VIII. ESC Requirement

A. Whether To Require Stability Control

B. Whether To Require ESC or RSC

C. Definition of ESC

D. Technical Documentation

IX. Vehicle Applicability and Phase-In

A. Trucks

1. Summary of the NPRM

2. Exclusions From ESC Requirement

3. Single-Unit Trucks

4. Compliance Dates

B. Buses

1. Summary of the NPRM

2. Buses Built on Truck Chassis

(a) Summary of NPRM

(b) Summary of Comments

3. Hydraulic-Braked Buses

4. School Buses

5. Transit Buses

6. Minimum Seating Capacity and Seating Configuration

7. Compliance Dates

8. Class 3 Through 6 Buses

C. Retrofitting

X. Performance Testing

A. NHTSA's Proposed Performance Tests

1. Characterization Test—SIS

2. Roll and Yaw Stability Test—SWD

3. Lateral Displacement

B. Comments on SIS and SWD Maneuvers

C. Alternative Maneuvers Considered in the NPRM

D. Comments on Alternative Test Maneuvers

E. NHTSA Examination and Testing of EMA Maneuvers

F. Roll Stability Performance Test—J-Turn Test

1. Rationale for Using J-Turn Test

2. Test Procedure and Performance Requirements

3. System Responsiveness

4. Engine Torque Reduction

5. Roll Stability Performance Requirements

G. Yaw Stability

H. Understeer

XI. Test Conditions and Equipment

A. Outriggers

B. Automated Steering Machine

C. Anti-Jackknife System

D. Control Trailer

E. Sensors

F. Ambient Conditions

G. Road Test Surface

H. Vehicle Test Weight

I. Tires

J. Mass Estimation Drive Cycle

K. Brake Conditioning

L. Compliance Options

M. Data Collection

XII. ESC Disablement

A. Summary of Comments

B. Response to Comments

XIII. ESC Malfunction Detection, Telltale, and Activation Indicator

A. ESC Malfunction Detection

B. ESC Malfunction Telltale

C. Combining ESC Malfunction Telltale With Related Systems

D. ESC Activation Indicator

XIV. Benefits and Costs

A. Target Crash Population

B. System Effectiveness

1. Summary of the NPRM

2. Summary of Comments and Response

(a) ATRI Study

(b) Bendix Study

3. Effectiveness Estimate

C. Benefits Estimates

1. Safety Benefits

2. Monetized Benefits

D. Cost Estimate

Truck Tractors

Large Buses

E. Cost Effectiveness

F. Comparison of Regulatory Alternatives

XV. Regulatory Analyses and Notices

A. Executive Order 12866, Executive Order 13563, and DOT Regulatory Policies and Procedures

B. Regulatory Flexibility Act

C. Executive Order 13132 (Federalism)

D. Executive Order 12988 (Civil Justice Reform)

E. Protection of Children From Environmental Health and Safety Risks

F. Paperwork Reduction Act

G. National Technology Transfer and Advancement Act

H. Unfunded Mandates Reform Act

I. National Environmental Policy Act

J. Incorporation by Reference

K. Regulatory Identifier Number (RIN)

L. Privacy Act

I. Executive Summary

This final rule establishes a new Federal Motor Vehicle Safety Standard (FMVSS) No. 136,

Electronic Stability Control Systems for Heavy Vehicles,

to reduce rollover and loss of directional control of truck tractors and large buses. The standard requires that truck tractors and certain large buses with a gross vehicle weight rating (GVWR) of greater than 11,793 kilograms (26,000 pounds) to be equipped with an electronic stability control (ESC) system that meets the equipment and performance criteria of the standard. ESC systems use engine torque control and computer-controlled

braking of individual wheels to assist the driver in maintaining control of the vehicle and maintaining its heading in situations in which the vehicle is becoming roll unstable (

i.e.,

wheel lift potentially leading to rollover) or experiencing loss of control (

i.e.,

deviation from driver's intended path due to understeer, oversteer, trailer swing or any other yaw motion leading to directional loss of control). In such situations, intervention by the ESC system can assist the driver in maintaining control of the vehicle, thereby preventing fatalities and injuries associated with vehicle rollover or collision.

This final rule is made pursuant to the authority granted to NHTSA under the National Traffic and Motor Vehicle Safety Act (“Motor Vehicle Safety Act”). Under 49 U.S. C. Chapter 301, Motor Vehicle Safety (49 U.S. C. 30101

et se.

), the Secretary of Transportation is responsible for prescribing motor vehicle safety standards that are practicable, meet the need for motor vehicle safety, and are stated in objective terms. The responsibility for promulgation of Federal motor vehicle safety standards is delegated to NHTSA. This rulemaking also completes NHTSA's rulemaking pursuant to a directive in the Moving Ahead for Progress in the 21st Century Act (MAP-21) that the Secretary consider requiring stability enhancing technology on motorcoaches.

Pub. L. 112-141 (July 6, 2012).

There have been two types of stability control systems developed for heavy vehicles. A roll stability control (RSC) system is designed to prevent rollover by decelerating the vehicle using braking and engine torque control. The other type of stability control system is ESC, which includes all of the functions of an RSC system plus the ability to mitigate severe oversteer or understeer conditions by automatically applying brake force at selected wheel-ends to help maintain directional control of a vehicle. To date, ESC and RSC systems for heavy vehicles have been developed for air-braked vehicles. Truck tractors and buses covered by today's final rule make up a large proportion of air-braked heavy vehicles and a large proportion of the heavy vehicles involved in both rollover crashes and total heavy vehicle crashes.

As a result of the data analysis research, we determined that ESC systems can be 40 to 56 percent effective in reducing first-event untripped rollovers and 14 percent effective in eliminating loss-of-control crashes caused by severe oversteer or understeer conditions. This estimate is based on an update of the estimate presented in a 2011 research note analyzing the effectiveness of ESC systems discussed in the Final Regulatory Impact Analysis (FRIA) accompanying this final rule.

See Wang, Jing-Shiam, “Effectiveness of Stability Control Systems for Truck Tractors” (January 2011) (DOT HS 811 437); Docket No. NHTSA-2010-0034-0043.

The agency considered requiring truck tractors and large buses to be equipped with RSC systems. When compared to the ESC requirement in this final rule, RSC systems would cost less than ESC systems, be slightly more cost-effective, but would produce net benefits that are much lower than the net benefits from this final rule. This is because RSC systems are less effective at preventing rollover crashes and much less effective at preventing loss-of-control crashes. We also considered requiring trailers to be equipped with RSC systems. However, this alternative would save many fewer lives, would not be cost-effective, and would not result in net benefits.

This final rule requires ESC systems to meet both definitional criteria and performance requirements. It is necessary to include definitional criteria and require compliance with them because developing separate performance tests to cover the wide array of possible operating ranges, roadways, and environmental conditions would be impractical. The definitional criteria are consistent with those recommended by SAE International and used by the United Nations (UN) Economic Commission for Europe (ECE), and similar to the definition of ESC in FMVSS No. 126, the agency's stability control standard for light vehicles. This definition describes an ESC system for heavy vehicles as one that will enhance both the roll and yaw stability of a vehicle using a computer-controlled system that can receive inputs such as the vehicle's lateral acceleration and yaw rate, and use the information to apply brakes individually, including trailer brakes, and modulate engine torque.

This final rule is applicable to all new typical three-axle truck tractors manufactured on or after August 1, 2017. We believe that two years of lead time is sufficient for these vehicles to be equipped with ESC, given that this is a common platform for which ESC systems are readily available today. We are allowing four years of lead time for all other truck tractors. These vehicles include two-axle vehicles, which have been more recently required to satisfy new, reduced minimum stopping distance requirements, and severe-service tractors, for which we believe two additional years of lead time is necessary to design and test ESC systems.

This final rule is applicable to buses over 14,969 kilograms (33,000 pounds) GVWR manufactured more than three years after the date of this final rule. Although we proposed a two-year lead time for buses in the NPRM, the Motorcoach Enhanced Safety Act mandates that new rules, including stability enhancing technology, be applicable to all buses manufactured more than three years after publication of a final rule. However, for buses with a GVWR greater than 11,793 kilograms (26,000 pounds) but not more than 14,969 kilograms (33,000 pounds), we believe that three years of lead time is not feasible. Some of these buses include vehicles with body-on-frame construction and hydraulic brakes, for which ESC system availability is not as widespread. Therefore, we are allowing four years of lead time for buses with a GVWR greater than 11,793 kilograms (26,000 pounds) but not more than 14,969 kilograms (33,000 pounds). We believe that including buses with body-on-frame construction and hydraulic brakes in this final rule will spur development of ESC systems for other hydraulic-braked vehicles, including vehicles with a GVWR of greater than 4,536 kilograms (10,000 pounds) but not more than 11,793 kilograms (26,000 pounds), which are not covered by this rulemaking.

We have chosen an alternative performance test to demonstrate an ESC system's ability to mitigate roll instability to what was proposed. After considering the public comments and conducting additional track testing, we have determined that a 150-foot-radius J-turn test maneuver is an efficient means to ensure vehicles maintain roll stability. Like the test maneuver in the NPRM, the J-turn test maneuver is among those available to manufacturers to demonstrate compliance with the UNECE mandate for ESC on trucks and buses.

The J-turn test maneuver, based on an alternative test discussed in the NPRM, involves accelerating to a constant speed on a straight stretch of high-friction track before entering into a 150-foot radius curve. After entering the curve, the driver attempts to maintain the lane. At a speed that is at up to 1.3 times the speed at which the ESC system activates, but in no case below 48.3 km/h (30 mph), an ESC system must activate the vehicle's service brakes to slow the vehicle's speed to 46.7 km/h (29 mph) within 3 seconds

after entering the curve and 45.1 km/h (28 mph) within 4 seconds after entering the curve. Additional J-turn tests are conducted to ensure that an ESC system is able to reduce engine torque.

The performance metric for the J-turn (reduction in forward speed) is easy to obtain and serves as a proxy for absolute lateral acceleration. Lateral acceleration on a fixed-radius curve is a function of forward velocity. On a 150-foot radius curve, a forward speed of 48.3 km/h (30 mph) corresponds to a lateral acceleration of approximately 0.4g. Based on prior NHTSA testing, we have found that 0.4g represents the margin of lateral stability on a typical fully loaded truck tractor with the loads having a high center of gravity (CG). That is, lateral acceleration levels greater than 0.4g (or forward speeds on a 150-foot radius curve of greater than 48.3 km/h (30 mph)) on a typical truck tractor are likely to lead to lateral instability, wheel lift, and possible rollover. However, lateral acceleration levels less than 0.4g (or forward speeds on a 150-foot radius curve of less than 48.3 km/h (30 mph)) on a typical truck tractor are unlikely to lead to lateral instability, wheel lift, and rollover.

This final rule includes a requirement proposed in the NPRM that an ESC system be able to mitigate yaw instability. This requirement is similar to one proposed in the NPRM, and adopted in this final rule, requiring an ESC system be able to mitigate understeer. However, this final rule does not include any performance test to evaluate the ability of an ESC system to mitigate yaw instability. Although the NPRM included the sine with dwell (SWD) maneuver to test both roll and yaw instability, we have decided not to include it in this final rule. The SWD maneuver is only a partial test of the ability to mitigate yaw instability. It tests an ESC system's ability to mitigate loss of control resulting from oversteer conditions, but not its ability to mitigate understeer, which is the most common loss-of-control scenario for heavy vehicles. NHTSA has been unable to develop a test for understeer mitigation. As argued by many commenters, performing the SWD maneuver entails substantial time and instrumentation burdens. We do not believe that this additional time and cost is justified solely to test an ESC system's ability to mitigate yaw instability caused by oversteer conditions when a majority of the benefits of this final rule are derived from rollover prevention and the majority of benefits attributed to prevented loss-of-control crashes in heavy vehicles are derived from understeer mitigation, which would not have been tested in the SWD maneuver. However, we are continuing to examine possible yaw performance maneuvers, including the SWD maneuver, to test yaw stability performance in the future.

The decision to adopt the J-turn test maneuver as the performance test in this final rule has caused us to reconsider test conditions and equipment. However, many aspects of testing remain identical to the proposal. For example, we will conduct performance testing on a high-friction surface. We believe that the potential for variance in surface friction on a low-friction surface may introduce variabilities in ESC testing that may lead to inconsistent results. We are still equipping all test vehicles with outriggers and truck tractors with anti-jackknife systems for the safety of test drivers.

On the other hand, many proposed aspects of testing had to be modified to accommodate the J-turn test maneuver. Because the J-turn test maneuver is a path-following maneuver, we are not using a steering wheel controller that was proposed in the NPRM. We noted potential variabilities in the proposed specification for the control trailer. However, because the performance metric for the J-turn test maneuver is different than the proposed SWD requirements, those variabilities identified in the NPRM that were related to the SWD maneuver are no longer relevant. We have modified the loading condition to load the vehicle to its GVWR because that is the most severe test condition with the J-turn test maneuver. Finally, the number of sensors used in testing is substantially reduced because the vehicle's actual lateral acceleration throughout the maneuver does not need to be measured.

We have considered comments on the issue of allowing ESC system disablement. This final rule does not allow the driver to disable the ESC system at speeds higher than 20 km/h (12.4 mph), which we have defined as the minimum speed at which an ESC system must operate. Many of the comments we received arguing in favor of allowing ESC system disablement were, in fact, arguing for disablement of traction control to allow a vehicle to start moving on certain surfaces with low friction such as on snow, ice, or off-road conditions. However, we do not believe that an ESC system would prevent a heavy vehicle from moving in these circumstances. Rather, we believe that manufacturers may wish to disable an automatic traction control system to allow the vehicle to move. NHTSA does not require traction control systems, nor does NHTSA prohibit the installation of an on/off switch for a traction control system. We understand that traction control systems are related to ESC systems in that they can control engine torque output and activate the brakes on individual wheel ends. However, we do not find these arguments to be a compelling reason to allow an ESC system deactivation switch or automatic deactivation of ESC systems at speeds above 20 km/h (12.4 mph).

This final rule requires that an ESC system be able to detect a malfunction and provide a driver with notification of a malfunction by means of a telltale. This requirement is similar to the malfunction detection and telltale requirements for light vehicles in FMVSS No. 126. After considering public comments, we have changed the vehicle depicted on the telltale to better represent the profile of a combination vehicle or bus rather than a passenger car.

Based on the agency's effectiveness estimates, this final rule will prevent 1,424 to 1,759 crashes per year resulting in 505 to 649 injuries and 40 to 49 fatalities. This final rule will also result in significant monetary savings as a result of the prevention of property damage and travel delays.

Without this final rule, we project that, in 2018, manufacturers would have equipped 33.9 percent of truck tractors with ESC systems, 21.3 percent of truck tractors would be equipped with RSC systems, and 80.0 percent of large buses would be equipped with ESC systems. Based on the agency's cost teardown study, the average ESC system cost is estimated to be $585 for truck tractors and $269 for large buses. The incremental cost of installing an ESC system in place of an RSC system on a truck tractor is estimated to be $194. Based upon the agency's estimate that 150,000 truck tractors and 2,200 buses covered by this final rule will be manufactured annually, the agency estimates the total technology cost of this final rule to be approximately $45.6 million.

This final rule is highly cost effective and beneficial. The net benefits of this final rule are estimated to range from $412 to $525 million at the 3 percent discount rate and $312 to $401 million at the 7 percent discount rate. The agency estimates that this rule will result in societal economic savings resulting from preventing crashes, reducing congestion, and preventing property damage, such that the net cost of this final rule range from $3.6 to $12.3 million at a 3 percent discount rate and from $12.3 to $19.2 million at 7 percent discount rate. As a result, the net cost per equivalent life saved ranges

from $0.1 to $0.3 million at the 3 percent discount rate and from $0.3 to $0.6 million at the 7 percent discount rate. The costs and benefits of this rule are summarized in Table 1.

