Federal Motor Vehicle Safety Standards; Electronic Stability Control Systems; Controls and Displays, 17236-17322 [07-1649]

Agencies

• National Highway Traffic Safety Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 72, Number 66 (Friday, April 6, 2007)]
[Rules and Regulations]
[Pages 17236-17322]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 07-1649]



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Part II





Department of Transportation





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National Highway Traffic Safety Administration



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49 CFR Parts 571 and 585



 Federal Motor Vehicle Safety Standards; Electronic Stability Control 
Systems; Controls and Displays; Final Rule

Federal Register / Vol. 72, No. 66 / Friday, April 6, 2007 / Rules 
and Regulations

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DEPARTMENT OF TRANSPORTATION

National Highway Traffic Safety Administration

49 CFR Parts 571 and 585

[Docket No. NHTSA-2007-27662]
RIN 2127-AJ77


Federal Motor Vehicle Safety Standards; Electronic Stability 
Control Systems; Controls and Displays

AGENCY: National Highway Traffic Safety Administration (NHTSA), DOT.

ACTION: Final rule.

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SUMMARY: As part of a comprehensive plan for reducing the serious risk 
of rollover crashes and the risk of death and serious injury in those 
crashes, this document establishes a new Federal motor vehicle safety 
standard (FMVSS) No. 126 to require electronic stability control (ESC) 
systems on passenger cars, multipurpose passenger vehicles, trucks, and 
buses with a gross vehicle weight rating of 4,536 Kg (10,000 pounds) or 
less. ESC systems use automatic computer-controlled braking of 
individual wheels to assist the driver in maintaining control in 
critical driving situations in which the vehicle is beginning to lose 
directional stability at the rear wheels (spin out) or directional 
control at the front wheels (plow out).
    Preventing single-vehicle loss-of-control crashes is the most 
effective way to reduce deaths resulting from rollover crashes. This is 
because most loss-of-control crashes culminate in the vehicle leaving 
the roadway, which dramatically increases the probability of a 
rollover. Based on the best available data, drawn from crash data 
studies, NHTSA estimates that the installation of ESC will reduce 
single-vehicle crashes of passenger cars by 34 percent and single 
vehicle crashes of sport utility vehicles (SUVs) by 59 percent, with a 
much greater reduction of rollover crashes. NHTSA estimates that ESC 
has the potential to prevent 71 percent of the passenger car rollovers 
and 84 percent of the SUV rollovers that would otherwise occur in 
single-vehicle crashes.
    NHTSA estimates that ESC would save 5,300 to 9,600 lives and 
prevent 156,000 to 238,000 injuries in all types of crashes annually 
once all light vehicles on the road are equipped with ESC systems. The 
agency further anticipates that ESC systems would substantially reduce 
(by 4,200 to 5,500) the more than 10,000 deaths each year on American 
roads resulting from rollover crashes.
    Manufacturers equipped about 29 percent of model year (MY) 2006 
light vehicles sold in the U.S. with ESC, and intend to increase the 
percentage to 71 percent by MY 2011. This rule requires installation of 
ESC in 100 percent of light vehicles by MY 2012 (with exceptions for 
some vehicles manufactured in stages or by small volume manufacturers). 
Once all light vehicles in the fleet have ESC, of the overall projected 
annual 5,300 to 9,600 highway deaths and 156,000 to 238,000 injuries 
prevented by stability control systems installed either voluntarily or 
under this rulemaking, we would attribute 1,547 to 2,534 prevented 
fatalities (including 1,171 to 1,465 involving rollover) to this 
rulemaking, in addition to the prevention of 46,896 to 65,801 injuries 
by increasing the percentage of light vehicles with ESC from 71 percent 
to 100 percent.

DATES: Effective Date: This final rule is effective June 5, 2007. The 
incorporation by reference of certain publications listed in the rule 
is approved by the Director of the Federal Register as of June 5, 2007.
    Compliance Date: Consistent with the phase-in commencing September 
1, 2008, all new light vehicles must be equipped with an ESC system 
that meets the requirements of the standard by September 1, 2011, with 
the following exceptions. Vehicle manufacturers need not meet the 
standard's requirements for control and display requirements for the 
ESC malfunction indicator telltale and ``ESC Off'' switch and telltale 
(if provided) until September 1, 2011 (i.e., at the end of the phase-
in), and vehicles produced by final-stage manufacturers and alterers 
must be equipped with a compliant ESC system (including the control and 
display requirements) by September 1, 2012. However, manufacturers may 
voluntarily certify vehicles to FMVSS No. 126 and earn carry-forward 
credits for compliant vehicles, produced in excess of the phase-in 
requirements, that are manufactured between June 5, 2007, and the 
conclusion of the phase-in.
    Petitions for Reconsideration: If you wish to submit a petition for 
reconsideration of this rule, your petition must be received by May 21, 
2007.

ADDRESSES: Petitions for reconsideration should refer to the docket 
number above and be submitted to: Administrator, Room 5220, National 
Highway Traffic Safety Administration, 400 Seventh Street, SW., 
Washington, DC 20590.
    See the SUPPLEMENTARY INFORMATION portion of this document (Section 
VI; Rulemaking Analyses and Notice) for DOT's Privacy Act Statement 
regarding documents submitted to the agency's dockets.

FOR FURTHER INFORMATION CONTACT: For non-legal issues, you may call Mr. 
Patrick Boyd, Office of Crash Avoidance Standards (Telephone: 202-366-
6346) (Fax: 202-366-7002).
    For legal issues, you may call Mr. Eric Stas, Office of the Chief 
Counsel (Telephone: 202-366-2992) (Fax: 202-366-3820).
    You may send mail to both of these officials at National Highway 
Traffic Safety Administration, 400 Seventh Street, SW., Washington, DC 
20590.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. Executive Summary
    A. Requirements of the Final Rule
    B. Lead Time and Phase-in
    C. Differences Between the Final Rule and the Notice of Proposed 
Rulemaking
    D. Impacts of ESC and of the Final Rule
II. Background
    A. Overview of the Safety Problem
    B. The Agency's Comprehensive Response to Rollover
    C. Congressional Mandate Under Section 10301 of the Safe, 
Accountable, Flexible, Efficient Transportation Equity Act: A Legacy 
for Users of 2005
    D. Electronic Stability Control as a Countermeasure to Address 
Single-Vehicle Crashes and Rollovers
III. September 2006 Notice of Proposed Rulemaking (NPRM) and Public 
Comments
    A. The NPRM
    B. Summary of the Public Comments on the NPRM
IV. The Final Rule and Response to Public Comments
    A. Summary of the Requirements
    B. Lead Time and Phase-in
    C. Response to Public Comments by Issue
    Major Issues
    1. Approach of the ESC NPRM
    (a) ESC Mandate vs. ESC Standardization
    (b) ESC as Part of a Comprehensive Rollover Safety Program
    (c) Need for Common Terminology
    2. The Definition of ``ESC System'' as the Basis of the Standard
    3. Stringency of the Standard
    4. Understeer Requirements
    5. Lateral Responsiveness Criteria
    6. Definition of ``ESC System'' and Required Equipment
    (a) Clarification of Performance Expectations
    (b) Clarification of Threshold Speed
    (c) Estimation of Sideslip--Request to Add Derivative
    (d) Request for Alternate Transducers
    (e) Interaction with Other Vehicle Systems
    (f) ESC Operation in Reverse
    7. ESC Performance Requirements
    (a) Definition for ``Lateral Acceleration''
    (b) Lateral Displacement Calculation
    (c) Yaw Rate Calculation

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    (d) Temperature and Pavement Specifications
    (e) Data Processing Issues
    (i) Determination of Beginning of Steering
    (ii) Determination of End of Steering
    (iii) Removing Offsets
    (iv) Use of Interpolation
    (v) Method for Determining Peak Steering Wheel Angle
    (vi) Need for a Common Data Processing Kernel
    (f) ESC Initialization Period
    (g) ESC Calibration
    Other Issues
    8. ESC Malfunction Detection Requirements
    (a) Types of Malfunctions to be Detected
    (b) Practicability Problems with Malfunction Detection
    (c) Monitoring When System is Off
    (d) Minimum Performance Level
    9. ESC Telltale Requirements
    (a) ESC Telltale
    (i) Telltale Symbol Text Enhancements
    (ii) Telltale Symbol Alternative: Substitute Text
    (iii) Waiver of Yellow Color Requirement for ESC Telltale When 
Message/Information Center is Used
    (iv) Telltale Illumination Strategy
    (v) Telltale Extinguishment
    (vi) Telltale Location
    (vii) Use of ESC Malfunction Telltale to Indicate Malfunctions 
of Related Systems/Functions
    (b) ``ESC Off'' Indication
    (i) ``ESC Off'' Symbol Alternative: Use of Text
    (ii) Waiver of Yellow Color Requirement When ``ESC Off'' is 
Indicated Via Message/Information Center Text
    (iii) ``ESC Off'' Telltale Clarification
    (iv) ``ESC Off'' Telltale Strategy
    (v) Use of Two-Part Telltales
    (vi) Conditions for Illumination of ``ESC Off'' Telltale: Speed
    (vii) Conditions for Illumination of ``ESC Off'' Telltale: 
Direction
    (c) Alerting the Driver of ESC Activation
    (i) Visual and Auditory Indications of ESC Activation
    (ii) Flashing Telltales as Activation Indication of Intervention 
by Related Systems/Functions
    (d) Bulb Check
    (i) Waiver of Bulb Check for Message/Information Centers
    (ii) Clarification Regarding Bulb Check
    10. System Disablement and the ``ESC Off'' Control
    (a) Provision of an ``ESC Off'' Control
    (b) Switch for Complete ESC Deactivation
    (c) ESC Operation After Malfunction and ``ESC Off'' Control 
Override
    (d) Default to ``ESC On'' Status
    (e) Operation of Vehicle in 4WD Low Modes
    (f) ``ESC Off'' Control Requirements
    (i) Labeling of the ``ESC Off'' Control
    (ii) Location of the ``ESC Off'' Control
    11. Test Procedures
    (a) Accuracy Requirements
    (b) Tolerances
    (c) Location of Lateral Accelerometer
    (d) Calculation of Lateral Displacement
    (e) Maximum Steering Angle
    (f) Vehicle Test Weight
    (g) Data Filtering
    (h) Outriggers
    (i) Ambient Temperature Range
    (j) Brake Temperatures
    (k) Wind Speed
    (l) Rounding of Steering Wheel Angle at 0.3 g
    (m) Vehicle Speed Specification for the Slowly Increasing Steer 
Test
    (n) Alternative Test Procedures
    (o) Representativeness of Real World Conditions
    12. Lead Time and Phase-in
    (a) Lead Time for ESC Telltale(s)
    (b) Phase-in Schedule
    13. Impacts on the Aftermarket
    (a) System Adaptability and Sharing ESC Information
    (b) ``Make Inoperative'' Prohibition
    (c) Pass-through Certification
    14. Compliance with Relevant Legal Requirements
    (a) Regulatory Flexibility Act
    (b) Executive Orders 12866 and 13258
    (c) Vehicle Safety Act
    15. ESC Outreach Efforts
    (a) ESC Test Procedures Workshop
    (b) Public Information Campaign
    16. Miscellaneous Issues
    (a) Linking Brake Light Illumination to ESC Activation
    (b) Vehicles with Dual Wheels on the Rear Axle
    (c) ESC Operation with Towed Trailers
    (d) Wheelchair-Accessible Vehicles
V. Benefits and Costs
    A. Summary
    B. ESC Benefits
    C. ESC Costs
VI. Regulatory Analyses and Notices
Appendix: Technical Explanation in Response to Comments on 
Understeer