Table 1—Estimated Annual Cost, Benefits, and Net Benefits of the Final Rule

[In millions of 2013 dollars]

Vehicle costs

Societal

economic

savings

VSL savings

Total

monetized

savings

Cost per equivalent live saved

Net benefits

At 3% Discount

$45.6

$33.3-$42.1

$424-$528

$458-$571

$0.1-$0.3

$412-$525

At 7% Discount

45.6

26.4-33.3

332-413

358-446

0.3-$.6

312-401

II. Statutory Authority

NHTSA is issuing this final rule under the National Traffic and Motor Vehicle Safety Act (“Motor Vehicle Safety Act”). Under 49 U.S.C. Chapter 301, Motor Vehicle Safety (49 U.S.C. 30101

et seq.

), the Secretary of Transportation is responsible for prescribing motor vehicle safety standards that are practicable, meet the need for motor vehicle safety, and are stated in objective terms. “Motor vehicle safety” is defined in the Motor Vehicle Safety Act as “the performance of a motor vehicle or motor vehicle equipment in a way that protects the public against unreasonable risk of accidents occurring because of the design, construction, or performance of a motor vehicle, and against unreasonable risk of death or injury in an accident, and includes nonoperational safety of a motor vehicle.” “Motor vehicle safety standard” means a minimum performance standard for motor vehicles or motor vehicle equipment. When prescribing such standards, the Secretary must consider all relevant, available motor vehicle safety information. The Secretary must also consider whether a standard is reasonable, practicable, and appropriate for the types of motor vehicles or motor vehicle equipment for which it is prescribed and the extent to which the standard will further the statutory purpose of reducing traffic accidents and associated deaths. The responsibility for promulgation of Federal motor vehicle safety standards is delegated to NHTSA.

On July 6, 2012, President Obama signed MAP-21, which incorporated in Subtitle G the “Motorcoach Enhanced Safety Act of 2012.” Section 32703(b)(3) of the Act states that, not later than two years after the date of enactment of the Act, the Secretary shall consider requiring motorcoaches to be equipped with stability enhancing technology, such as electronic stability control and torque vectoring, to reduce the number and frequency of rollover crashes of motorcoaches. The Secretary was directed to prescribe regulations that address stability enhancing technology if the Secretary determines that such standards meet the requirements and considerations set forth in subsections (a) and (b) of 49 U.S.C. 30111. These requirements are discussed in the preceding paragraph.

The Motorcoach Enhanced Safety Act directs the Secretary to consider various other motorcoach rulemakings, in provided timeframes, related to safety belts,

improved roof support standards, advanced glazing standards and other portal improvements to prevent partial and complete ejection of motorcoach passengers, tire pressure monitoring systems, and tire performance standards. The Act also includes provisions on fire research, interior impact protection, enhanced seating designs, and collision avoidance systems, and the consideration of rulemaking based on such research. There also are provisions in the Motorcoach Enhanced Safety Act relating to improved oversight of motorcoach service providers, including enhancements to driver licensing and training programs and motorcoach inspection programs.

Pursuant to the Motor Vehicle Safety Act and the Motorcoach Enhanced Safety Act, NHTSA published a final rule requiring lap/shoulder seat belts for each passenger seating position on all new over-the-road buses, and in new buses other than over-the-road buses with a GVWR greater than 11,793 kilograms (26,000 pounds) beginning on November 26, 2016. 78 FR 70415 (Nov. 25, 2013).

In section 32702, “Definitions,” of the Motorcoach Enhanced Safety Act, the Act states at section 32702(6) that “the term `motorcoach' has the meaning given the term `over-the-road bus' in section 3038(a)(3) of the Transportation Equity Act for the 21st Century (TEA-21) (49 U.S.C. 5310 note), but does not include a bus used in public transportation provided by, or on behalf of, a public transportation agency; or a school bus, including a multifunction school activity bus.” Section 3038(a)(3) states: “The term `over-the-road bus' means a bus characterized by an elevated passenger deck located over a baggage compartment.”

Under section 32703(e)(1) of the Motorcoach Enhanced Safety Act, any regulation prescribed in accordance with section 32703(b) (and several other subsections) shall apply to all motorcoaches manufactured more than three years after the date on which the regulation is published as a final rule, take into account the impact to seating capacity of changes to size and weight of motorcoaches and the ability to comply with State and Federal size and weight requirements, and be based on the best available science.

Prior to enactment of the Motorcoach Enhanced Safety Act, the agency's May 23, 2012 NPRM proposed requiring truck tractors and large buses with a GVWR of greater than 11,793 kg (26,000 lb.) to be equipped with stability enhancing technology. Thus, the agency had already considered requiring motorcoaches to have stability enhancing technology, and had proposed requiring the same, prior to the enactment of the Motorcoach Enhanced Safety Act.

The agency does not interpret the Motorcoach Enhanced Safety Act on its own as a mandate to require stability enhancing technology on over-the-road buses. With respect to rollover crash avoidance, section 32703(b)(3) of the Motorcoach Enhanced Safety Act directs the agency to “consider requiring” stability enhancing technology such as electronic stability control or torque vectoring on over-the-road buses. However, the agency was also directed in section 32703(b) to prescribe a regulation if the Secretary determines that such standards meet the requirements and considerations for issuing a motor vehicle safety standard under the Motor Vehicle Safety Act. The Motorcoach Enhanced Safety Act does not provide independent statutory authority to require stability enhancing technologies on over-the-road buses.

Thus, any mandate requiring stability enhancing technology pursuant to the Motorcoach Enhanced Safety Act is dependent on satisfying the considerations and requirements of the Motor Vehicle Safety Act.

In contrast, the Motorcoach Enhanced Safety Act specifically mandated that the agency prescribe

regulations requiring safety belts to be installed at each designated seating position on all over-the-road buses.

In issuing this final rule, we took into account the considerations of section 32703(e)(1) of the Motorcoach Enhanced Safety Act regarding the implementation of regulations prescribed in accordance with subsection (b)(3). Unlike subsection (b)(3), subsection (e)(1) does not use permissive language. Because this final rule is issued in accordance with subsection (b)(3), we believe the considerations regarding the application of regulations in subsection (e)(1) must be addressed in this rulemaking. Nonetheless, because the Motorcoach Enhanced Safety Act contains no independent statutory authority in support of a mandate for stability enhancing technology, the considerations in subsection (e)(1) are constrained by the agency's authority to issue standards under the Motor Vehicle Safety Act. Therefore, where the considerations in subsection (e)(1) conflict with any requirements and considerations set forth in subsections (a) and (b) of 49 U.S.C. 30111, the requirements of the Motor Vehicle Safety Act supersede the Motorcoach Enhanced Safety Act.

See section IX.B below for such a finding with respect to the application of this final rule to buses with a GVWR of 14,969 kilograms (33,000 pounds) or less.

This final rule is practicable, meets a need for motor vehicle safety, and is stated in objective terms. With respect to the considerations of the Motorcoach Enhanced Safety Act, we believe that Congress intended that a final rule based on the 2012 NPRM would complete the rulemaking proceeding specified in section 32703(b)(3) of the Act. Electronic stability control will reduce the number and frequency of rollover crashes of motorcoaches. This rulemaking is based on the best available science. Further, we have considered the impact to seating capacity and changes to size and weight of motorcoaches, and we believe that this rule will have no effect on these considerations. ESC systems will add less than 10 pounds of additional weight to over-the-road buses.

“Report: Cost and Weight Analysis of Electronic Stability Control (ESC) and Roll Stability Control for Heavy Trucks,” Docket No. NHTSA-2011-0066-0034.

Although the Motorcoach Enhanced Safety Act also suggested torque vectoring as a possible technology to consider requiring on motorcoaches, we did not propose requiring torque vectoring in the May 2012 NPRM, and it is beyond the scope of this rulemaking proceeding. Even if it was within scope to require torque vectoring, the agency would not do so in this rulemaking. The agency's understanding of torque vectoring is that it is a technology that allows a vehicle's differential or brakes to vary the power supplied to the drive axle wheel end. In contrast, ESC systems activate the vehicle's service brakes to vary the braking on each wheel end combined with the ability to reduce engine torque (which reduces power on drive axle wheel ends). In the May 2012 NPRM, we noted that, all things being equal, a vehicle entering a curve at a higher speed is more likely to roll over than a vehicle entering a curve at a lower speed.

Once a vehicle is about to enter a curve at a high enough speed that would generate sufficient lateral acceleration to cause a possible rollover, the most effective manner to vary the individual wheel speeds in an attempt to prevent the rollover is primarily through the activation of a vehicle's service brakes along with the decrease in engine power and the use of engine braking. Torque vectoring systems that are differential-based would not provide adequate braking power and would be less effective than ESC at slowing a vehicle down to allow it to maneuver a curve without rolling over. Likewise, brake-based torque vectoring systems would be less effective than ESC for braking in a curve. In brake-based systems, the inside wheels are braked during cornering in order to prevent any loss of traction, which could result because there is less weight on those wheel during cornering. ESC provides braking to both the inside and outside wheels of the vehicle resulting in better brake performance.

77 FR 30771.

III. Background

In the NPRM, we provided a detailed explanation of how rollovers occur, how stability control technologies such as roll stability control and electronic stability control function and reduce rollover, examples of situations in which stability control systems may not be effective, and the differences between stability enhancing technology on light vehicles and heavy vehicles.

This section is a summary of that information.

77 FR 30771-74.

A turning maneuver initiated by the driver's steering input results in a vehicle response that can be broken down into two phases. As the steering wheel is turned, the displacement of the front wheels generates a slip angle at the front wheels and a lateral force is generated. That lateral force leads to vehicle rotation, and the vehicle starts rotating about its center of gravity. Then, the vehicle's yaw causes the rear wheels to experience a slip angle. That causes a lateral force to be generated at the rear tires, which causes vehicle rotation. All of these actions establish a steady-state turn in which lateral acceleration and yaw rate are constant. In combination vehicles, which typically consist of a tractor towing a trailer, an additional phase is the turning response of the trailer, which is similar to, but slightly delayed, when compared to the turning response of the tractor.

If the lateral forces generated at either the front or the rear wheels exceed the friction limits between the road surface and the tires, the result will be a vehicle loss-of-control in the form of severe understeer (loss of traction at the steer tires) or severe oversteer (loss of traction at the rear tires). In a combination vehicle, a loss of traction at the trailer wheels would result in the trailer swinging out of its intended path. Conversely, rollover conditions occur on a vehicle when high lateral forces are generated at the tires from steering or sliding and result in a vehicle lateral acceleration that exceeds the rollover threshold of the vehicle.

High lateral acceleration is one of the primary causes of rollovers. Figure 1 depicts a simplified untripped rollover condition. As shown, when the lateral force (

i.e.,

lateral acceleration) is sufficiently large and exceeds the roll stability threshold of the tractor-trailer combination vehicle, the vehicle will roll over. Many factors related to the drivers' maneuvers, heavy vehicle loading conditions, vehicle handling characteristics, roadway design, and road surface properties would result in various lateral accelerations and influences on the rollover propensity of a vehicle. For example, given other factors are equal, a vehicle entering a curve at a higher speed has a higher lateral acceleration and, as a result, is more likely to roll than a vehicle entering the curve at a lower speed. Also, transporting a high-CG load would increase the rollover probability more than transporting a relatively lower CG load.

ER23JN15.008

Stability control technologies help a driver maintain directional control and help to reduce roll instability. Two types of heavy vehicle stability control technologies have been developed. One such technology is roll stability control or RSC. RSC systems are available for truck tractors and for trailers. A tractor-based RSC system consists of an electronic control unit (ECU) that is mounted on a vehicle and continually monitors the vehicle's speed and lateral acceleration based on an accelerometer, and estimates vehicle mass based on engine torque information.

The ECU continuously estimates the roll stability threshold of a vehicle, which is the lateral acceleration above which a combination vehicle will roll over. When the vehicle's lateral acceleration approaches the roll stability threshold, the RSC system intervenes. Depending on how quickly the vehicle is approaching the estimated rollover threshold, the RSC system intervenes by one or more of the following actions: Decreasing engine power, using engine braking, applying the tractor's drive-axle brakes, or applying the trailer's brakes. When RSC systems apply the trailer's brakes, they use a pulse modulation protocol to prevent wheel lockup because tractor stability control systems cannot currently detect whether or not the trailer is equipped with ABS.

RSC systems are not presently available for large buses.

An RSC system can reduce rollovers, but is not designed to help to maintain directional control of a truck tractor. Nevertheless, RSC systems may provide some additional ability to maintain directional control in some scenarios, such as in a low-center-of-gravity scenario, where an increase in a lateral acceleration may lead to yaw instability rather than roll instability.

In comparison, a trailer-based RSC system has an ECU mounted on the trailer, which typically monitors the trailer's wheel speeds, the trailer's suspension to estimate the trailer's loading condition, and the trailer's lateral acceleration. A trailer-based RSC system works similarly to a tractor-based system. However, a trailer-based RSC system can only apply the trailer brakes to slow a combination vehicle, whereas a tractor-based RSC system can apply brakes on both the tractor and trailer.

The other type of stability control systems available for truck tractors and large buses is an ESC system. An ESC system incorporates all of the inputs of an RSC system. However, it also has two additional sensors to monitor a vehicle for loss of directional control, which may result due to either understeer or oversteer. The first additional sensor is a steering wheel angle sensor, which senses the driver's steering input.

10 11

The other is a yaw rate sensor, which measures the actual turning movement of the vehicle. These system inputs are monitored by the system's ECU, which estimates when the vehicle's directional response begins to deviate from the driver's steering command, either by oversteer or understeer. An ESC system intervenes to restore directional control by taking one or more of the following actions: Decreasing engine power, using engine braking, selectively applying the brakes on the truck tractor to create a counter-yaw moment to turn the vehicle back to its steered direction, or applying the brakes on the trailer. An ESC system enhances the RSC functions because it has the added information from the steering wheel angle and yaw rate sensors, as well as more braking power because of its additional capability to apply the tractor's steer axle brakes.

Because ESC systems must monitor steering inputs from the tractor, ESC systems are not available for trailers.

Some RSC systems also use a steering wheel angle sensor, which allows the system to identify potential roll instability events earlier.

This is a design strategy to avoid the unintended consequences of applying the brakes on the steering axle without knowing where the driver is steering the vehicle.

Figure 2 illustrates the oversteering and understeering conditions. While Figure 2 may suggest that a particular vehicle loses control due to either oversteer or understeer, it is quite possible that a vehicle could require both understeering and oversteering interventions during progressive phases of a complex crash avoidance maneuver such as a double lane change.

ER23JN15.009

Understeering.

The left side of Figure 2 shows a truck tractor whose driver has lost directional control during an attempt to drive around a right curve. The ESC system momentarily applies the right rear brake, creating a clockwise rotational force, to turn the heading of the vehicle back to the correct path. It will also reduce engine power to gently slow the vehicle and, if necessary, apply additional brakes (while maintaining the uneven brake force to create the necessary yaw moment).

Oversteering.

The right side of Figure 2 shows that the truck tractor whose driver has lost directional control during an attempt to drive around a right curve. In a vehicle equipped with ESC, the system immediately detects that the vehicle's heading is changing more quickly than appropriate for the driver's intended path (

i.e.,

the yaw rate is too high). To counter the clockwise rotation of the vehicle, it momentarily applies the left front brake, thus creating a counter-clockwise counter-rotational force and turning the heading of the vehicle back to the correct path. It will also reduce engine power to gently slow the vehicle and, if necessary, apply additional brakes (while maintaining the uneven brake force to create the necessary yaw moment). The ESC activation can be so subtle that the driver does not perceive the need for steering corrections.

A stability control system will not prevent all rollover and loss-of-control crashes. A stability control system has the capability to prevent many untripped on-road rollovers and first-event loss-of-control events. Nevertheless, there are real-world situations in which stability control systems may not be as effective in avoiding a potential crash. Such situations include:

• Off-road maneuvers in which a vehicle departs the roadway and encounters a steep incline or an unpaved surface that significantly reduces the predictability of the vehicle's handling

• Entry speeds that are much too high for a curved roadway or entrance/exit ramp

• Cargo load shifts or liquid sloshing within the trailer during a steering maneuver

• Vehicle tripped by a curb or other roadside object or barrier

• Truck rollovers that are the result of collisions with other motor vehicles

• Inoperative antilock braking systems—the performance of stability control systems depends on the proper functioning of ABS

• Brakes that are out-of-adjustment or other defects or malfunctions in the ESC, RSC, or brake system.

• Maneuvers during tire tread separation or sudden tire deflation events.

On April 6, 2007, the agency published a final rule that established FMVSS No. 126,

Electronic Stability Control Systems,

which requires all passenger cars, multipurpose passenger vehicles, trucks and buses with a GVWR of 4,536 kg (10,000 lb.) or less to be equipped with an electronic stability control system beginning in model year 2012.

The system must be capable of applying brake torques individually at all four wheels, and must comply with the performance criteria established for stability and responsiveness when subjected to the sine with dwell steering maneuver test. For light vehicles, the focus of the FMVSS No. 126 is on addressing yaw instability, which can assist the driver in preventing the vehicle from leaving the roadway, thereby preventing fatalities and injuries associated with crashes involving tripped rollover, which often occur when light vehicles run off the road. The standard does not include any equipment or performance requirements for roll stability.

72 FR 17236.

The dynamics of light vehicles and heavy vehicles differ in many respects. First, on light vehicles, the yaw stability threshold is typically lower than the roll stability threshold. This means that a light vehicle making a crash avoidance maneuver, such as a lane change on a dry road, is more likely to reach its yaw stability threshold and lose directional control before it reaches its roll stability threshold and rolls over. On a heavy vehicle, however, the roll stability threshold is lower than the yaw stability threshold in most operating conditions, primarily because of its higher center-of-gravity height.

As a result, there is a greater propensity for a heavy vehicle, particularly in a loaded condition, to roll during a severe crash avoidance maneuver or when negotiating a curve, than to become yaw unstable, as compared with light vehicles.

One instance where a heavy vehicle's yaw stability threshold might be higher than its roll stability threshold is in an unloaded condition on a low-friction road surface.

Second, a tractor-trailer combination unit is comprised of a power unit and one or more trailing units with one or more articulation points. In contrast, although a light vehicle may occasionally tow a trailer, a light vehicle is usually a single rigid unit. The tractor and the trailer have different center-of-gravity heights and different lateral acceleration threshold limits for rollover. A combination vehicle rollover frequently begins with the trailer where the rollover is initiated by trailer wheel lift.

Third, due to greater length, mass, and mass moments of inertia of heavy vehicles, they respond more slowly to steering inputs than do light vehicles. The longer wheelbase of a heavy vehicle, compared with a light vehicle, results in a slower response time, which gives the stability control system the opportunity to intervene and prevent rollovers.

Finally, the larger number of wheels on a heavy vehicle, as compared to a light vehicle, makes heavy vehicles less

likely to become yaw unstable on dry road surface conditions.

IV. Safety Need

A. Heavy Vehicle Crash Problem

This section presents data on the safety problem associated with rollover and loss of control of heavy vehicles. The information has been updated from similar information contained in the NPRM. For the specific target population used to support the agency's system effectiveness and estimated benefits, see Section XIV.

The

Traffic Safety Facts 2012

reports that tractor trailer combination vehicles are involved in about 72 percent of the fatal crashes involving large trucks, annually.

According to FMCSA's

Large Truck and Bus Crash Facts 2011,

these vehicles had a fatal crash involvement rate of 1.46 crashes per 100 million vehicle miles traveled during 2011, whereas single-unit trucks had a fatal crash involvement rate of 1.00 crashes per 100 million vehicle miles traveled.

Combination vehicles represent about 24 percent of large trucks registered but travel 61 percent of the large truck miles, annually. Traffic tie-ups resulting from loss-of-control and rollover crashes also contribute to in millions of dollars of lost productivity and excess energy consumption each year.

DOT HS 812 032,

available at http://www-nrd.nhtsa.dot.gov/Pubs/812032.pdf.

FMCSA-RRA-13-049 (Oct. 2013),

available at http://www.fmcsa.dot.gov/sites/fmcsa.dot.gov/files/docs/LargeTruckandBusCrashFacts2011.pdf.

According to

Traffic Safety Facts 2012,

the overall crash problem for tractor trailer combination vehicles in that year was approximately 180,000 crashes, 42,000 of which involve injury. The overall crash problem for single-unit trucks is nearly as large—in 2012, there were approximately 154,000 crashes, 35,000 of which were injury crashes. However, the fatal crash involvement for truck tractors is much higher. In 2011, there were 2,736 fatal combination truck crashes and 1,066 fatal single-unit truck crashes.

The rollover crash problem for combination trucks is much greater than for single-unit trucks. In 2011, there were approximately 8,000 crashes involving combination truck rollover and 5,000 crashes involving single-unit truck rollover. As a percentage of all crashes, combination trucks are involved in rollover crashes at a higher rate compared to single-unit trucks. Approximately 4.6 percent of all combination truck crashes were rollovers, but 3.2 percent of single-unit truck crashes were rollovers. Combination trucks were involved in 3,000 injury crashes and 373 fatal crashes, and single-unit trucks were involved in 3,000 injury crashes and 194 fatal crashes.

According to FMCSA's

Large Truck and Bus Crash Facts 2011,

cross-country intercity buses were involved in 39 of the 242 fatal bus crashes in 2011. The bus types presented in the crash data include school buses, cross-country intercity buses, transit buses, van-based buses, and other buses. From 2002 to 2011, cross-country intercity buses, on average, accounted for approximately 12 percent of all buses involved in fatal crashes, whereas transit buses and school buses accounted for 34 percent and 40 percent, respectively, of all buses involved in fatal crashes. However, most of the transit bus and school bus crashes are not rollover or loss-of-control crashes that ESC systems are capable of preventing. Fatal rollover and loss-of-control crashes are a subset of these crashes.

There are many more fatalities in buses with a GVWR greater than 11,793 kg (26,000 lb.) compared to buses with a GVWR between 4,536 kg and 11,793 kg (10,000 lb. and 26,000 lb.).

In the 10-year period between 2000 and 2009, there were 42 fatalities on buses with a GVWR between 4,536 kg and 11,793 kg (10,000 lb. and 26,000 lb.) compared to 209 fatalities on buses with a GVWR greater than 11,793 kg (26,000 lb.). Among buses with a GVWR of greater than 11,793 kg (26,000 lb.), over 70 percent of the fatalities were cross-country intercity bus occupants, “other buses,” and “unknown buses.”

Thus, although these buses are only involved in 12 percent of fatal crashes involving buses, they represent the majority of fatalities from bus crashes.

This data was taken from the FARS database and was presented in the final rule requiring that seat belts be installed on certain buses. See 78 FR 70415, 70423-26 (Nov. 25, 2013).

The FARS database has five bus body type categories: (1) Cross-country/intercity bus, (2) transit bus, (3) school bus, (4) other bus, and (5) unknown bus. Transit bus and school bus body types were excluded from the analysis because they are easily recognized and categorized as such by crash investigators and those coding the FARS data. Thus, those vehicles are unlikely to be miscoded as other buses.

Furthermore, the size of the rollover crash problem for cross-country intercity buses is greater than in other buses. According to FARS data from 2000 to 2009, there were 114 occupant fatalities as a result of rollover events on cross-country intercity buses, “other buses,” and “unknown buses” with a GVWR of greater than 11,793 kg (26,000 lb.), which represents 55 percent of bus fatalities on those bus types.

B. Contributing Factors in Rollover and Loss-of-Control Crashes

Many factors related to heavy vehicle operation, as well as factors related to roadway design and road surface properties, can cause heavy vehicles to become yaw unstable or to roll. Listed below are several real-world situations in which stability control systems may prevent or lessen the severity of such crashes.

• Speed too high to negotiate a curve—The entry speed of vehicle is too high to safely negotiate a curve. When the lateral acceleration of a vehicle during a steering maneuver exceeds the vehicle's roll or yaw stability threshold, a rollover or loss of control is initiated. Curves can present both roll and yaw instability issues to these types of vehicles due to varying heights of loads (low versus high, empty versus full) and road surface friction levels (

e.g.,

wet, dry, icy, snowy).

• Road design configuration—Some drivers may misjudge the curvature of ramps and not brake sufficiently to negotiate the curve safely. This includes driving on ramps with decreasing radius curves as well as operating on curves and ramps with improper signage. A vehicle traveling on a curve with a decrease in super-elevation (banking) at the end of a ramp where it merges with the roadway causes an increase in vehicle lateral acceleration, which may increase even more if the driver accelerates the vehicle in preparation to merge.

• Sudden steering maneuvers to avoid a crash—The driver makes an abrupt steering maneuver, such as a single- or double-lane-change maneuver, or attempts to perform an off-road recovery maneuver, generating a lateral acceleration that is sufficiently high to cause roll or yaw instability. Maneuvering a vehicle on off-road, unpaved surfaces such as grass or gravel may require a larger steering input (larger wheel slip angle) to achieve a given vehicle response, and this can lead to a large increase in lateral acceleration once the vehicle returns to the paved surface. This increase in lateral acceleration can cause the vehicle to exceed its roll or yaw stability threshold.

• Loading conditions—A loss of yaw stability due to severe over-steering is more likely to occur when a vehicle is in a lightly loaded condition and has a lower center-of-gravity height than it would have when fully loaded. Heavy vehicle rollovers are much more likely to occur when the vehicle is in a fully loaded condition, which results in a high center of gravity for the vehicle.

Cargo placed off-center in the trailer may result in the vehicle being less stable in one direction than in the other. It is also possible that improperly secured cargo can shift while the vehicle is negotiating a curve, thereby reducing roll or yaw stability. Sloshing can occur in tankers transporting liquid bulk cargoes, which is of particular concern when the tank is partially full because the vehicle may experience significantly reduced roll stability during certain maneuvers.

• Road surface conditions—The road surface condition can also play a role in the loss of control a vehicle experiences. On a dry, high-friction asphalt or concrete surface, a tractor trailer combination vehicle executing a severe turning maneuver is likely to experience a high lateral acceleration, which may lead to roll or yaw instability. However, a similar maneuver performed on a wet or slippery road surface is not as likely to experience the high lateral acceleration because of less available tire traction. Hence, the vehicle is more likely to be yaw unstable than roll unstable.

C. NTSB Safety Recommendations

The National Transportation Safety Board (NTSB) has issued several safety recommendations relevant to ESC systems on heavy and other vehicles. One is H-08-15, which addresses ESC systems and collision warning systems with active braking on commercial vehicles. Recommendations H-11-07 and H-11-08 specifically address stability control systems on commercial motor vehicles and buses with a GVWR above 10,000 pounds. Two other safety recommendations, H-01-06 and H-01-07, relate to adaptive cruise control and collision warning systems on commercial vehicles and are indirectly related to ESC on heavy vehicles because these technologies require the ability to apply brakes without driver input.

• H-08-15: Determine whether equipping commercial vehicles with collision warning systems with active braking

and electronic stability control systems will reduce commercial vehicle accidents. If these technologies are determined to be effective in reducing accidents, require their use on commercial vehicles.

Active braking involves using the vehicle's brakes to maintain a certain, preset distance between vehicles.

• H-11-07: Develop stability control system performance standards for all commercial motor vehicles and buses with a gross vehicle weight rating greater than 10,000 pounds, regardless of whether the vehicles are equipped with a hydraulic or pneumatic brake system.

• H-11-08: Once the performance standards from Safety Recommendation H-11-07 have been developed, require the installation of stability control systems on all newly manufactured commercial vehicles with a GVWR greater than 10,000 pounds.

D. Motorcoach Safety Plan

In November 2009, the U.S. Department of Transportation Motorcoach Safety Action Plan was issued.

Among other things, the Motorcoach Safety Action Plan includes an action item for NHTSA to assess the safety benefits for stability control on large buses and develop objective performance standards for these systems.

Consistent with that plan, NHTSA made a decision to pursue a stability control requirement for large buses.

See

supra,

note 6.

Id.

at 28-29.

In March 2011, NHTSA issued its latest Vehicle Safety and Fuel Economy Rulemaking and Research Priority Plan (Priority Plan).

The Priority Plan describes the agency plans for rulemaking and research for calendar years 2011 to 2013. The Priority Plan includes stability control on truck tractors and large buses, and states that the agency plans to develop test procedures for a Federal motor vehicle safety standard on stability control for truck tractors, with the countermeasures of roll stability control and electronic stability control, which are aimed at addressing rollover and loss-of-control crashes.

See Docket No. NHTSA-2009-0108-0032.

E. International Regulation

The United Nations (UN) Economic Commission for Europe (ECE) Regulation 13, Uniform Provisions Concerning the Approval of Vehicles of Categories M, N and O with Regard to

Braking,

has been amended to include Annex 21,

Special Requirements for Vehicles Equipped with a Vehicle Stability Function.

Annex 21's requirements apply to trucks with a GVWR greater than 3,500 kg (7,716 lb.), buses with a seating capacity of 10 or more (including the driver), and trailers with a GVWR greater than 3,500 kg (7,716 lb.). Trucks and buses are required to be equipped with a stability system that includes rollover control and directional control, while trailers are required to have a stability system that includes only rollover control. The directional control function must be demonstrated in one of eight tests, and the rollover control function must be demonstrated in one of two tests. For compliance purposes, the ECE regulation requires a road test to be performed with the function enabled and disabled, or as an alternative, accepts results from a computer simulation. No test procedure or pass/fail criterion is included in the regulation, but it is left to the discretion of the Type Approval Testing Authority in agreement with the vehicle manufacturer to show that the system is functional. The implementation date of Annex 21 was 2012 for most vehicles, with a phase-in based on the vehicle type.

V. Summary of the May 2012 NPRM

Since 2006, the agency has been involved in testing truck tractors and large buses with stability control systems. To evaluate these systems, NHTSA sponsored studies of crash data in order to examine the potential safety benefits of stability control systems. NHTSA and industry representatives separately evaluated data on dynamic test maneuvers. At the same time, the agency launched a three-phase testing program to improve its understanding of how stability control systems in truck tractors and buses work and to develop dynamic test maneuvers to challenge roll propensity and yaw stability. By combining the studies of the crash data with the testing data, the agency is able to evaluate the potential effectiveness of stability control systems for truck tractors and large buses.

The agency conducted a three-phase testing program for truck tractors and large buses that was described at length in the NPRM and in published reports in order to develop one or more test maneuvers to ensure that ESC systems can reduce vehicle instability. As a result of the agency's testing program and the test data received from industry, the agency was able to develop reliable and repeatable test maneuvers that could demonstrate a stability control system's ability to prevent rollover and loss of directional control among the varied configurations of truck tractors and buses in the fleet.

After considering and evaluating several test maneuvers, the agency proposed using two test maneuvers for performance testing: The slowly increasing steer (SIS) maneuver and the sine with dwell (SWD) maneuver. The SIS maneuver is a characterization maneuver used to determine the amount of steering input required by the SWD maneuver. By determining the relationship between a vehicle's steering wheel angle and the lateral acceleration,

the SIS maneuver normalizes the severity of the SWD maneuver. The SIS maneuver was also proposed to be used to ensure that the system has the ability to reduce engine torque.