I. Executive Summary

    As part of a comprehensive plan \1\ that seeks to reduce the 
serious risk of rollover crashes and the risk of death and serious 
injury in those crashes, and that includes a number of complementary 
rulemaking actions, this rule establishes Federal Motor Vehicle Safety 
Standard (FMVSS) No. 126, Electronic Stability Control Systems, which 
requires passenger cars, multipurpose passenger vehicles (MPVs), 
trucks, and buses that have a gross vehicle weight rating (GVWR) of 
4,536 kg (10,000 pounds) or less to be equipped with an ESC system that 
meets the requirements of the standard. ESC systems use automatic, 
computer-controlled braking of individual wheels to assist the driver 
in maintaining control (and the vehicle's intended heading) in 
situations where the vehicle is beginning to lose directional stability 
(e.g., where the driver misjudges the severity of a curve or over-
corrects in an emergency situation). In such situations (which occur 
with considerable frequency), intervention by the ESC system can assist 
the driver in preventing the vehicle from leaving the roadway, thereby 
preventing fatalities and injuries associated with crashes involving 
vehicle rollover or collision with various objects (e.g., trees, 
highway infrastructure, other vehicles).
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    \1\ 70 FR 49223, at 49229 (August 23, 2005).
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    Based upon current estimates regarding the effectiveness of ESC 
systems, we believe that an ESC standard could save thousands of lives 
each year, providing potentially the greatest safety benefits produced 
by any safety device since the introduction of seat belts. The 
following discussion highlights the research and regulatory efforts 
that have culminated in this safety standard.
    Since the early 1990's, NHTSA has been actively engaged in finding 
ways to address the problem of vehicle rollover, because crashes 
involving rollover are responsible for a disproportionate number of 
fatalities and serious injuries (over 10,000 of the 33,000 fatalities 
of vehicle occupants in 2004). Although various options were explored, 
the agency ultimately chose to add a rollover resistance component to 
its New Car Assessment Program (NCAP) consumer information program in 
2001. In response to NCAP's market-based incentives, vehicle 
manufacturers made modifications to their product lines to increase 
their vehicles' geometric stability and rollover resistance by 
utilizing wider track widths (typically associated with passenger cars) 
on many of their newer sport utility vehicles (SUVs) and by making 
other improvements to truck-based SUVs during major redesigns (e.g., 
introduction of roll stability control). This approach was successful 
in terms of reducing the much higher rollover rate of SUVs and other 
high-center-of-gravity vehicles, as compared to passenger cars. 
However, manipulating vehicle configuration alone cannot entirely 
resolve the rollover problem (particularly when consumers continue to 
demand vehicles with greater carrying capacity and higher ground 
clearance).
    Accordingly, the agency began exploring technologies that could 
confront the issue of vehicle rollover from a different perspective or 
line of inquiry, which led to today's final rule. We believe that the 
ESC requirement offers a complementary approach that may provide 
substantial benefits to drivers of both passenger cars and LTVs (light 
trucks/vans). Undoubtedly, keeping vehicles from leaving the roadway is 
the best way to prevent deaths and injuries associated with rollover, 
as well as other types of

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crashes. Based on its crash data studies, NHTSA estimates that the 
installation of ESC systems will reduce single vehicle crashes of 
passenger cars by 34 percent and single vehicle crashes of sport 
utility vehicles (SUVs) by 59 percent. Its effectiveness is especially 
great for single-vehicle crashes resulting in rollover, where ESC 
systems were estimated to prevent 71 percent of passenger car rollovers 
and 84 percent of SUV rollovers in single vehicle crashes (see Section 
V).
    In short, we believe that preventing single-vehicle loss-of-control 
crashes is the most effective way to reduce rollover deaths, and we 
believe that ESC offers considerable promise in terms of meeting this 
important safety objective while maintaining a broad range of vehicle 
choice for consumers. In fact, among the agency's ongoing and planned 
rulemakings, it is the single most effective way of reducing the total 
number of traffic deaths. It is also the most cost-effective of those 
rulemakings.
    We note that this final rule also satisfies the recent mandate in 
section 10301 of the Safe, Accountable, Flexible, Efficient 
Transportation Equity Act: A Legacy for Users of 2005 (SAFETEA-LU).\2\ 
That provision requires the Secretary of Transportation to ``establish 
performance criteria to reduce the occurrence of rollovers consistent 
with stability enhancing technologies'' and to ``issue a proposed rule 
* * * by October 1, 2006, and a final rule by April 1, 2009.'' In light 
of the tremendous life-saving potential anticipated to be associated 
with a requirement for ESC to be standard equipment on all light 
vehicles, the agency determined that, consistent with its mission to 
save lives, prevent injuries and reduce economic costs due to road 
traffic crashes, it was important to issue a final rule as soon as 
possible and accelerate the rate of installation. Accordingly, today's 
final rule is being published well in advance of the statutory deadline 
under SAFETEA-LU.
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    \2\ Pub. L. 109-59, 119 Stat. 1144 (2005).
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    The balance of this notice discusses (1) The background regarding 
the size of the safety problem, the agency's comprehensive response to 
rollover-related safety problems, the agency's mandate under SAFETEA-
LU, and ESC systems as a countermeasure to address single-vehicle 
crashes and rollovers (see Section II); (2) the agency's September 2006 
NPRM for ESC and public comments on that proposal (see Section III); 
(3) the requirements and implementation of the final rule, including a 
detailed discussion regarding resolution of the issues raised in public 
comments (see Section IV); and (4) costs and benefits associated with 
the final rule (see Section V). However, before turning to this more 
detailed analysis, we summarize the key points of the final rule, 
including the requirements for ESC systems under FMVSS No. 126, lead 
time and phase-in, differences between the final rule and the NPRM, and 
the anticipated impacts of the final rule.