Using a steering wheel angle derived from the SIS maneuver, the agency proposed conducting the sine with dwell maneuver. The SWD test maneuver challenges both roll and yaw stability by subjecting the vehicle to a sinusoidal input. This maneuver would be repeated for two series of test runs (first in the counterclockwise direction and then in the clockwise direction) at several target steering wheel angles from 30 to 130 percent of the angle derived in the SIS maneuver.

We proposed measuring, recording, and processing lateral acceleration, yaw rate, and engine torque data derived from the SIS and SWD maneuvers to determine four performance metrics: Lateral acceleration ratio (LAR), yaw rate ratio (YRR), lateral displacement, and engine torque reduction. The LAR and YRR metrics ensure that the system reduces lateral acceleration and yaw rate, respectively, after an aggressive steering input, thereby preventing rollover and loss of control, respectively. The lateral displacement metric ensures that the stability control system is not set to intervene solely by making the vehicle nonresponsive to driver input. The engine torque reduction metric ensures that the system has the capability to automatically reduce engine torque in response to high lateral acceleration and yaw rate conditions.

The agency also considered several test maneuvers based on its own work and that of industry. In particular, the agency's research included both a J-turn maneuver and a ramp steer maneuver (RSM) for evaluating roll stability. The J-turn maneuver is a path-following maneuver where a vehicle is driven on a test course consisting of a straight lane followed by a fixed radius curve. The steering wheel angle is determined by the driver making adjustments and corrections to maintain the fixed path. In the RSM maneuver, a vehicle is driven at a constant speed and a steering wheel input that is based on the steering wheel angle derived from the SIS maneuver. The steering wheel angle is then held for a period of time before it is returned to zero. In both the J-turn and RSM maneuvers, a stability control system acts to reduce lateral acceleration, and thereby wheel lift and roll instability, by applying selective braking. A vehicle without a stability control system being tested with these maneuvers would exhibit high levels of lateral acceleration and potentially experience wheel lift or rollover.

The NPRM also set forth the test conditions that the agency would use to ensure safety and demonstrate sufficient performance. All vehicles were proposed to be tested using outriggers for the safety of the test driver. The agency proposed using an automated steering controller for the RSM, SIS, and SWD maneuvers to ensure reproducible and repeatable test execution performance. The agency proposed testing truck tractors with an unbraked control trailer to eliminate the effect of the trailer's brakes on testing. The agency also proposed a test to ensure that system malfunction is detected.

The NPRM proposed that a final rule would take effect for most truck tractors and applicable buses produced two years after publication of a final rule. We stated that two years of lead time would be necessary to ensure sufficient availability of stability control systems from suppliers of these systems and to complete necessary engineering on all vehicles. For three-axle tractors with one drive axle, tractors with four or more axles, and severe service tractors, we proposed allowing two years of additional lead time. We stated this additional time would be necessary to develop, test, and equip these vehicles with ESC systems. Although the agency has statutory authority to require retrofitting of in-service truck tractors, trailers, and large buses, the agency did not propose to require retrofitting, but sought comment on its feasibility, given the integrated aspects of a stability control system.

VI. Overview of the Comments

This section presents a brief overview of the comments received in response to the NPRM. The comments are addressed in detail in the section related to the subject of the comment. However, those comments that merely advocated the adoption or rejection of the proposal or some aspect thereof without any underlying explanation are not addressed further.

We also conducted a public hearing on July 24, 2012 in Washington, D.C.

Summaries of the oral testimony and a transcript of the hearing are both available in the docket.

Although we have considered the public hearing testimony as if it was a written comment received in the docket, much of the testimony was duplicated in the written comments. We have discussed public hearing testimony below only where that testimony was not reflected in written comments received by the agency.

Notice of the hearing was published in the

Federal Register

on July 2, 2012. 77 FR 39206.

Summaries of the oral testimony provided by the presenters are contained in Docket No. NHTSA-2012-0065-0049. A transcript of the public hearing is contained in Docket No. NHTSA-2012-0065-0056.

In addition to the comments received at the public hearing, we received written comments from 43 individuals or entities. The commenters represented wide-ranging interests, including individuals, truck drivers, truck fleet operators, vehicle component manufacturers, truck and bus manufacturers, and safety advocacy organizations. The identity of the 46 commenters, their self-identified interest or affiliation, if given, where the comments can be located in the docket are cited in Table 2.

Three commenters presented comments only at the public hearing.

Table 2—List of Commenters and Location of Comments in the Docket

Commenter

Docket Number

Vehicle Manufacturers:

Blue Bird Body Company (Blue Bird)

NHTSA-2012-0065-0034

Daimler Trucks North America LLC (Daimler)

NHTSA-2012-0065-0028

EvoBus GmbH

NHTSA-2012-0065-0027

Fire Apparatus Manufacturer's Association

NHTSA-2012-0065-0014

Navistar, Inc.

NHTSA-2012-0065-0039

Schneider National Inc. (Schneider)

NHTSA-2012-0065-0033

Temsa Global (Temsa)

NHTSA-2012-0065-0019

Truck & Engine Manufacturers Association (EMA)

NHTSA-2012-0065-0044

Volvo Group

NHTSA-2012-0065-0031

Component Manufacturers:

Bendix Commercial Vehicle Systems

NHTSA-2012-0065-0046

NHTSA-2012-0065-0048

NHTSA-2012-0065-0055

Heavy Duty Brake Manufacturers Council (HDBMC)

NHTSA-2012-0065-0041

Meritor WABCO

NHTSA-2012-0065-0035

Robert Bosch LLC (Bosch)

NHTSA-2012-0065-0036

Drivers and Fleet Operators:

American Trucking Associations, Inc. (ATA), including report of the American Transportation Research Institute (ATRI)

NHTSA-2012-0065-0016

NHTSA-2012-0065-0030

NHTSA-2012-0065-0057

Associated Logging Contractors—Idaho

NHTSA-2012-0065-0042

John Boyle

NHTSA-2012-0065-0017

Jim Burg, James Burg Trucking Company

NHTSA-2012-0065-0056 (public hearing)

John H. Hill, The Hill Group

NHTSA-2012-0065-0056 (public hearing)

Alexander J. MacDonald

NHTSA-2012-0065-0005

National Ready Mixed Concrete Association

NHTSA-2012-0065-0038

National School Transportation Association

NHTSA-2012-0065-0037

Owner-Operator Independent Drivers Association (OOIDA)

NHTSA-2012-0065-0024

Skagit Transportation Inc

NHTSA-2012-0065-0006

Bob Waterman

NHTSA-2012-0065-0052

Safety Organizations:

AAA Public Affairs (AAA)

NHTSA-2012-0065-0043

Advocates for Highway and Auto Safety (Advocates)

NHTSA-2012-0065-0047

American Highway Users Alliance

NHTSA-2012-0065-0040

Commercial Vehicle Safety Alliance (CVSA)

NHTSA-2012-0065-0050

Consumers Union

NHTSA-2012-0065-0053

Insurance Institute for Highway Safety (IIHS)

NHTSA-2012-0065-0021

Kentucky Injury Prevention and Research Center

NHTSA-2012-0065-0007

National Association for Pupil Transport (NAPT)

NHTSA-2012-0065-0023

National Transportation Safety Board (NTSB)

NHTSA-2012-0065-0015

Road Safe America

NHTSA-2012-0065-0004

Other Organizations and Private Individuals:

American Association for Justice (AAJ)

NHTSA-2012-0065-0020

American Trauma Society

NHTSA-2012-0065-0009

Justin C. Barriault

NHTSA-2012-0065-0010

Robert M. Chin

NHTSA-2012-0065-0011

Jerry R. Curry

NHTSA-2012-0065-0018

Jerry J. Evans

NHTSA-2012-0065-0003

Fried Rogers Goldberg, LLC

NHTSA-2012-0065-0025

Nadya V. Gerber

NHTSA-2012-0065-0012

The Martec Group, Inc. (Martec)

NHTSA-2012-0065-0051

Mercatus Center at George Mason University (Mercatus)

NHTSA-2012-0065-0022

Josh A. Sullivan

NHTSA-2012-0065-0013

Hon. Betty Sutton

NHTSA-2012-0065-0056

(public hearing)

VII. Key Differences Between the Final Rule and the NRPM

This section summarizes the significant differences between the NPRM and this final rule. Less significant changes are noted in the appropriate sections of the preamble.

The most significant change between the NPRM and the final rule is that the agency has chosen an alternative performance test maneuver to demonstrate an ESC system's ability to maintain vehicle stability. After considering public comments and conducting additional track testing, we have adopted a 150-foot J-turn maneuver as the performance test maneuver in this final rule. In the NPRM, we proposed using a slowly increasing steer (SIS) maneuver as a characterization maneuver and a sine with dwell (SWD) maneuver as a roll and yaw performance maneuver. The 150-foot J-turn test maneuver is discussed in the NPRM and is a variation of an alternative test maneuver proposed in the NPRM.

Because the 150-foot J-turn test maneuver only tests an ESC system's ability to mitigate roll instability and the agency lacks any alternative test maneuver to test an ESC system's ability to mitigate yaw instability, this final rule does not include a performance test to evaluate yaw instability. However, this final rule carries forward the requirement that an ESC system be capable of mitigating yaw instability.

The 150-foot J-turn maneuver also uses a different performance metric than the SWD maneuver. The SWD maneuver's performance criteria were the change in lateral acceleration and yaw rate through the maneuver. In this final rule, we are using a simpler metric—reduction in forward speed.

The change in performance test maneuver has also led to changes in the test conditions and equipment. Because the test maneuver in this final rule is conducted over a fixed path, rather than fixed steering used for the SWD maneuver, an automated steering wheel controller will not be used for the J-turn maneuver. We have also modified the

loading condition for vehicles to test them at GVWR. We have also reduced the instrumentation requirements in light of the simpler performance metric.

VIII. ESC Requirement

A. Whether to Require Stability Control

In the May 2012 NPRM, the agency proposed to require that all truck tractors and certain buses with a GVWR of more than 11,793 kg (26,000 lb.) to be equipped with ESC. The agency preliminarily found that the proposed standard met the need for motor vehicle safety.

That finding was based upon the safety problem discussed in the NPRM and summarized in section IV above.

Moreover, the agency found that requiring ESC systems on truck tractors and certain large buses would be cost-effective.

77 FR 30788.

77 FR 30769-71.

77 FR 30791.

We received many comments addressing the general question of whether stability control systems should be required on truck tractors and large buses. Several commenters questioned the need for a stability control mandate on truck tractors and certain large buses and recommended against adopting a final rule requiring any type of stability control system. A consistent theme in many of the comments received from private individuals was also expressed in the comment from Yankee Trucks. These commenters argued that the decision to include ESC should be decided by the vehicle's end user.

Other commenters such as Mercatus and OOIDA were concerned that NHTSA failed to look at alternative methods to improve motor vehicle safety problems caused by rollover and loss-of-control crashes. Mercatus suggested that NHTSA failed to look at driver fatigue detection, road condition sensors, improved safety procedures, or driver training, which might be less costly. OOIDA highlighted driver training, enforcement of traffic laws, driver incentives, improved crashworthiness, and road signage as alternative ways to deal with the rollover problem. Several other commenters highlighted driver training and accountability related to both driving and vehicle loading as alternative methods that could prevent rollover and loss-of-control crashes. The Boyle Brothers, OOIDA, and several individual commenters both noted that stability control systems would not prevent crashes caused by driving too fast for conditions. Both Mercatus and OOIDA believe that alternative measures are less costly than a stability control mandate at preventing rollover and loss-of-control crashes.

Individual commenters, many of whom identified themselves as truck drivers, also questioned the safety of stability control systems and their ability to prevent crashes. One commenter believes that stability control systems are unsafe based on personal experience because it often engaged the service brakes in curves. Another commenter was concerned that drivers would become too dependent on stability control systems and cause them to drive through curves faster with the system than without.

OOIDA and many individual commenters were concerned about the total cost of the rule and whether the benefits justified the costs. Relatedly, several commenters raised concerns that stability control systems would add complexity to the brake system by requiring additional parts, and thus, higher repair costs. Yankee Trucks also raised concerns that if a stability control system malfunctions, ABS would also not function. OOIDA claimed that a stability control requirement would cause drivers and truck companies to keep existing vehicles in service longer or even go out of business due to the added costs of stability control and other regulatory mandates.

Some commenters also expressed concerns that stability control technologies could have negative effects on safety. For example, individual commenters questioned whether it was safe to have stability control systems braking the vehicle automatically in wet conditions or on curves. Associated Logging Contractors opposed a mandate because it believes that a stability control requirement may cause safety issues on forest roads, which are different from highways.

Commenters from a wide variety of backgrounds supported a stability control mandate. These organizations include organizations such as Road Safe America, the Kentucky Injury Prevention and Research Center, the American Trauma Society, the American Association for Justice, Advocates, the American Highway Users Alliance, AAA, the Commercial Vehicle Safety Alliance, and Consumers Union. Business associations representing brake suppliers (HDBMC), truck manufacturers (EMA), and truck fleet operators (ATA) all supported a stability control mandate. Brake suppliers such as Bosch, Bendix, and Meritor WABCO also supported a stability control mandate. Individual truck and bus manufacturers who commented also such as Daimler, Volvo, and Navistar supported a stability control mandate. Some motor carriers who commented also supported a stability control mandate. The NTSB and a former Member of Congress, Betty Sutton, both supported a stability control mandate. Many individual commenters also supported a stability control mandate.

Although these commenters come from varied backgrounds, their reasons for supporting a stability control mandate were generally consistent. Commenters supporting a mandate generally cited research from NHTSA, the manufacturing industry, and others regarding the effectiveness of stability control systems, and their ability to prevent rollover and loss-of-control crashes and save lives. IIHS, for example, cited its own research suggesting that having ESC systems on all truck tractors could prevent as many as 295 fatal crashes each year. Some individual commenters also cited personal experience with stability control systems. John Hill observed that the cost of a stability control system on a vehicle is comparable to the cost to the government of a single compliance review of a motor carrier's safety practices. These commenters generally agreed that the benefits of a stability control mandate far exceed its costs.

After considering all public comments, the agency is proceeding with adopting FMVSS No. 136 to require all truck tractors and certain large buses with a GVWR of more than 11,793 kg (26,000 lb.) to have stability control systems. This decision is largely driven by the data before the agency. In developing the proposal, the agency analyzed crash data to identify risks not addressed in existing FMVSSs. These safety risks include rollover and loss-of-control crashes that are caused by many factors including traveling at a speed too high to negotiate a curve, sudden steering maneuvers to avoid a crash, loading conditions, road surface conditions, and road design configuration. The agency's research, described at length in the NPRM, shows that stability control technologies could prevent crashes in these situations.

With respect to the comments suggesting that vehicles braking during a curve or on wet conditions could have adverse safety consequences, we observe that an ESC system is designed to slow the vehicle in a curve in order to reduce the lateral acceleration and allow the operator to maintain roll and yaw control of the vehicle only in situations where instability is imminent. After careful qualitative and quantitative assessment, we have concluded that requiring stability

control systems will improve the overall safety of the vehicle.

Regarding other possible improvements to reduce crashes, we do not disagree that many of the suggestions regarding driver training, enforcements, and crashworthiness of trucks and buses could improve motor vehicle safety and (except for the latter) reduce vehicle rollover and loss-of-control crashes. However, driver training and enforcement of traffic safety laws are outside of NHTSA's regulatory authority under the Safety Act. Moreover, the commenters advocating these alterative means to address the safety problem did not provide data to support their conclusions that their alternatives would be less costly or more cost-effective than a stability control mandate. Although the issues related to costs and benefits will be addressed more specifically in section XIV below, the agency has concluded that requiring ESC systems on truck tractors and certain large buses is cost-effective and the most effective means to address the safety problem identified in this rulemaking.