A. Requirements of the Final Rule

    After careful consideration of all available information, including 
the public comments, the agency has decided to adopt in the ESC final 
rule most of the elements of the proposed rule. Consistent with 
SAFETEA-LU, NHTSA is requiring all light vehicles to be equipped with 
an ESC system with, at the minimum, the capabilities of current 
production systems. We believe that a requirement for such ESC systems 
is desirable in terms of both ensuring technological feasibility and 
providing the desired safety benefits in a cost-effective manner. 
Although vehicle manufacturers have been increasing the portion of the 
light vehicle fleet equipped with ESC, we believe that given the 
relatively high cost of this technology, a mandatory standard is 
necessary to maximize the safety benefits associated with electronic 
stability control, and is required by SAFETEA-LU.
    In order to realize these benefits, we have decided to require 
vehicles to be equipped with an ESC system meeting definitional 
requirements and to pass a dynamic test. The definitional requirements 
specify the necessary elements of a stability control system that is 
capable of both effective oversteer and understeer intervention. These 
requirements are necessary due to the extreme difficulty in 
establishing tests adequate, by themselves, to ensure the desired level 
of ESC functionality in a variety of circumstances.\3\ The test that we 
are adopting is necessary to ensure that the ESC system is robust and 
meets a level of performance at least comparable to that of current ESC 
systems. This approach is similar to the one we took, for similar 
reasons, in 1995 in mandating antilock brakes for medium and heavy 
vehicles pursuant to the Intermodal Surface Transportation Efficiency 
Act (ISTEA) of 1991.\4\
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    \3\ An equipment requirement is necessary because it would be 
almost impossible to devise a single performance test that could not 
be met through some action by the manufacturer other than providing 
an ESC system. Establishing a battery of performance tests to 
achieve our intended results is not possible at this time because we 
have not been able to develop a practical, repeatable limit-
understeer test, and there are no applicable tests in vehicle 
dynamics literature. Although the agency has undertaken its own 
preliminary research efforts related to understeer, the complexity 
of such research would require several years of additional work 
before any conclusions could be reached regarding an ESC understeer 
performance test.
    Given this, the agency determined that it had three available 
options: (1) Delay the ESC final rule and conduct research and 
development; (2) drop the understeer requirement and amend the 
standard once an ESC performance test is developed; or (3) include a 
requirement for understeer as part of the definition of ``ESC 
System,'' along with requiring specific components that will permit 
the system to intervene in excessive understeer situations.
    The agency eliminated the first and second options on the 
grounds of safety.
    The agency believes that the third option, adopting an 
understeer requirement as part of the definition of ``ESC System,'' 
along with a requirement for specific equipment suitable for that 
purpose, will accomplish the purposes of the statutory mandate. Such 
requirement is objective in terms of explaining to manufacturers 
what type of performance is required and the minimal equipment 
necessary for that purpose. The agency can verify that the system 
has the necessary hardware and logic for understeer mitigation. 
Since the necessary components for effective understeer intervention 
are already present on all ESC systems, we believe that 
manufacturers are highly unlikely to decrease their ESC systems' 
understeer capabilities simply because the standard does not have a 
specific test for understeer. The agency believes that its chosen 
approach will ensure that vehicle manufacturers maintain understeer 
intervention as a feature of the ESC system, without delaying the 
life-saving benefits of the ESC rule. In the meantime, the agency 
will conduct additional research in the area of ESC understeer 
intervention and consider taking additional action, as appropriate.
    Even with an understeer test, the ultimate practicability of a 
standard without an equipment requirement remains in doubt because 
of the possible large number of test conditions that would be 
required.
    \4\ 60 FR 13216 (March 10, 1995).
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    These requirements are summarized below:
     Consistent with the definition of ESC contained in a 
voluntary consensus standard, the Society of Automotive Engineers \5\ 
(SAE) Surface Vehicle Information Report J2564 (rev. June 2004), we are 
requiring vehicles covered under the standard to be equipped with an 
ESC system that:
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    \5\ The Society of Automotive Engineers is an association of 
engineers, business executives, educators, and students who share 
information and exchange ideas for advancing the engineering of 
mobility systems. SAE currently has over 90,000 members in 
approximately 97 countries. The organization's activities include 
development of standards, events, and technical information and 
expertise used in designing, building, maintaining, and operating 
self-propelled vehicles for use on land or sea, in air or space. See 
https://www.sae.org.
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    (1) Augments vehicle directional stability by applying and 
adjusting the vehicle brake torques individually to induce a correcting 
yaw moment to a vehicle;
    (2) Is computer-controlled, with the computer using a closed-loop

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algorithm \6\ to limit vehicle oversteer and to limit vehicle 
understeer;
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    \6\ A ``closed-loop algorithm'' is a cycle of operations 
followed by a computer that includes automatic adjustments based on 
the result of previous operations or other changing conditions.
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    (3) Has a means to determine vehicle yaw rate \7\ and to estimate 
its sideslip \8\ or the time derivative of sideslip;
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    \7\ ``Yaw rate'' means the rate of change of the vehicle's 
heading angle measured in degrees/second of rotation about a 
vertical axis through the vehicle's center of gravity.
    \8\ ``Sideslip'' means the arctangent of the lateral velocity of 
the center of gravity of the vehicle divided by the longitudinal 
velocity of the center of gravity.
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    (4) Has a means to monitor driver steering input;
    (5) Has an algorithm to determine the need, and a means to modify 
engine torque, as necessary, to assist the driver in maintaining 
control of the vehicle, and
    (6) Is operational over the full speed range of the vehicle (except 
at vehicle speeds less than 15 km/h (9.3 mph) or when being driven in 
reverse).
     The ESC system, as defined above, is also required to be 
capable of applying brake torques individually at all four wheels and 
to have an algorithm that utilizes this capability.\9\ Except for the 
situations specifically set forth in part (6) of the definition of 
``ESC System'' above, the system is also required to be operational 
during all phases of driving, including acceleration, coasting, and 
deceleration (including braking). It is also required to be capable of 
activation even if the anti-lock brake system or traction control 
system is also activated.
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    \9\ The standard was developed based on new vehicles produced in 
2005 and 2006. The definition of ESC is limited to four-wheel ESC 
systems because existing two-wheel ESC systems are not capable of 
understeer invention or four-wheel automatic braking during an 
intervention, even though these systems also produced substantial 
(but lesser) benefits.
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     In order to ensure that a vehicle is equipped with an ESC 
system that meets the definition of ``ESC System'' under S4, the final 
rule requires vehicle manufacturers to submit, upon the request of 
NHTSA's Office of Vehicle Safety Compliance, ESC system technical 
documentation as to when understeer intervention is appropriate for a 
given vehicle (see S5.6). Specifically, NHTSA may seek information such 
as a system diagram that identifies all ESC components, a written 
explanation describing the ESC system's basic operational 
characteristics, a logic diagram supporting the explanation of system 
operations, and a discussion of the pertinent inputs to the vehicle 
computer or calculations within the computer and how its algorithm uses 
that information and controls ESC system hardware to limit vehicle 
understeer.
     We are also requiring vehicles covered under the standard 
to meet a performance test. It must satisfy the standard's stability 
criteria and responsiveness criterion when subjected to the sine with 
dwell steering maneuver test. This test involves a vehicle's coasting 
at an initial speed of 50 mph while a steering machine steers the 
vehicle with a steering wheel pattern as shown in Figure 2 of the 
regulatory text. The test maneuver is then repeated over a series of 
increasing maximum steering angles. This test maneuver was selected 
over a number of other alternatives because we decided that it has the 
best set of characteristics, including severity of the test, 
repeatability and reproducibility of results, and the ability to 
address lateral stability and responsiveness.
    The maneuver is severe enough to produce spinout for most vehicles 
without ESC. The stability criteria for the test measure how quickly 
the vehicle stops rotating after the steering wheel is returned to the 
straight-ahead position. A vehicle that continues to rotate for an 
extended period after the driver steers straight is out of control, 
which is what ESC is designed to prevent. The quantitative stability 
criteria are expressed in terms of the percent of the peak yaw rate 
after maximum steering that persists at a period of time after the 
steering wheel has been returned to straight ahead. They require that 
the vehicle yaw rate decrease to no more than 35 percent of the peak 
value after one second and that it continue to drop to no more than 20 
percent after 1.75 seconds. Since a vehicle that simply responds very 
little to steering commands could meet the stability criteria, a 
minimum responsiveness criterion is applied to the same test.
     Because the benefits of the ESC system can only be 
realized if the system is functioning properly, we are requiring that a 
telltale be mounted inside the occupant compartment in front of and in 
clear view of the driver. The ESC malfunction telltale is required to 
illuminate after the occurrence of one or more malfunctions that affect 
the generation or transmission of control or response signals in the 
vehicle's ESC system. Such telltale must remain continuously 
illuminated for as long as the malfunction(s) exists, whenever the 
ignition locking system is in the ``On'' (``Run'') position.
     In certain circumstances, drivers may have legitimate 
reasons to disengage the ESC system or limit its ability to intervene, 
such as when the vehicle is stuck in sand/gravel, is being used while 
equipped with snow chains, or is being run on a track for maximum 
performance. Accordingly, under this final rule, vehicle manufacturers 
may include a driver-selectable switch that places the ESC system in a 
mode in which it does not satisfy the performance requirements of the 
standard (e.g., ``sport'' mode or full-off mode). However, if the 
vehicle manufacturer chooses this option, it must ensure that the ESC 
system always returns to the fully-functional default mode at the 
initiation of each new ignition cycle, regardless of the mode the 
driver had previously selected (with certain exceptions for low speed 
off-road axle/transfer case selections that turn off ESC, but cannot be 
reset electronically). If the vehicle manufacturer chooses this option, 
it must also provide an ``ESC Off'' control and a telltale that is 
mounted inside the occupant compartment in front of and in clear view 
of the driver. Such telltale must remain continuously illuminated for 
as long as the ESC is in a mode that renders it unable to meet the 
performance requirements of the standard, whenever the ignition locking 
system is in the ``On'' (``Run'') position.
     We are not requiring the ESC system to be equipped with a 
roll stability control system. Roll stability control systems involve 
relatively new technology. There is currently an insufficient body of 
data to judge the efficacy of such systems. However, the agency will 
continue to monitor the development of these systems.

B. Lead Time and Phase-In

    In order to provide the public with what are expected to be the 
significant safety benefits of ESC systems as rapidly as possible, 
compliance with this final rule is set to commence on September 1, 
2008. That date marks the start of a three-year phase-in period. 
Subject to the special provisions discussed below, NHTSA has decided to 
require compliance in accordance with the following schedule: 55 
percent of a vehicle manufacturer's light vehicles manufactured during 
the period from September 1, 2008 to August 31, 2009; 75 percent of 
those manufactured during the period from September 1, 2009 to August 
31, 2010; 95 percent of those manufactured during the period from 
September 1, 2010 to August 31, 2011, and all light vehicles 
thereafter.
    For the reasons discussed in detail in Section IV.B of this notice, 
we believe that it is practicable for vehicle manufacturers to meet the 
requirements of the phase-in discussed above, subject

[[Page 17240]]

to the exceptions below. Because ESC is so cost-effective and has such 
high benefits in terms of potential fatalities and injuries that may be 
prevented, the agency has decided that it is important to require ESC 
installation in light vehicles as quickly as possible. Given the 
product plans we have from six vehicle manufacturers, and the desire to 
provide manufacturers with flexibility by having a carry-forward 
provision, we have chosen the most aggressive phase-in alternative that 
we believe is reasonable (i.e., 55/75/95%). In doing so, we have 
carefully considered the financial and technological practicability of 
the final rule (in keeping with our statutory mandate), while at the 
same time facilitating ESC installation in the light vehicle fleet as 
expeditiously as possible.
    With the above said, the agency has decided that it is appropriate 
to provide the following exceptions to the phase-in. First, we have 
decided to defer the standard's requirements related to the ESC 
telltales and controls until the end of the phase-in (i.e., September 
1, 2011 for most manufacturers; September 1, 2012 for final-stage 
manufacturers and alterers). Although vehicle manufacturers generally 
commented that they could bring their ESC systems into full compliance 
(including the control and telltale requirements), they stated that 
additional lead time would be necessary to accomplish those changes, 
suggesting that they could do so by the end of the phase-in. As a 
complicating matter, vehicle manufacturers and their trade associations 
explained that even though most current ESC systems would largely meet 
the performance requirements of the proposed standard, manufacturers' 
inability to meet the proposed control and display requirements would 
prevent them from earning the carry-forward credits needed to comply 
with the ESC phase-in schedule. Our analysis demonstrates that the 
safety benefits associated with early introduction of ESC systems, even 
without standardized controls and displays, far outweigh the benefits 
of delaying the standard until all systems can fully meet the control 
and display requirements (see FRIA's lead time/phase-in discussion). 
Accordingly, we believe that it is preferable to move rapidly to 
implement the standard, but to delay the compliance date only for the 
ESC control and telltale requirements.
    As proposed, vehicle manufacturers may earn carry-forward credits 
for compliant vehicles, produced in excess of the phase-in 
requirements, which are manufactured between the effective date of the 
final rule and the conclusion of the phase-in period.\10\
---------------------------------------------------------------------------

    \10\ We note that carry-forward credits may not be used to defer 
the mandatory compliance date of September 1, 2011 for all covered 
vehicles.
---------------------------------------------------------------------------

    This final rule excludes small volume manufacturers (i.e., 
manufacturers producing less than 5,000 vehicles for sale in the U.S. 
market in one year) from the phase-in, instead requiring those 
manufacturers to fully comply with the standard beginning on September 
1, 2011.
    In addition, consistent with the policy set forth in NHTSA's 
February 14, 2005 final rule on certification requirements for vehicles 
built in two or more stages and altered vehicles (70 FR 7414), final-
stage manufacturers and alterers are excluded from the requirements of 
the phase-in and are permitted an additional one year for compliance 
(i.e., until September 1, 2012). However, final-stage manufacturers and 
alterers may voluntarily certify compliance with the standard prior to 
this date.