B. Whether to Require ESC or RSC

The agency proposed to require that truck tractors and large buses be equipped with ESC systems rather than RSC systems. An ESC system is capable of all of the functions of an RSC system. In addition, an ESC system has the additional ability to detect yaw instability, provide braking at front wheels, and detect the steering wheel angle. These additions, as demonstrated by NHTSA's testing, allow an ESC system to have better rollover prevention performance than an RSC system in addition to the yaw instability prevention component. This is because the steering wheel angle sensor allows the ESC system to anticipate changes in lateral acceleration based upon driver input and to intervene with engine torque reduction or selective braking sooner, rather than waiting for the lateral acceleration sensors to detect potential instability.

The NPRM stated that mandating ESC systems rather than RSC systems will prevent more crashes, injuries, and fatalities. The additional benefits from ESC systems can be attributed to both the ESC's system's ability to intervene sooner and its ability to prevent yaw instability that would lead to loss-of-control crashes.

The NPRM stated that mandating ESC systems rather than RSC systems will result in higher initial costs to manufacturers. Moreover, while our benefit and cost estimates led to the preliminary conclusion that mandating RSC systems would be more cost-effective than mandating ESC systems, mandating ESC systems would result in higher net benefits.

Several commenters agreed with NHTSA's proposal to require ESC systems rather than RSC systems. Jerry Curry and Bendix specifically mentioned that ESC systems should be required instead of RSC systems. Mr. Curry and IIHS also commented that RSC systems would not be the best platform to use when considering future technological advances. John Hill similarly observed that ESC systems have the potential to support future collision avoidance and crash mitigation technologies. Mr. Hill also observed that loss-of-control crashes can be difficult to identify and classify. Road Safe America, Mr. MacDonald, and AAA said the agency should require ESC equipment on truck tractors and buses. IIHS and Jim Burg recommended requiring ESC systems over RSC systems because loss-of-control collisions can be reduced using ESC systems. Volvo, while not expressly advocating for an ESC mandate, stated that it had investigated the use of RSC systems, but found they were unable to provide stability control in a wide range of driving conditions and environments that its customers operate.

In its comment, Bendix stated that an ESC system has an effectiveness that is 31% greater than a RSC system. Bendix also commented that ESC systems provide “more information about what the vehicle is doing” because these systems include two additional sensors. Bendix also said that ESC systems provide more effective interventions through selective application of all available vehicle brakes.

Other commenters supported RSC as a minimum requirement rather than ESC. Schneider, for example, asserted that it considered purchasing vehicles with ESC system, but determined that ESC systems would provide a negligible benefit at substantially higher costs when compared to RSC. ATA also asserted that marginal benefit of ESC over RSC is not justified by the added cost based on current information. ATA cited the variability of the truck-tractor industry in four areas: (1) Private trucking vs. for-hire companies; (2) the size of loads; (3) the type of truck and trailer being used (

e.g.,

box, van, refrigerated, liquid and bulk tankers); and by operation (

e.g.,

agricultural, long haul, short haul, over size, overweight, etc.). ATA believes this diversity may warrant choosing ESC or RSC depending on the individual vehicle.

Both Schneider and ATA cited a study by the American Transportation Research Institute (ATRI) that surveyed stability control technology used in the trucking industry. This study collected crash and financial data from the trucking industry, including information regarding whether the vehicle was equipped with an ESC system, an RSC system, or no stability control system at all. The sample included 135,712 trucks, of which 68,647 had RSC systems, 39,529 had ESC systems, and 27,536 had no stability control systems. The study included unit costs of stability systems, average annual miles per tractor, the total number of safety incidents (including rollover crashes), and the average cost of each incident. The crash analysis concluded that industry-wide installation of RSC systems would result in fewer rollover, jackknife, and tow/stuck crashes compared to industry-wide installation of ESC systems.

NHTSA agrees with those commenters recommending ESC systems instead of RSC systems. However, we are not relying on the assertions of Mr. Curry, Mr. Hill and IIHS that ESC systems provide a better platform for future technological advances. We believe the justification for ESC systems is satisfied using benefits estimates for today's ESC systems, without having to consider possible future advances such as forward collision mitigation systems. Similarly, we are not relying on Bendix's assessment of ESC system effectiveness. While Bendix's analysis of the effectiveness of ESC and RSC systems is addressed in more detail in section XIV below, we believe that our own analysis based on an effectiveness study conducted by University of Michigan Transportation Research Institute (UMTRI) and Meritor WABCO is a more accurate assessment of the effectiveness of ESC and RSC systems. Although both NHTSA and Bendix reached the conclusion that ESC systems will be more effective than RSC systems at preventing rollover crashes, we believe that Bendix's method of determining system effectiveness is arbitrarily biased in favor of ESC systems.

Regarding ATA's assertion of the variability of trucks, we agree that truck tractors are varied and that some of those variations affect vehicle stability. However, we believe that variability justifies choosing to require ESC systems rather than RSC systems. In particular, ATA observed that trucks carry various loads, implying that certain kinds of loads may be more suited to ESC systems whereas other

loads may only require RSC systems to achieve equal effectiveness. However, the nature of the trucking industry is such that a truck tractor may end up towing many different types of trailers in its lifetime, including flatbed trailers, box trailers, and tanker trailers. A vehicle manufacturer is unlikely to know at the time of a vehicle's production whether a specific truck tractor is going to be carrying loads that are more likely to cause a rollover or loss-of-control crash because the load has a high center of gravity or has the potential to slosh. The only way to ensure that the vehicles that ATA believes would perform better with ESC systems is to require all truck tractors to be equipped with ESC systems.

The ATRI study will be addressed more specifically in the benefits and costs discussion in section XIV below and in the FRIA accompanying this final rule. However, for the purpose of determining whether to require ESC systems or RSC systems, the ATRI study's suggestion that RSC systems would be more beneficial than ESC systems reflects the specific truck carriers they studied, but does not necessarily constitute a representative sample of the truck fleet. ATRI's conclusion is contrary to NHTSA's own findings that ESC systems are more effective and have greater net benefits than RSC systems. First, as explained above, ESC systems contain all of the functions of RSC systems, plus have additional sensors such as a steering wheel angle sensor, to allow a system to intervene based on a predicted rise in lateral acceleration rather than waiting for the lateral acceleration to rise. Second, ESC systems have the capability to braking all of the vehicle's axles, whereas an RSC system is generally unable to brake the steering axle of the vehicle. Third, although NHTSA's own research found that one RSC system performed as well or slightly better than an ESC system under certain conditions, we attributed the performance difference to that particular RSC system being programmed to brake more aggressively than the ESC system on the same vehicle.

For these reasons, we conclude that the ATRI study is not representative of the entire trucking industry or the performance of ESC systems compared to RSC systems.

77 FR 30779.

Based on the foregoing, this final rule will require that truck tractors and certain buses be equipped with ESC systems rather than RSC systems. As discussed in section XIV below, RSC systems are less beneficial than ESC systems in reducing rollover crashes and much less beneficial in addressing loss-of-control crashes. Although RSC systems are slightly more cost beneficial than ESC systems, ESC systems provide substantially higher net benefits because ESC systems will prevent many more crashes.

NHTSA has concluded that the additional safety benefits of ESC systems in both rollover and loss-of-control crashes justify the additional cost of ESC systems compared to RSC systems.

Cost-effectiveness is measured in terms of lower cost per equivalent life saved. For more discussion of the costs and benefits of this rule see Section XIV, below, and the Final Regulatory Impact Analysis accompanying this final rule, which has been placed in the docket.

C. Definition of ESC

The NPRM included definitional criteria in the proposed regulatory text. We reasoned that, relying solely on performance-based tests without mandating any specific equipment may require a battery of tests to cover the complete operating range of the vehicle. Given the wide array of possible configurations and operating ranges for heavy vehicles, the agency did not believe it was practical to develop performance tests that address the full range of possibilities and remain cost-effective. Accordingly, the agency proposed to include definitional criteria in the NPRM, which included equipment that would be required as part of a compliant ESC system.

We note that, when developing the ESC requirement for light vehicles, the agency chose to include such a requirement in FMVSS No. 126.

Similar requirements exist in the light vehicle ESC requirements. See 49 CFR 571.126, S4.

SAE International has a Recommended Practice on

Brake Systems Definitions-Truck and Bus,

J2627 (Aug. 2009), which includes a definition of Electronic Stability Control and Roll Stability Control. SAE International's definition of an ESC system requires that a system have an electronic control unit that considers wheel speed, yaw rate, lateral acceleration, and steering angle and that the system must intervene and control engine torque and auxiliary brake systems to correct the vehicle's path.

The UN ECE Regulation 13 definition for the electronic stability control system, promulgated in Annex 21, includes the following functional attributes for directional control: Sensing yaw rate, lateral acceleration, wheel speeds, braking input and steering input; and the ability to control engine power output. For vehicles with rollover control, the functions required by the stability control include: Sensing lateral acceleration and wheel speeds; and the ability to control engine power output.

In developing a definition for ESC, the agency reviewed the functional attributes contained in SAE J2627 and the requirements of Annex 21 of UN ECE Regulation 13, and incorporated parts of both of definitions the NPRM. The proposed definition was similar in wording to the definition from FMVSS No. 126, which specifies certain features that must be present, that ESC be capable of applying all the brakes individually on the vehicle, and that it have a computer using a closed-loop algorithm to limit vehicle oversteer and understeer when appropriate. Unlike the light vehicle standard, which focuses on yaw stability, the NPRM proposed to require a stability control system that also helps to mitigate roll instability conditions.

Furthermore, the proposed definition required that the ESC system must be operational during all phases of driving, including acceleration, coasting, deceleration, and braking, except when the vehicle is below a low-speed threshold where loss of control or rollover is unlikely. According to information the agency obtained from vehicle manufacturers and ESC system suppliers, the low speed threshold for a stability control system is 10 km/h (6.2 mph) for yaw stability control and 20 km/h (12.4 mph) for roll stability control. For the purposes of the NPRM, the agency set a single threshold of 20 km/h (12.4 mph) as the speed below which ESC is not required to be operational.

The benefit of an ESC system is that it will reduce vehicle rollovers and loss of control under a wide variety of vehicle operational and environmental conditions. However, the performance tests in the NPRM would only evaluate ESC system performance under very specific conditions. To ensure that a vehicle is equipped with an ESC system that met the proposed definition, we proposed that vehicle manufacturers make available to the agency documentation that would enable NHTSA to ascertain that the system includes the components and performs the functions of an ESC system.

Meritor WABCO, HDBMC, and Bendix recommended a change to the definition of an ESC system. Where the definition required that the system both augment vehicle directional stability and enhance rollover stability by applying and adjusting brake torques, the commenters recommended that the words “having the capability of” be added to each instance. Bendix also recommended that each instance of

“brake torque” should be changed to “deceleration torque.”

We agree with the commenters' recommendation to change the requirement that ESC systems augment vehicle directional stability and enhance rollover stability by “applying and adjusting vehicle brake torques” to “having the capability of applying and adjusting vehicle brake torques.” The wording in the NPRM could be construed to require brake torques to be applied simultaneously at each wheel position for correcting yaw moment or reduce lateral acceleration. This was not our intention. Rather, we intended to require that brake torque at each wheel position be capable of being applied and adjusted individually. In analogous portions of the ESC system definition, we use the words “has a means,” which is similar in meaning to “capable.”

However, we are not making Bendix's suggested change of the term “brake torque” to “deceleration torque.” We are not sure that Bendix's suggested language would be functionally different than the proposal and cannot see how it adds clarity. We are specifically interested in requiring that systems be capable of controlling the brakes independently at each wheel end on at least one front and at least one rear axle of the vehicle.

Bendix also recommended a change to the requirement that the system enhance vehicle directional stability by applying and adjusting the vehicle brake torques. Bendix requested that NHTSA clarify that the “vehicle” referred to in this requirement is the truck tractor or bus and not the trailer. That is, Bendix wanted to ensure that the trailer is omitted from the vehicle directional stability requirements. Bendix noted that the requirements regarding the system's ability to control trailer brakes is addressed elsewhere.

We agree with Bendix's recommendation. It was not our intention to include trailers in the requirement that vehicles be capable of maintaining directional stability. Bendix is correct that there could to be some confusion with the proposed requirement because a trailer is also a motor vehicle and consequently, the proposed requirement that vehicles have the capability to maintain directional stability and the roll stability may be misinterpreted to apply to a trailer. Therefore, we have revised the ESC definition to specify that truck tractors and buses must have the means to apply and adjust vehicle brake torques on at least one front and at least one rear axle.

Regarding the definitional criteria for mass estimation, Meritor WABCO, HDBMC, and Bendix suggested an addition to the requirement that a system have a means to estimate the vehicle (or combination vehicle) mass. The commenters request that NHTSA include language allowing a system to automatically obtain the vehicle's mass.

NHTSA is not making the suggested change. The suggested change would require a system to have a means to estimate or automatically obtain vehicle mass. We do not believe there is a manner in which to automatically obtain the vehicle's mass short of weighing it on a scale. Any other calculation of the vehicle's mass is an estimate. We note that the means for obtaining the vehicle's mass is not prescribed. The requirement is necessary to ensure that the ESC system is capable of using the vehicle mass data in the closed-loop algorithm of its computer to apply and adjust the vehicle brake torques for enhancing rollover stability and inducing correcting yaw moment. Adding “automatically obtain” to the definition does not improve or clarify the requirement to have a means of estimating vehicle mass.

In summary, NHTSA continues to believe that the definitional criteria, including required equipment and system capabilities, are necessary to ensure that ESC systems perform as they are intended and as they currently perform. These criteria are objective in terms of explaining to manufacturers what type of performance is required and the minimal equipment necessary for that purpose.

D. Technical Documentation

The NPRM proposed requiring that the vehicle manufacturer provide a system diagram that identifies all ESC system hardware; a written explanation, with logic diagrams included, describing the ESC system's basic operational characteristics; and a discussion of the pertinent inputs to the computer and how its algorithm uses that information to prevent rollover and limit oversteer and understeer. Because the proposed definition for ESC systems on truck tractors included the capability to provide brake pressure to a towed vehicle, the agency proposed requiring that, as part of the system documentation, the manufacturer include the information that shows how the tractor provides brake pressure to a towed trailer under the appropriate conditions.

Volvo questioned the need for manufacturers to submit technical documentation to NHTSA, stating that NHTSA has relied on the manufacturer's certification that the system meets the FMVSSs. HDBMC and Bendix requested confirmation that this technical documentation would be considered proprietary information and would not be released to the public. Finally, Bendix was concerned about the acceptance criteria for the evaluation of the submitted technical documentation. Bendix stated that there was no objective acceptance criteria in the proposed standard and recommended that the agency add acceptance criteria.

Upon consideration of the comments, we have decided to remove from the regulatory text references to specific documentation that NHTSA would request from manufacturers. However, NHTSA's Office of Vehicle of Safety Compliance often requests, as part of its testing to verify compliance with the FMVSSs, certain information from manufacturers. For example, NHTSA may ask how a manufacturer's system meets the definition of an “ESC System” set forth in this final rule. Information such as the technical documentation that was listed in the regulatory text of the NPRM may be included in or responsive to such a request. Of course, a manufacturer's inability to demonstrate that its system meets the definition of an “ESC System” could lead to a finding of noncompliance with S5.1 of FMVSS No. 136.