C. Differences Between the Final Rule and the Notice of Proposed 
Rulemaking

    As noted above, NHTSA has decided to adopt most of the provisions 
in the NPRM as part of this final rule. We made a number of changes in 
response to the public comments on the NPRM. The main differences 
between the NPRM and the final rule involve an increase in the 
percentages of FMVSS No. 126-compliant vehicles that must be produced 
during the phase-in period, a delay in the requirements for 
standardized symbols and acronyms for ESC controls and displays until 
the end of the phase-in, and the inclusion of engine control as part of 
the standard's definition of ``ESC system.''
    The following points briefly describe the main differences between 
the NPRM and this final rule.
     In order to increase fleet installation of life-saving ESC 
systems, the phase-in schedule for ESC is being accelerated to require 
55 percent phase-in in the first year, 75 percent in the second year, 
and 95 percent in the third year, rather than the 30 percent, 60 
percent, and 90 percent schedule that was proposed (see S8.1, S8.2, and 
S8.3 in the regulatory text of this final rule).
     The effective date for the requirement to use standardized 
symbols and acronyms as well as certain malfunction detection and ``ESC 
Off'' control functions has been moved to the end of the phase-in 
period. This was done in recognition of the fact that manufacturers 
will be relying on the carry-forward and compliance credits for 
vehicles in current production that pass all the ESC performance 
requirements, but currently lack the standardized controls and displays 
features proposed in the NPRM (see S5.3.1, S5.3.2; S5.3.4; S5.3.9; 
S5.4.2; S5.5.2; S5.5.3; S5.5.6).
     The definition of ``ESC System'' has been changed to 
require ESC systems with engine control, a feature that allows the ESC 
system to reduce vehicle speed during an intervention by cutting engine 
power as well as by brake application (see S4 ESC (5)). It was a 
feature on most vehicles in the crash data analysis and on all the 
vehicles in the ESC cost study.
     The definition of ``ESC System'' has been changed to 
delete the word ``as appropriate'' from the description of when the 
system must intervene to mitigate vehicle understeer (see S4 ESC (2)). 
Instead, in order to ensure that a vehicle is equipped with an ESC 
system that meets the definition of ``ESC System'' under S4, we have 
decided to require vehicle manufacturers to submit, upon the request of 
NHTSA's Office of Vehicle Safety Compliance, ESC system technical 
documentation as to when understeer intervention is appropriate for a 
given vehicle (see S5.6). Specifically, NHTSA may seek information such 
as a system diagram that identifies all ESC components, a written 
explanation describing the ESC system's basic operational 
characteristics, a logic diagram supporting the explanation of system 
operations, and a discussion of the pertinent inputs to the vehicle 
computer or calculations within the computer and how its algorithm uses 
that information and controls ESC system hardware to limit vehicle 
understeer.\11\
---------------------------------------------------------------------------

    \11\ We note here that we anticipate that much of this 
information is proprietary and would be submitted under a request 
for confidential treatment pursuant to 49 CFR Part 512.
---------------------------------------------------------------------------

     The ``ESC System'' definition and performance requirements 
have been changed to refer to generating brake torques at all four 
wheels individually, rather than applying individual brakes, so that 
the action of regenerative braking by electric motors is included (see 
S4 ESC (1); S5.1.1).
     The definition of ``ESC System'' has been further changed 
to recognize that some systems operate by estimating the time 
derivative of side slip, rather than by measuring side slip directly. 
The final rule also defines the low speed threshold for ESC operation 
as 15 km/h (see S4 ESC (3), (6)).
     The responsiveness criterion has been changed to a two-
stage criterion with a lower lateral displacement requirement for large 
vehicles (i.e., ones

[[Page 17241]]

over 7,716 pounds GVWR). It is applied during tests with a peak 
commanded steering angle of five times or greater than the steering 
wheel angle necessary to produce 0.3g steady-state lateral 
acceleration. This is a change from applying it simply for tests with 
steering wheel angles greater than 180 degrees. It compensates for the 
slower steering gear ratios of large vehicles. (see S5.2; S5.2.3; 
S6.3.5).
     Low-speed four-wheel-drive (4WD) modes that have the side 
effect of turning off ESC and that are selected by mechanical controls 
that cannot be automatically reset electrically are excluded from the 
requirement for automatic ESC restoration at the next ignition cycle 
(see S5.4.1).
     Under the final rule, outriggers will be used for testing 
of trucks, MPVs, and buses, and the maximum weight and roll moment of 
inertia are also specified for outriggers (see S6.3.4).
     The ESC malfunction detection test procedure has been 
modified to include a short driving and turning procedure so that ESC 
systems with self-diagnostics requiring vehicle motion can accomplish 
their function (see S7.10.2).

D. Impacts of ESC and of the Final Rule

    Based on its analysis of the best available data, NHTSA estimates 
that ESC--both installed voluntarily and under this regulatory 
mandate--will save 5,300 to 9,600 lives and prevent 156,000 to 238,000 
injuries in all types of crashes annually once all light vehicles on 
the road are equipped with ESC systems. A large portion of these 
savings will come from preventing large numbers of rollover crashes. 
ESC systems will substantially reduce (by 4,200 to 5,500) the more than 
10,000 deaths that occur on American roads each year as a result of 
rollover crashes.
    Manufacturers installed ESC in about 29 percent of model year (MY) 
2006 light vehicles sold in the U.S., and intend to increase the 
percentage of ESC installation in light vehicles to 71 percent by MY 
2011. This rule accelerates that rate of installation by requiring a 
100 percent installation rate by MY 2012 (with exceptions for some 
vehicles manufactured in stages or by small volume manufacturers). We 
took that step because, in response to public comments and our review 
of vehicle manufacturers' production plans, we determined that it is 
practicable to increase the percentage of new light vehicles that must 
comply with Standard No. 126 under the phase-in, thereby accelerating 
the benefits expected to be provided by ESC systems.
    As the discussion below demonstrates, ESC not only has a very 
significant life-saving and injury-preventing potential in absolute 
terms, but it also achieves these benefits in a very cost-effective 
manner vis-[agrave]-vis other agency rulemakings. ESC offers 
consistently strong benefits and cost-effectiveness across all types of 
light vehicles, including passenger cars, SUVs, vans, and pick-up 
trucks. Of the 5,300 to 9,600 highway deaths and 156,000 to 238,000 
MAIS 1-5 injuries that we project will be prevented annually for all 
types of crashes once all light vehicles on the road are equipped with 
ESC, we attribute 1,547 to 2,534 prevented fatalities (including 1,171 
to 1,465 involving rollover) to this rulemaking, in addition to the 
prevention of 46,896 to 65,801 injuries.
    The agency estimates that the production-weighted, average cost per 
vehicle to meet the proposed standard's requirements will be $58 ($90.3 
per passenger car and $29.2 per light truck).\12\ These are incremental 
costs over the manufacturers' MY 2011 plans for installation of ABS, 
which is expected to be installed in almost 93 percent of the light 
vehicle fleet, and ESC, which is expected to be installed in 71 percent 
of the light vehicle fleet. Vehicle costs are estimated to be $368 (in 
2005$) for anti-lock brakes (ABS) and an additional $111 for ESC, for a 
total system cost of $479 per vehicle. The total annual vehicle cost of 
this regulation, based on ESC installation beyond manufacturers' 
planned percentages, is expected to be approximately $985 million.
---------------------------------------------------------------------------