IX. Vehicle Applicability and Phase-In

A. Trucks

1. Summary of the NPRM

Vehicles with a GVWR greater than 10,000 pounds include a large variety of vehicles ranging from medium duty pickup trucks to different types of single-unit trucks, buses, trailers and truck tractors. Vehicles with a GVWR of greater than 10,000 pounds are divided into Classes 3 through 8. Class 7 vehicles are those with a GVWR greater than 11,793 kilograms (26,000 pounds) and up to 14,969 kilograms (33,000 pounds), and Class 8 vehicles are those with a GVWR greater than 14,969 kilograms (33,000 pounds).

About 85 percent of truck tractors sold annually in the U.S. are air-braked three-axle (6×4) tractors with a front axle that has a GAWR of 14,600 pounds or less and with two rear drive axles that have a combined GAWR of 45,000 pounds or less, which we will refer to as “typical 6×4 tractors.” Other truck tractors, including two-axle (4×2) tractors, tractors with four or more axles, and severe service tractors, represent about 15 percent of the truck-tractor market in the U.S.

In the NPRM, the agency proposed that truck tractors with a GVWR greater

than 11,793 kilograms (26,000 pounds) would be required to have ESC systems. The agency did not propose requiring stability control systems on trailers, primarily because trailer-based RSC systems were determined by the agency research to be much less effective than tractor-based RSC or ESC systems in preventing rollover. Trailer-based RSC systems are capable of applying braking only on the trailer's brakes. Tractor-based systems can command more braking authority by using both the tractor and trailer brakes. As a result, trailer-based RSC systems do not appear to provide additional safety benefits when used in combination with tractor-based RSC or ESC systems. In addition, the typical service life of a trailer is 20 to 25 years compared with about 8 to 10 years for a truck tractor. Because new tractors are added to the U.S. fleet at a faster rate than new trailers, the safety benefits from stability control systems would be achieved at a faster rate by requiring stability control systems to be installed on a tractor.

Our proposed rule also excluded certain types of low-volume, highly specialized vehicle types. In these cases, the vehicle's speed capability does not allow it to operate at speeds where roll or yaw instability is likely to occur. These exclusions were drawn from FMVSS No. 121,

Air brake systems,

which exclude any vehicle equipped with an axle that has a gross axle weight rating of 29,000 pounds or more; any truck or bus that has a speed attainable in two miles of not more than 33 mph; and any truck that has a speed attainable in two miles of not more than 45 mph, an unloaded vehicle weight that is not less than 95 percent of its GVWR, and no capacity to carry occupants other than the driver and operating crew.

2. Exclusions From ESC Requirement

The Fire Apparatus Manufacturers' Association (FAMA) was generally supportive of the rule. However, they stated that the rule would not be feasible if it is interpreted to apply to a Tractor Drawn Aerial Apparatus. As FAMA explained, this apparatus is a combination vehicle used for firefighting, which are used in many large urban fire departments. The distinguishing feature of this vehicle is that it has two drivers, one in the truck tractor and one in the trailer. FAMA believes that an ESC algorithm on such a vehicle would be very complex because it would need to consider two steering wheels rather than one. FAMA suggested that NHTSA exclude from a final rule any combination vehicle that requires more than one operator to steer it.

The agency is not adding the exclusion suggested by FAMA. Although FAMA stated that its vehicles would not be subject to the exclusion of vehicles with an axle having a gross axle weight rating of 29,000 pounds or more, it is not clear that this or other exclusions do not apply. Moreover, absent specific information that more fully explains why an exclusion is necessary and not overly broad, NHTSA cannot agree that an exclusion for all combination vehicles that require more than one operator to steer it is necessary.

Furthermore, the scope of the exclusion suggested by FAMA is not consistent with the scope of the rule. Specifically, this final rule, like the NPRM, applies to truck tractors, not trailers. However, the suggested exclusion would apply to combination vehicles, which include both a truck tractor and a trailer. That is, the presence of a trailer would form the basis for the exclusion. If this exclusion was added to the final rule, then the basis for the exclusion would be dependent on the trailer that is attached to the vehicle. This would be confusing and unnecessarily complicate enforcement.

Finally, FAMA has not articulated why its vehicles cannot be equipped with ESC systems. Because the ESC requirement applies only to the truck tractor, the system would only need to take account of one steering wheel input. There would be no requirement that the vehicle respond to any inputs from the trailer. Moreover, NHTSA would conduct compliance testing of the truck tractor using the control trailer specified in the test procedure, not a trailer with a steering wheel.

Several commenters suggested that the agency reduce the scope of the ESC requirement. EMA requested that NHTSA exclude all severe duty trucks from the scope of a final rule. It reasoned that manufacturers offer multiple configurations of truck tractors with different wheelbases, axle, and suspension combinations. Furthermore, it claimed that manufacturers often build only a few vehicles in each configuration and in some cases of severe duty trucks, may only build a single vehicle in a particular configuration.

The agency is not excluding severe duty trucks as EMA suggests. Currently, manufacturers are able to produce products in small volumes that meet all the requirements of the Federal Motor Vehicle Safety Standards (FMVSS). The addition of the ESC rule will not unduly burden the manufacturers with regard to their small volume products. EMA's actions related to this rulemaking support this conclusion. For example, EMA provided test data to the agency after performing multiple test maneuvers with severe duty trucks equipped with ESC systems. EMA also included the test results from the severe duty trucks to form its recommended test criteria for an alternate roll stability test.

Meritor WABCO requested NHTSA to add the words “pneumatically braked” to the definitions of truck tractors and buses in the ESC rule. Similarly, EMA recommended that NHTSA include the ESC requirements within FMVSS No. 121 rather than in a separate standard.

We are not expressly limiting the scope of the final rule to air braked vehicles. Although Class 8 vehicles typically use pneumatic or air brakes, Class 7 vehicles vary between either air or hydraulic brakes. The scope of the NPRM includes all truck tractors and Class 7 and 8 buses, which showed the greatest rollover problem of all the buses according to our research. In order to address the safety problem with these classes of buses, the ESC rule must include both air and hydraulic brakes. Limiting the scope of this rulemaking to air braked vehicles could provide an incentive for some manufacturers to equip vehicles with hydraulic brakes rather than air brakes to circumvent an ESC system requirement.

3. Single-Unit Trucks

The agency did not propose to include single-unit trucks with a GVWR over 4,536 kg (10,000 pounds). Several commenters recommended expanding the scope of the rule to include straight trucks. Skagit, NTSB, IIHS, and NAPT all suggested that ESC should be mandated on all commercial vehicles greater than 10,000 pounds GVWR, including straight trucks. Advocates recommended that NHTSA should consider the FMCSA study stating the number of fatalities by single-unit trucks, based on data from 2008, are 1,147 each year. Bosch stated that the rule should be expanded to cover all vehicles over 10,000 pounds GVWR vehicles, including hydraulic-braked vehicles, because this segment accounts for a large number of commercial and load bearing vehicles on the U.S. roads. Bosch claims that a mandate with a phase-in period is needed to facilitate industry development of ESC systems on these vehicles. On the other hand, Bendix recommended that “[t]he decision by the agency regarding if and when to consider rulemaking on single-unit trucks should be based on the same

level of research undertaken for tractor and coach.”

We are not expanding the scope of this rulemaking to include single-unit trucks. We believe that a level of research closer to what we had to support the NPRM for truck tractors and large buses is necessary before NHTSA would propose to mandate ESC on all single-unit trucks. After publishing the NPRM, we began a research and testing program to study the safety benefits and performance criteria of ESC systems on single-unit trucks. The research is not yet complete. Furthermore, as we stated in the NPRM, the complexity of the single-unit truck population and the limited crash data available present a significant challenge to determining the effectiveness of stability control on these vehicles. At this time, we will not include single-unit trucks in the ESC rule. However, we believe including buses with hydraulic brakes in this final rule will spur development of ESC systems for other hydraulic-braked vehicles, including trucks with a GVWR of greater than 4,536 kilograms (10,000 pounds) but not more than 11,793 kilograms (26,000 pounds).

4. Compliance Dates

The agency proposed that all new typical 6×4 truck tractors would be required to meet the proposed standard beginning two years after a final rule is published. Because there are currently only two suppliers of truck tractor and large bus stability control systems, Bendix and Meritor WABCO, we reasoned that the industry would require lead time to ensure that the necessary production stability control systems are available to manufacturers. NHTSA also proposed a two-year lead time for two-axle tractors.

For severe service tractors and tractors with four axles or more, which represent about 5 percent of annual truck tractor sales, the agency believed additional lead time was necessary to develop, test, and equip these vehicles with a stability control system. Therefore, we proposed to require that severe service tractors and other atypical tractors be equipped with ESC systems beginning four years after the final rule is published.

Four commenters addressed the compliance dates for trucks proposed by the NPRM. Daimler requested an additional lead time for ESC implementation because it said that it only has RSC systems developed on some models and needs more time to design and validate ESC on all of its models.

In its comment, EMA mentioned that this ESC rule should align with the implementation dates of the new FMVSS No. 121 stopping distance requirements to give manufacturers the opportunity to refine the braking systems prior to the implementation of this ESC rule. EMA said it is impractical for manufacturers to certify compliance tests using the tests in the NPRM for all typical 6×4 tractors within 2 years of the final rule. Moreover, EMA said that tractors with four or more axles and severe service tractors have not been evaluated using the tests in the NPRM and likely would need additional lead time. However, EMA did not specify how much additional lead time was necessary. Finally, EMA and Bendix recommended including two-axle tractors in the longer lead time period because it appears to be an error.

In contrast, HDBMC stated its belief that the suppliers of ESC systems are prepared to meet the anticipated deployment demands by the implementation dates proposed.

We recognize the recent changes to the stopping distance requirements in FMVSS No. 121 affected truck tractors. Truck tractors, other than three-axle truck tractors, were recently subjected to the reduced stopping distance changes that went into effect on August 1, 2013. Manufacturers of these truck tractors were given two additional years beyond the timeframe for three-axle truck tractors to comply with the amendments to FMVSS No. 121. We agree with Daimler and EMA that at least four years of lead time is warranted for all truck tractors other than typical 6×4 tractors (three-axle truck tractors with a front axle that has a GAWR of 6,622 kg (14,600 pounds) or less and with two rear drive axles that have a combined GAWR of 20,412 kg (45,000 pounds) or less). Although HDMA said that its member companies are ready to supply brake components by the implementation dates proposed, we realize that truck tractor manufacturers need extra time to integrate the ESC systems into their products and to perform the necessary testing to ensure compliance. In addition, manufacturers recently made brake system changes to these models of truck tractors in order to comply with the new requirements in the FMVSS No. 121 amendments. We recognize that ESC systems must be integrated into the brake systems, and we expect that manufacturers may need to modify the brake systems for a second time.

B. Buses

1. Summary of the NPRM

The NPRM proposed that certain buses would be required to be equipped with ESC systems. The applicability of the proposal to buses mirrored the applicability of the agency's proposal that certain large buses be equipped with seat belts.

The proposal for seat belts was applicable to buses with a gross vehicle weight rating (GVWR) of 11,793 kilograms (26,000 pounds) or greater, 16 or more designated seating positions (including the driver), and at least 2 rows of passenger seats that are rearward of the driver's seating position and are forward-facing or can convert to forward-facing without the use of tools.” That proposal excluded school buses and urban transit buses sold for operation in urban transportation along a fixed route with frequent stops. The agency proposed a very similar applicability in the NPRM for this rulemaking.

We believed that the proposal encompassed the category of “cross-country intercity buses” represented in the FARS and FMCSA data (identified in section II.A above) that had a higher involvement of crashes that ESC systems are capable of preventing.

75 FR 50958 (Aug. 18, 2010).

The primary difference is that the ESC proposal was not made applicable to buses with a GVWR of exactly 11,793 kilograms (26,000 pounds) in order to exclude Class 6 vehicles from the proposal.

2. Buses Built on Truck Chassis

(a) Summary of NPRM

The agency tested three air-braked buses, all of which had a GVWR over 14,969 kg (33,000 lb.) (Class 8). Nevertheless, the agency included Class 7 buses (buses with a GVWR of more than 11,793 kg (26,000 lb.) but not greater than 14,969 kg (33,000 lb.). We reasoned that, although many Class 7 buses are built on chassis similar to those of single-unit trucks for which ESC has not been widely developed, and we are not aware of any Class 7 bus that is equipped or currently available with ESC. Class 7 buses represent less than 20 percent of the market. Although the agency was not aware of any Class 7 bus currently available with ESC, we were aware that stability control systems are available on a limited number of Class 8 single-unit trucks, such as concrete trucks, refuse trucks, and other air-braked trucks, and that the same technology could be developed for use on Class 7 buses, which we believed were also air-braked vehicles. We also believed that the manufacturers of Class 7 buses would need additional lead time to have the ESC systems developed, tested and installed on their vehicles. Hence, for large buses, the agency proposed an effective date of two years after the final rule is published,

primarily to accommodate manufacturers of Class 7 buses.

However, we sought comment on the feasibility of including Class 7 buses that are built on chassis similar to those of single-unit trucks within two years. We noted that, although we believed that Class 7 buses were primarily air braked and that ESC systems were readily available for air-braked buses, system availability for any hydraulic-braked buses that may be covered may be more limited. We requested that, if hydraulic-braked buses were covered by the proposal, commenters address manners in which hydraulic-braked buses may be differentiated for exclusion or a different phase-in period.

(b) Summary of Comments

Several commenters raised issues related to the NPRM's definition for large buses. EMA and Navistar commented that the “large bus” definition should not include commercial buses, which are buses greater than 11,793 kg (26,000 lb.), but are not traditional intercity buses. They claimed that many of these buses are built on truck chassis and are different than the Class 8 buses tested by NHTSA. They stated that these buses are built in multiple stages by multiple manufacturers, which would make compliance certification difficult.

According to Navistar, NHTSA did not “reach out” to Navistar regarding its commercial buses because it claimed NHTSA was not aware of its Class 8 commercial buses from the sole fact that they were not specifically mentioned in list of bus manufacturers included in the NPRM.

In its comments, EMA opined that non-motorcoach buses with a GVWR over 11,793 kg (26,000 lb.) are more closely related to single-unit trucks. It also commented that some of the same issues related to requiring ESC systems on single-unit trucks are also present for large buses.

EMA stated that consistent with the Motorcoach Enhanced Safety Act (part of MAP-21), it considered the term “motorcoach” to have the same meaning as “over-the-road-bus,” which “means a bus characterized by an elevated passenger deck located over a baggage compartment.”

EMA and Daimler also commented that a “motorcoach” has some, if not all, of the following attributes: a GVWR greater than 33,000 pounds (Class 8); air disc brakes; passenger deck floor more than 45 inches above the ground; rear engine configuration; monocoque

construction; 40 or more passenger seats; no provisions for standee passengers; and one passenger entrance and exit door. EMA asserted that NHTSA did not study ESC on other non-motorcoach buses, and therefore, the rule should not apply to those buses.

The rulemaking requirements of the Motorcoach Enhanced Safety Act are addressed in section II above.

Monocoque means a type of vehicular construction in which the body is combined with the chassis as a single unit.

NHTSA is not changing the general applicability of the ESC requirement to buses. As we stated in the NPRM, we intended the applicability of the ESC requirement to buses to be similar to the applicability of the agency's requirement that buses have seat belts at each passenger seating position. In both rulemakings, the target vehicles were high occupancy buses associated with a known fatality and injury risk. The buses typically carried a large number of passengers and were operated at highway speeds. We examined the involvement of high occupancy buses in fatal crashes over a 10-year period (FARS data files, for the NPRM, 1999-2008). In this examination of high occupancy bus data, we inspected crash data for buses with a GVWR greater than 4,536 kg (10,000 lb.). We analyzed the construction type and various attributes of the vehicles. The 2000-2009 FARS data show that for buses over 4,536 kg (10,000 lb.), there were 49 passenger fatalities in buses with a GVWR less than 11,793 kg (26,000 lb.), but there were 209 in buses with a GVWR greater than 11,793 kg (26,000 lb.).