    \12\ We note that the costs for passenger cars are higher 
because a greater portion of those vehicles require installation of 
ABS in addition to ESC.
---------------------------------------------------------------------------

    In terms of cost-effectiveness, this final rule is expected to save 
1,547 to 2,534 lives and prevent 46,896 to 65,801 injuries at a cost of 
$0.18 to $0.33 million per equivalent life saved at a 3 percent 
discount rate and $0.26 to $0.45 million at a 7 percent discount rate.
    The final rule is highly cost-effective even when passenger cars 
are considered alone. The passenger car portion of the final rule will 
save 945 lives and prevent 32,196 injuries at a cost of $0.38 million 
per equivalent life saved at a 3 percent discount rate and $0.50 at a 7 
percent discount rate.

II. Background

A. Overview of the Safety Problem

    The following discussion explains the nature and scope of the 
safety problem which the agency seeks to address through this 
rulemaking for ESC, based upon our analysis of recent single-vehicle 
crash and rollover statistics. About one in seven light vehicles 
involved in police-reported crashes collides with something other than 
another vehicle. However, the proportion of these single-vehicle 
crashes increases steadily with increasing crash severity, and almost 
half of serious and fatal injuries occur in single-vehicle crashes. We 
can describe the relationship between crash severity and the number of 
vehicles involved in the crash using information from the agency's 
crash data programs. We limit our discussion here to ``light 
vehicles,'' which consist of passenger cars, multipurpose passenger 
vehicles (MPVs), trucks, and buses with a gross vehicle weight rating 
(GVWR) of 4,536 kilograms (10,000 pounds) or less.\13\
---------------------------------------------------------------------------

    \13\ For brevity, we use the term ``light trucks'' in this 
document to refer to multipurpose passenger vehicles (e.g., vans, 
minivans, and SUVs), trucks, and buses with a GVWR of 4,536 
kilograms (10,000 pounds) or less.
---------------------------------------------------------------------------

    The 2000-2005 data from the National Automotive Sampling System 
(NASS) Crashworthiness Data System (CDS) and 2005 data from the 
Fatality Analysis Reporting System (FARS) were combined to estimate the 
current target population for this rulemaking. It includes 27,680 
people who were killed as occupants of light vehicles (both single-
vehicle and multi-vehicle crashes). Over half of these (15,191) 
occurred in single-vehicle crashes. Of these, 8,596 occurred in 
rollovers. About 1.0 million injuries (AIS 1-5) occurred in crashes 
that could be affected by ESC, almost 458,000 in single vehicle crashes 
(of which almost half were in rollovers). Multi-vehicle crashes that 
could be affected by ESC accounted for 12,485 fatalities and almost 
547,000 injuries.
    Rollover crashes are complex events that reflect the interaction of 
driver, road, vehicle, and environmental factors. We can describe the 
relationship between these factors and the risk of rollover using 
information from the agency's crash data programs.
    According to 2005 data from FARS, 10,836 people were killed as 
occupants in light vehicle rollover crashes, which represents 34 
percent of all occupants killed that year in crashes. Of those, 8,769 
were killed in single-vehicle rollover crashes. Seventy-four percent of 
the people who died in single-vehicle rollover crashes were not using a 
seat belt, and 61 percent were partially or completely ejected from the 
vehicle (including 50 percent who were completely ejected). FARS shows 
that 55 percent of light vehicle occupant fatalities in single-vehicle 
crashes involved a rollover event.

[[Page 17242]]

    Using data from the 2000-2004 NASS CDS files, we estimate that 
266,000 light vehicles were towed from a police-reported rollover crash 
each year (on average), and that 29,000 occupants of these vehicles 
were seriously injured. Of these 266,000 light vehicle rollover 
crashes, 219,000 were single-vehicle crashes. Sixty-one percent of 
those people who suffered a serious injury in a single-vehicle tow-away 
rollover crash were not using a seat belt, and 52 percent were 
partially or completely ejected (including 41 percent who were 
completely ejected). Estimates from NASS CDS indicate that 82 percent 
of tow-away rollovers were single-vehicle crashes, and that 88 percent 
(197,000) of the single-vehicle rollover crashes occurred after the 
vehicle left the roadway. An audit of 1992-96 NASS CDS data showed that 
about 95 percent of rollovers in single-vehicle crashes were tripped by 
mechanisms such as curbs, soft soil, pot holes, guard rails, and wheel 
rims digging into the pavement, rather than by tire/road interface 
friction as in the case of untripped rollover events.