Moreover, MAP-21, which was enacted after publication of the NPRM, requires the Secretary to consider requiring ESC systems on certain large buses if the Secretary determines that such a requirement is consistent with the requirements of the Motor Vehicle Safety Act. We believe that mandating ESC systems on the buses covered by the NPRM, subject to some minor changes discussed below, is consistent with those requirements. That is, this standard is practicable, meets the need for motor vehicle safety, and may be stated in objective terms. We believe that ESC systems are currently available for must buses covered by this final rule and can be developed for the others. Moreover, the safety problem discussed in Section IV.D above highlights the rollover problem in buses with a GVWR greater than 11,793 kg (26,000 lb.).

NHTSA has decided to adopt the proposal to require all buses with a GVWR over 11,793 kg (26,000 lb.), subject to some modified exclusions for school buses, transit buses, and perimeter seating buses. In Section V.B.1 of the NPRM, NHTSA mentioned the rationale for not including a requirement for ESC on single-unit trucks with a GVWR over 4,536 kilograms (10,000 pounds) at this time.

The rationale was primarily based on the differences between truck tractors and single-unit trucks; it was not intended and did not mention the differences between buses built on truck chassis and buses built with monocoque construction. Although the NPRM stated that single-unit trucks as a whole are more complex and diverse than truck tractors, this does not necessarily apply to buses built on truck chassis. Among the different bodies that could be assembled on a truck chassis, a bus body presents a degree of complexity and diversity that is substantially less than the other truck bodies. For example, a bus body presents a scenario where center-of-gravity height and cargo type are more easily calculated because the bus is limited to transporting people and their luggage rather than varied cargo. The chassis supplier for a bus would be more likely to have knowledge of critical vehicle design parameters that affect ESC calibration.

77 FR 30789.

NHTSA reviewed various definitions used in motorcoach safety legislation including the “over-the-road bus” definition in TEA-21 that was referenced in MAP-21. Similar to the final rule requiring seat belts on certain buses, we are not limiting the applicability of the ESC requirement to TEA-21's definition of over-the-road buses.

We believe that the definitions referring to over-the-road buses or over-the-road bus service are too narrow, because a number of intercity transport buses involved in fatal crashes were body-on-chassis buses that lacked an elevated passenger deck over a baggage compartment. Further, definitions based on the intended use of the vehicle could pose difficulties for manufacturers and dealers, because the intended use of a vehicle might not be known at the time of vehicle manufacture or sale. We want to make sure as reasonably possible that the buses we most wanted to affect (high-capacity buses associated with known fatality and injury risks) would meet the “motorcoach” safety standards, without having to depend on the state of knowledge of persons in the manufacturing and distribution chain about the prospective use of the bus.

78 FR 70429.

Currently, there is no common Departmental or industry definition of “motorcoach.” FMCSA does not have a

definition for motorcoach in its regulations, but it considers a “motorcoach” to be an over-the-road bus. As noted above, over-the-road buses are a subset of the buses NHTSA believes should be regulated as “motorcoaches,” encompassing a part of but not enough of the heavy bus safety problem we seek to address.

We reviewed the underlying chassis structure of high-occupancy vehicles involved in fatal crashes. Some had a monocoque structure with a luggage compartment under the elevated passenger deck (“over-the-road buses”). However, an elevated passenger deck over a baggage compartment was not an element common to the buses involved in fatal intercity transport. In FARS data for buses with a GVWR greater than 11,793 kg (26,000 lb.), 36 percent of the fatalities were in the other bus and unknown bus categories,

i.e.,

not in the over-the-road bus category. Some buses were built using body-on-chassis configurations.

We believe that body-on-chassis configurations are newer entrants into the motorcoach services market. They appear to be increasing in number. A cursory review of the types of buses being used in the Washington, DC area for motorcoach services showed that traditional motorcoaches are generally used for fixed-route services between major metropolitan areas. However, for charter, tour, and commuter transportation from outlying areas, many bus types are used. Some are of monocoque structure, while others are of body-on-chassis structure.

The agency tested Class 8 buses, those with a GVWR greater than 14,969 kg (33,000 lb.), because these buses have larger dimensions and masses than Class 7 buses, and it places them on the most severe end of the spectrum. The performance criteria were created based on the testing of the larger Class 8 buses, and the agency has made a reasoned determination that the criteria are applicable for Class 7 buses, as well. If a Class 8 bus with a larger GVWR can pass the minimum performance criteria for ESC systems, a Class 7 bus with a smaller GVWR can reasonably be required to meet the same criteria.

Despite the fact that some of these buses are built in multiple stages by multiple manufacturers, the agency does not agree that compliance with the ESC standard will be very difficult. Presently, manufacturers building buses in various stages must provide an incomplete vehicle document (49 CFR part 568) to subsequent manufacturers listing each standard that applies. One example of a standard that must be documented is FMVSS No. 121, Air Brake Systems. A number of factors such as GVWR, GAWR, and any other specific conditions given by the manufacturer must be considered when determining if a bus will be compliant with the braking requirements after it is built. Likewise, the agency expects manufacturers to give similar conditions of final manufacture under which the manufacturer specifies that the completed vehicle will conform to the ESC standard. The agency considers that burden of bus manufacturers to comply with the ESC rule will not be more difficult than the current burden of complying with the air brake requirements in FMVSS No. 121.

3. Hydraulic-Braked Buses

In the NPRM, we requested comment on manners in which hydraulic-braked buses may be differentiated, such as by exclusion or a different phase-in period for the ESC rule. Six commenters provided statements about hydraulic-braked buses and how they should be excluded. Specifically, Blue Bird opposes an ESC mandate on hydraulic-braked buses with a GVWR of 36,200 pounds and less. It also commented that the agency should wait until ESC systems are developed and fully evaluated for hydraulic-braked medium or heavy buses and not include hydraulic-braked buses as part of the ESC rule at this time. Blue Bird, Daimler, Meritor WABCO, Navistar, and EMA all commented that they are not aware of any ESC systems available for hydraulic-braked buses covered by the NPRM. Meritor WABCO recommended that NHTSA exclude vehicles that are not “pneumatically braked.” Finally, both Daimler and EMA stated that they want the ESC regulation to extend only to motorcoaches over 33,000 pounds.

NHTSA has no convincing evidence to exclude hydraulic-braked buses from this ESC rule. The NPRM proposed to require ESC on both Class 7 and Class 8 buses. The mandate in the Motorcoach Enhanced Safety Act makes no differentiation between Class 7 and Class 8 buses. In order to address the rollover and loss-of-control safety problems with these classes of buses, the ESC rule must include both air and hydraulic brakes.

Based on feedback received from the commenters, we recognize that Class 7 buses are composed of both air- and hydraulic-braked vehicles. We recognize that manufacturers who produce large buses equipped with hydraulic-powered brakes might need extra time to ensure the proper integration between the ESC system and the vehicle's chassis, engine, and braking system. Rather than exclude hydraulic-braked buses from the rule entirely, NHTSA will extend the compliance date for buses that may be equipped with hydraulic brakes. NHTSA acknowledges that ESC systems are still in development for large buses with hydraulic-braked buses, and therefore, manufacturers and suppliers need additional time to implement this new technology. However, whether the bus is equipped with air brakes or hydraulic brakes, we expect the performance requirements to apply because they are based on the stability of the bus as defined by its attributes such as geometry, mass, inertia, and center-of-gravity height. There is a negligible change in these attributes between an air-braked and a hydraulic-braked bus.

4. School Buses

Six commenters recommended that NHTSA include a requirement that school buses be equipped with ESC systems in the final rule. Consumers Union commented that ESC technology should be required for school buses in order to set a precedent for future crash avoidance technologies. Martec recommended that ESC be required on all buses because it claims that “large school buses satisfy multiple criteria described by NHTSA in its 2011-2013 Rulemaking and Research Priority Plan: the addition of ESC/RSC to school buses would offer large safety benefits, would apply to high-occupancy vehicles, and would apply to a vulnerable population—children.” Skagit, NTSB, and IIHS all want ESC to be mandated on all buses greater than 10,000 lb., including school buses.

Conversely, Daimler and NSTA both agreed that NHTSA not include school buses in a final rule mandating ESC systems on large buses. NSTA asserted that, if school buses were subject to an ESC mandate, the costs to purchase school buses would increase. NSTA is concerned that the added costs would reduce the number of school buses on the road, and, consequently, reduce the number of children riding buses to school. NTSA claims that students riding school buses are eight times safer than riding in the family vehicle because school buses travel at lower speeds and largely in residential areas.

As in the NPRM, we are excluding school buses from the ESC requirement. Each NHTSA rulemaking must address a present safety need and be justified by present safety benefits. We cannot accept Consumers Union's recommendation to do rulemaking now based on speculative benefits of ESC systems on school buses. According to FARS data between 2000 and 2009, among the large buses, more than 70%

of fatalities on large buses with a GVWR greater than 11,793 kg (26,000 lb.) were related to cross-country intercity bus crashes. Similarly, we stated in the NPRM that FMCSA's Large Truck and Bus Crash Facts 2008 indicates that most of the school bus crashes are not rollover or loss-of-control crashes that ESC systems are capable of preventing. For these reasons, we will not require school buses to be equipped with ESC at this time.

Navistar, EMA, and Daimler requested that the school bus exclusion extend into its line of school bus derivatives. Navistar and EMA reasoned that some commercial buses are built on truck chassis. Because of their similarities to school buses, they reasoned that those buses should be exempted from the ESC rule. According to Daimler, school bus derivatives are vehicles built with hydraulic brakes, and no ESC system is available on these types of hydraulic brakes in the market today.

We disagree with Daimler, EMA, and Navistar that the school bus exception should extend to other buses that are similar or “derivatives” as Daimler stated. If the commenters' reasoning was adopted, any manufacturer could offer a school bus version of a particular bus model and claim that the school bus exception should apply because of the artificially created similarities. This would create an unintended loophole for the ESC requirement and potentially undermine the rule.

5. Transit Buses

The NPRM proposed to exclude from the ESC system requirements urban transit buses sold for operation in urban transportation along a fixed route with frequent stops. EMA and Volvo suggested that we exclude certain buses based on the intended use of the vehicle in public transit. Volvo requested that the agency base the exclusion on the Federal Transit Administration's (FTA) bus procurement guidelines. Volvo suggested excluding “urban transit buses which may be used on suburban express service and general service on urban arterial streets along a fixed route with frequent stops.” Similarly, EMA suggested adding to the exclusion for transit buses “urban transit buses used in suburban express service.” Conversely, Volvo stated during the public hearing that it was practical and technologically feasible to equip its urban buses with ESC, but it did not want to do so because it did not perceive a safety need.

The Motorcoach Enhanced Safety Act excludes from its mandate to consider requiring ESC systems on large buses a bus used in public transportation provided by, or on behalf of, a public transportation agency. However, as we explained in the previous section regarding school buses, an exclusion based on the intended use of the vehicle could pose difficulties for manufacturers and dealers, because the intended use of a vehicle might not be known at the time of vehicle manufacture or sale. Consequently, we will not adopt the recommendation suggested by EMA and Volvo to exclude urban transit buses used in suburban express service.

The final rule requiring seat belts at all passenger seating position on certain buses noted that commenters on that NPRM were troubled that the proposed transit bus exclusion was not sufficiently clear. To make the definition more clear, the final rule made clarifications that we believe are also warranted in this final rule requiring ESC systems on certain buses.

First, we made the regulatory text clearer in describing a “transit bus” by referring to a structural feature (a stop-request system) that buses must have to be a “transit bus.” A “stop-request system” means a vehicle-integrated system for passenger use to signal to a vehicle operator that a stop is requested. Second, we expanded the description of a transit bus by recognizing that a transit bus could be sold for public transportation provided not only by, but also on behalf of, a State or local government, for example, by a contractor.

78 FR 70438.

Finally, we made clear that over-the-road buses, as defined by TEA-21, do not qualify as “transit buses,” even if the over-the-road bus has a stop-request system or is sold for public transportation provided by or on behalf of a State or local government. This final clarification ensures both that a manufacturer cannot integrate a simple stop-request system on any bus and make it subject to the transit bus exclusion. We recognize that any over-the-road bus used for public transportation provided by or on behalf of a State or local government is likely to be used as a commuter express bus that would carry large numbers of passengers over long distances at highway speeds. However, this use case is similar to the use of over-the-road buses by private companies in intercity service.

6. Minimum Seating Capacity and Seating Configuration

The NPRM also excluded buses that had fewer than 16 designated seating positions (DSPs), including the driver. This reference was included in the seat belt NPRM based on FMCSA's definition of a “commercial motor vehicle,” for purposes of FMCSA's commercial driver's license requirements.

In the final rule, however, NHTSA noted that FMCSA's regulations state that buses with a GVWR greater than 11,793 kg (26,000 lb.) are commercial vehicles under the commercial driver's license regulations, regardless of the number of DSPs. Accordingly, that exclusion was removed from the final rule.

75 FR 50969.

78 FR 70433.

EMA and Daimler suggested that the rule exclude all buses with fewer than 40 passenger seats, which they imply would exclude buses that are not considered “motorcoaches.” However, neither EMA nor Daimler included any explanation for why 40 passenger seats is an appropriate cutoff for an ESC system requirement, and we can perceive none. We do not believe that a minimum number of passenger seats would serve to include or exclude buses that are being driven at long distances or at highway speeds.

The NPRM also proposed to exclude buses with fewer than two rows of passenger seats that are rearward of the driver's seating position and are forward-facing or can convert to forward-facing without the use of tools. This reference was included in the large bus seat belt NPRM to distinguish buses with perimeter seating such as those used to transport passengers in airports between the terminal and locations such as a rental car facility or long term parking.

These buses typically have a single forward-facing row of seats in the back of the vehicle and seats along one or both sides of the bus. These buses typically carry people for a relatively short period, often transport standees, generally accommodate baggage and other items, and are designed for rapid boarding and alighting. These buses were excluded because we believed they would be used for relatively short distances on set routes, which are not widely exposed to general traffic.

78 FR 70434.

In the seat belt final rule, the agency simplified the exclusion by defining these vehicles as perimeter seating buses and excluding them from the seat belt requirement rather than specifying the number of rows and seats that a bus has. Second, we referred to the maximum number of forward-facing DSPs that the vehicle may have rather than the number of “rows” it may have. We made this change because there is no definition of “row” generally

applicable to the FMVSSs and it was difficult to define “row” for the purpose of excluding perimeter-seating buses using plain language. Thus, we defined a “perimeter-seating bus” as a bus with 7 or fewer DSPs rearward of the driver's seating position that are forward-facing or can convert to forward-facing without the use of tools, and excluded perimeter-seating buses from the seat belt requirement.

See 78 FR 70434-35.

We believe that this exclusion is similarly applicable to the ESC system requirement, and we are adopting in this final rule the simplified language used in the seat belt final rule. A perimeter-seating bus typically carries people for short distances on set routes and is often less exposed to general traffic than transit buses. However, consistent with the Motorcoach Enhanced Safety Act, we are not excluding from the ESC system requirement perimeter-seating buses that are also over-the-road buses. Some of these buses may include vehicles often referred to as “limo buses” or “party buses.” These vehicles may also be used as touring or entertainment buses with eating and sleeping accommodations that are used by celebrities and entertainers on tour. We expect that these types of buses will be used for intercity travel and driven at highway speeds.

7. Compliance Dates

The NPRM proposed that buses meet the ESC system requirements two years after publication of a final rule implementing the proposal. Although we did not receive any comments specifically addressing the compliance date for large buses, the Motorcoach Enhanced Safety Act specifically states that a stability enhancing requirement shall apply to all motorcoaches manufactured more than 3 years after the date on which the regulation is published as a final rule. Based on the Congressional determination that any enhancing stability technology rulemaking shall apply to all over-the-road buses manufactured more than 3 years after the final rule is published, we will allow bus manufacturers that amount of time inasmuch as a three-year lead time is practical.

With respect to Class 7 buses, the agency has determined that a three-year compliance date is not practical. The scope of this final rule includes buses that are hydraulic-braked. We recognize the manufacturers of hydraulic-braked buses will likely require extra time to ensure system availability and that the ESC system is properly integrated with the vehicle. Based on the comments received from the bus industry, Class 7 buses are equipped with both air and hydraulic brakes. Rather than differentiate between brake systems of the Class 7 buses, we believe it would be better to base the compliance date requirements on GVWR. This will also address the concerns of manufacturers of buses built on truck chassis, for which ESC systems may not currently be equipped. We believe that at least four years of lead time are necessary to ensure that suppliers have ESC systems available for hydraulic-braked large buses. Accordingly, this final rule allows Class 7 bus manufacturers four years of lead time before the requirements of this final rule become applicable.

8. Class 3 Through 6 Buses

Some of the commenters recommended that we expand the scope to include mid-size buses which are typically built on single-unit truck frames. Skagit, NTSB, IIHS, NAPT, Advocates, and Bosch all suggested that ESC should be mandated on all buses greater than 10,000 pounds. The NTSB estimated that 11,600 mid-size buses (buses with a GVWR between 10,000 pounds and 26,000 pounds) are produced each year. Advocates recommended that NHTSA should consider the NTSB recommendation that all buses over 10,000 pounds GVWR should be equipped with stability control systems. Bosch stated that the agency should develop a performance standard to cover vehicles in Classes 3 through 7 with hydraulic brakes because this segment accounts for a large number of commercial and load bearing vehicles on the U.S. roads. Bosch claims that a standard with a phase-in period is needed to facilitate industry development of ESC systems for these vehicles. Bosch also cites Annex 21 of UN ECE Regulation 13, which requires ESC on buses operating in the European Union.

We are not expanding the scope of this rule to include vehicles with a GVWR of 11,793 kilograms (26,000 pounds) or less. After publishing the NPRM, we began a research program to study the safety benefits and performance criteria of ESC systems on single-unit trucks, which includes mid-size buses. The research is not yet complete on single-unit trucks or smaller buses. However, we believe including buses with hydraulic brakes in this final rule will spur development of ESC systems for other hydraulic-braked vehicles, including buses with a GVWR of greater than 4,536 kilograms (10,000 pounds) but not more than 11,793 kilograms (26,000 pounds).

C. Retrofitting

NHTSA considered proposing to require retrofitting of in-service truck tractors, trailers, and large buses with stability control systems. The Secretary has the statutory authority to promulgate safety standards for “commercial motor vehicles and equipment subsequent to initial manufacture.”

The Secretary has delegated authority to NHTSA to promulgate safety standards for commercial motor vehicles and equipment subsequent to initial manufacture when the standards are based upon and similar to an FMVSS promulgated, either simultaneously or previously, under chapter 301 of title 49, U.S.C.

Additionally, the Federal Motor Carrier Safety Administration (FMCSA) is authorized to promulgate and enforce vehicle safety regulations, including those aimed at maintaining commercial motor vehicles so they continue to comply with the safety standards applicable to commercial motor vehicles at the time they were manufactured.

See Motor Carrier Safety Improvement Act of 1999, section 101(f), Pub. L. 106-159 (Dec. 9, 1999).

See 49 CFR 1.50(n).

Although the NPRM did not propose requiring truck tractors, trailers, or large buses to be equipped with stability control systems “subsequent to initial manufacture,” we requested public comment on several issues related to retrofitting in-service truck tractors, trailers, and buses:

• The extent to which a proposal to retrofit in-service vehicles with stability control systems would be complex and costly because of the integration between a stability control system and the vehicle's chassis, engine, and braking systems.

• The changes necessary to an originally manufactured vehicle's systems that interface with a stability control system, such as plumbing for new air brake valves and lines and a new electronic control unit for a revised antilock brake system.

• The additional requirements that would have to be established to ensure that stability control components are at an acceptable level of performance for a compliance test, given the uniqueness of the maintenance condition for vehicles in service, particularly for items such as tires and brake components that are important for ESC performance.

• The original manufacture date of vehicles that should be subject to any retrofitting requirements.

• Whether the performance requirements for retrofitted vehicles should be less stringent or equally stringent as for new vehicles, and, if less stringent, the appropriate level of stringency.

• The cost of retrofitting a stability control system on a vehicle, which we believe would exceed the cost of including stability control on a new vehicle.

Several commenters addressed issues related to retrofitting in-service vehicles with ESC systems. We received comments both favoring and opposing retrofitting.

Road Safe America, NTSB, and Advocates supported a requirement for ESC to be retrofitted to existing heavy vehicles. Road Safe America recommended that RSC systems be retrofitted on all existing truck trailers. NTSB cited its recommendation that RSC systems be retrofitted on in-use cargo tank trailers. In its comments, Advocates said that there should be a retrofit requirement to install ESC systems on all in-service vehicles. Advocates stated that the failure to require retrofitting could significantly delay fleet penetration of ESC systems because of the extended service life of the affected vehicles.

Many more commenters were opposed to a retrofit requirement for ESC systems. IIHS stated that ESC systems should not be required to be retrofitted at this time, but that the agency should explore the feasibility creating a requirement in the future. American Highway Users requested that there should be no retrofit requirements for existing vehicles in order to incorporate ESC systems and would oppose any efforts to implement a retrofit requirement. In its comment, ATA did not support a retrofit requirement for ESC systems because it claims there is an average of a 4-5 year turnover for a majority of Class 7 and Class 8 tractors. Volvo commented that there should not be a retrofit of trucks because the changes to the vehicle are too significant, and there is no way to assure the quality of the retrofit.

Meritor WABCO stated that there should not be a retrofit of vehicles because, as a system supplier, it does not offer an ESC system retrofit option. Meritor WABCO also specified that ESC systems must be engineered and validated for each vehicle model and parts must be added, which would be difficult to do on in-service vehicles. Meritor WABCO further stated that an ESC system requires a steering wheel angle sensor, which is difficult to design for in-service vehicles. Meritor WABCO also expressed concern about the possibility of incomplete or incorrect retrofit installations if retrofits are required.

The National Ready Mix Concrete Association argued that there should not be an ESC system retrofit requirement on single-unit trucks or truck tractors because retrofit costs will be higher on existing trucks than installations on new trucks. They further stated that a variety of improvised techniques are needed when doing retrofit installations, and these techniques result in higher maintenance costs. They were also concerned that a retrofitted system would not work on some older trucks because of unworkable truck designs and interference with safety and electronic features.

HDBMC stated that there should be no retrofit requirement because retrofitting of ESC systems is impractical and difficult. HDBMC cited the challenges of ESC system retrofitting, which include: (1) Compatibility of the vehicle; (2) computer hardware and software issues; (3) issues with new component installation; (4) vehicle downtime to make the conversion; (5) testing and validation; and (6) further unknown variables.

EMA asserted that it would be unsafe to implement a retrofit requirement because ESC systems are not currently installed over existing components. EMA also believes that aftermarket facilities do not have the capability to design, test, and implement ESC systems. EMA stated that rotational sensors, yaw rate, and lateral accelerometers must be mounted close to the vehicle's center of yaw rotation, or complex calculations must be used to compensate for any deviations in the mounting. Finally, EMA commented that the necessary components for an ESC system do not exist for older vehicle models.

Bendix commented that it had, for the purposes of research and development, retrofitted ESC to more than 25 vehicles. Bendix estimated that retrofitting in-service vehicles would take between 80 and 120 person-hours for installation because each installation would have to be customized and there would be little or no OEM support.

After considering the public comments, NHTSA has decided not to include a retrofit requirement in this final rule. NHTSA recognizes that the costs and safety risks of mandating an ESC system retrofit may exceed the benefits. Those commenters supporting an ESC system retrofit did not provide any information to mitigate issues such as: (1) The complexity and cost to retrofit in-service vehicles with ESC systems; (2) the changes necessary to integrate the ESC system to the vehicle's chassis, engine, and braking system; (3) the changes necessary on the in-service vehicle to interface with the ESC system such as plumbing for new air brake valves and lines and a new electronic control unit for the ABS system; and (4) the additional requirements for in-service vehicles considering the uniqueness of the maintenance condition of the tire and brake components. Considering that the potential safety risks and certain high costs associated with a requirement to retrofit in-service vehicles with ESC systems greatly exceed the benefits, NHTSA has not included a retrofit requirement in this final rule.

X. Performance Testing

A. NHTSA's Proposed Performance Tests

The agency's research initially focused on a variety of maneuvers that we could use to evaluate the roll stability performance and the yaw stability performance of truck tractors and large buses. Several of these maneuvers were also tested by industry and some of them are allowed for use in testing for compliance to the UN ECE stability control regulation. The agency's goal was to develop one or more maneuvers that showed the most promise as repeatable and reproducible roll and yaw performance tests for which objective pass/fail criteria could be developed. Based on the agency's own testing and the results from industry-provided test data, two stability performance tests were proposed to evaluate ESC systems on truck tractors and large buses—the SIS test and the SWD test.

1. Characterization Test—SIS

The agency proposed using the slowly increasing steer maneuver (SIS) as a characterization test to determine the unique dynamic characteristics of a vehicle. This maneuver would allow the agency to determine the relationship between the steering wheel angle and lateral acceleration of a vehicle. Also as part of the SIS characterization test, the ability of the ESC system to reduce engine torque is determined. During each of the SIS maneuvers, ESC activation is confirmed by verifying that the system automatically reduces the driver requested engine torque output. The NPRM proposed that, for each of the SIS maneuver test runs, the commanded engine torque and the driver requested torque signals must diverge at least 10 percent for 1.5 seconds after the beginning of ESC system activation. This test

demonstrates that the ESC system has the capability to reduce engine torque, as required in the functional definition. The vehicles that the agency tested were all able to meet this proposed performance level.

2. Roll and Yaw Stability Test—SWD

In the NPRM, we proposed using the sine with dwell maneuver (SWD) to test the ability of an ESC system to mitigate conditions that would lead to rollover or loss of control. Conceptually, the steering profile of this maneuver is similar to that expected to be used by real drivers during some crash avoidance maneuvers. As the agency found in the light vehicle ESC research program, the severity of the SWD maneuver makes it a rigorous test, while maintaining steering rates within the capabilities of human drivers. We believed that the maneuver is severe enough to produce rollover or vehicle loss-of-control without a functioning ESC system on the vehicle.

The agency's test program was able to develop test parameters for the SWD maneuver so that both roll stability and yaw stability could be evaluated using a single loading condition and test maneuver. Previously, the SWD maneuver had typically been used to evaluate only the yaw instability of a vehicle. NHTSA evaluated several loading conditions and found that a loading condition of 80 percent of the tractor's GVWR enabled us to evaluate both the yaw and roll stability control of the ESC system.

For a truck tractor, the agency would conduct the SWD test with the truck tractor coupled to an unbraked control trailer and loaded with ballast directly over the kingpin. The combination vehicle would be loaded to 80 percent of the tractor's GVWR. For a bus, the vehicle is loaded with a 68 kilogram (150 pound) ballast in each of the vehicle's designated seating positions, which would bring the vehicle's weight to less than its GVWR. The test vehicles were proposed to be equipped with outriggers to prevent the trailer from rolling over in case the ESC system does not function properly.

The SWD test would be conducted at a speed of 72 km/h (45 mph). An automated steering machine would be used to initiate the steering maneuver. Each vehicle is subjected to two series of test runs. One series uses counterclockwise steering for the first half-cycle, and the other series uses clockwise steering for the first half-cycle. The steering amplitude for the initial run of each series is 0.3A, where A is the steering wheel angle determined from the SIS maneuver. In each of the successive test runs, the steering amplitude would be increased by increments of 0.1A until a steering amplitude of 1.3A or 400 degrees, whichever is less, is achieved. Upon completion the test runs, the agency would conduct post-processing of the yaw rate and lateral acceleration data to determine the lateral acceleration ratio, yaw rate ratio, and lateral displacement, as discussed below.

The lateral acceleration ratio (LAR) is a performance metric developed to evaluate the ability of a vehicle's ESC system to prevent rollovers. Lateral acceleration is measured on a bus or a tractor and corrected for the vehicle's roll angle. As a performance metric, the lateral acceleration value is normalized by dividing it by the maximum lateral acceleration that was determined at any time between 1.0 seconds after the beginning of steering and the completion of steering. The two proposed performance criteria are described below:

• A vehicle must have a LAR of 30 percent or less 0.75 seconds after completion of steer.

• A vehicle must have a LAR of 10 percent or less at 1.5 seconds after completion of steer.

The yaw rate ratio (YRR) is a performance metric used to evaluate the ability of a vehicle's ESC system to prevent yaw instability. The YRR expresses the lateral stability criteria for the sine with dwell test to measure how quickly the vehicle stops turning, or rotating about its vertical axis, after the steering wheel is returned to the straight-ahead position. The lateral stability criterion, expressed in terms of YRR, is the percent of peak yaw rate that is present at designated times after completion of steer. This performance metric is identical to the metric used in the light vehicle ESC system performance requirement in FMVSS No. 126. The two proposed performance criteria are described below:

• A vehicle must have a YRR of 40 percent or less 0.75 seconds after completion of steer.

• A vehicle must have a YRR of 15 percent or less at 1.5 seconds after completion of steer.

3. Lateral Displacement

Lateral displacement is a performance metric used to evaluate the responsiveness of a vehicle, which relates to its ability to steer around objects. Stability control intervention has the potential to significantly increase the stability of the vehicle in which it is installed. However, we believe that these improvements in vehicle stability should not come at the expense of poor lateral displacement in response to the driver's steering input.

A hypothetical way to pass a stability control performance test would be to make either the vehicle or its stability control system intervene simply by making the vehicle poorly responsive to the speed and steering inputs required by the test. An extreme example of this potential lack of responsiveness would occur if an ESC system locked both front wheels as the driver begins a severe avoidance maneuver that might lead to vehicle rollover. Front wheel lockup would create an understeer condition in the vehicle, which would result in the vehicle plowing straight ahead and colliding with an object the driver was trying to avoid. It is very likely that front wheel lockup would reduce the roll instability of the vehicle since the lateral acceleration would be reduced. This is clearly, however, not a desirable compromise.

Because a vehicle that simply responds poorly to steering commands may be able to meet the stability criteria proposed in the NPRM, a minimum responsiveness criterion was also proposed for the SWD test. The proposed lateral displacement criterion was that a truck tractor equipped with stability control must have a lateral displacement of 2.13 meters (7 feet) or more at 1.5 seconds from the beginning of steer, measured during the sine with dwell maneuver. For a bus, the proposed performance criterion is a lateral displacement of 1.52 meters (5 feet) or more at 1.5 seconds after the beginning of steer. The lateral displacement criteria is less for a bus because a large bus has a longer wheelbase than a truck tractor and higher steering ratio, which makes it less responsive than a truck tractor.

B. Comments on SIS and SWD Maneuvers

The agency received many comments, particularly from representatives of ESC system, truck tractor, and bus manufacturers specifically addressing the slowly increasing steer and sine with dwell ma

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