B. The Agency's Comprehensive Response to Rollover

    As mentioned above, this final rule for ESC is but one part of the 
agency's comprehensive plan to address the issue of vehicle rollover. 
The following discussion provides background on NHTSA's comprehensive 
plan to reduce rollover crashes. In 2002, the agency formed an 
Integrated Project Team (IPT) to examine the rollover problem and to 
make recommendations on how to reduce rollovers and to improve safety 
when rollovers nevertheless occur. In June 2003, based on the work of 
that team, the agency published a report titled, ``Initiatives to 
Address the Mitigation of Vehicle Rollover.'' \14\ The report 
recommended improving vehicle stability, ejection mitigation, roof 
crush resistance, as well as road improvements and behavioral 
strategies aimed at consumer education.
---------------------------------------------------------------------------

    \14\ See Docket Number NHTSA 2003-14622-1.
---------------------------------------------------------------------------

    Since then, the agency has been working to implement these 
recommendations as part of its comprehensive agency plan for reducing 
the serious risk of rollover crashes and the risk of death and serious 
injury when rollover crashes do occur. It is evident that the most 
effective way to reduce deaths and injuries in rollover crashes is to 
prevent the rollover crash from occurring. This final rule adopting a 
new Federal motor vehicle safety standard for electronic stability 
control systems is one key part of that comprehensive agency plan.
    Moreover, we note that the agency also published a notice of 
proposed rulemaking in the Federal Register in August 2005, seeking to 
upgrade our safety standard on roof crush resistance (FMVSS No. 216); 
that notice, like the present one, contains an in-depth discussion of 
the rollover problem and the countermeasures which the agency intends 
to pursue as part of its comprehensive response to the rollover problem 
(see 70 FR 49223 (August 23, 2005)).

C. Congressional Mandate Under Section 10301 of the Safe, Accountable, 
Flexible, Efficient Transportation Equity Act: A Legacy for Users of 
2005

    During the course of the ongoing agency's research into ESC 
systems, Congress passed the Safe, Accountable, Flexible, Efficient 
Transportation Equity Act: A Legacy for Users of 2005 (SAFETEA-LU).\15\ 
Section 10301 of that Act contains legislative mandates for the agency 
to initiate a number of rulemakings, including ones for rollover 
prevention and occupant ejection prevention. In relevant part, that 
provision states:
---------------------------------------------------------------------------

    \15\ Pub. L. 109-59, 119 Stat. 1144 (2005).

    (a) In General.--The Secretary [of Transportation] shall 
initiate rulemaking proceedings, for the purpose of establishing 
rules or standards that will reduce vehicle rollover crashes and 
mitigate deaths and injuries associated with such crashes for motor 
vehicles with a gross vehicle weight rating of not more than 10,000 
pounds.
    (b) Rollover Prevention.--One of the rulemaking proceedings 
initiated under subsection (a) shall be to establish performance 
criteria to reduce the occurrence of rollovers consistent with 
stability enhancing technologies. The Secretary shall issue a 
proposed rule in this proceeding by rule by October 1, 2006, and a 
final rule by April 1, 2009.

    This SAFETEA-LU mandate is consistent with the agency's efforts 
under its Comprehensive Rollover Safety Program (discussed above). The 
agency's research efforts had already identified electronic stability 
control systems as a mature and effective technology which has had 
adequate time to be analyzed in both the scientific literature, as well 
as by NHTSA researchers. These research results strongly suggest that 
fleet-wide installation of ESC systems should yield tremendous benefits 
in terms of the prevention of fatalities and injuries. Although the 
agency considered other potential ``stability enhancing technologies,'' 
there was no evidence to demonstrate that they would meet the need for 
motor vehicle safety (see Section IV.C.3 below). Accordingly, the 
agency has determined that adopting a requirement for installation of 
ESC systems in light vehicles would be consistent with the statutory 
mandate under section 10301 of SAFETEA-LU. Under our interpretation of 
that statutory provision, Congress provided the agency discretion to 
evaluate various stability enhancing technologies and to adopt a 
requirement for a system that the agency determines would best reduce 
the occurrence of rollovers. The agency agrees with Congress regarding 
the tremendous life-saving potential associated with ESC as a proven 
stability enhancing technology, and because of the agency's prior 
efforts, it was possible to publish today's final rule well in advance 
of the statutory deadline under SAFETEA-LU.
    As this final rule makes clear, the agency has decided to implement 
the statutory mandate contained in section 10301 of SAFETEA-LU through 
promulgation of a Federal motor vehicle safety standard for ESC 
pursuant to 49 U.S.C. Chapter 301, Motor Vehicle Safety. Adoption of an 
FMVSS for ESC meets the statutory directive to ``establish performance 
criteria'' consistent with stability enhancing technologies. 
Furthermore, this approach is consistent with the agency's 
implementation of the statutory mandate for tire pressure monitoring 
systems contained in section 13\16\ of the Transportation Recall 
Enhancement, Accountability, and Documentation (TREAD) Act.\17\
---------------------------------------------------------------------------

    \16\ See 49 U.S.C. 30123 note (2003).
    \17\ Pub. L. 106-414, 114 Stat. 1800 (2000).
---------------------------------------------------------------------------

D. Electronic Stability Control as a Countermeasure to Address Single-
Vehicle Crashes and Rollovers

General Principles of ESC System Operation
    Although Electronic Stability Control (ESC) systems have been known 
by a number of different trade names such as Vehicle Stability Control 
(VSC), Electronic Stability Program (ESP), StabiliTrak and Vehicle 
Stability Enhancement (VSE), their function and performance are 
similar. They are systems that use computer control of individual wheel 
brakes to help the driver maintain control of the vehicle during 
extreme maneuvers by keeping the vehicle headed in the direction the 
driver is steering even when the vehicle nears or reaches the limits of 
road traction.
    When a driver attempts an ``extreme maneuver'' (e.g., one initiated 
to avoid

[[Page 17243]]

a crash or due to misjudgment of the severity of a curve), the driver 
may lose control if the vehicle responds differently as it nears the 
limits of road traction than it does during ordinary driving. The 
driver's loss of control can result in either the rear of the vehicle 
``spinning out'' or the front of the vehicle ``plowing out.'' As long 
as there is sufficient road traction, a highly skilled driver may be 
able to maintain control in many extreme maneuvers using 
countersteering (i.e., momentarily turning away from the intended 
direction) and other techniques. However, average drivers in a panic 
situation in which the vehicle is beginning to spin out would be 
unlikely to countersteer to regain control.
    ESC uses automatic braking of individual wheels to adjust the 
vehicle's heading if it departs from the direction the driver is 
steering. Thus, it prevents the heading from changing too quickly 
(spinning out) or not quickly enough (plowing out). Although it cannot 
increase the available traction, ESC affords the driver the maximum 
possibility of keeping the vehicle under control and on the road in an 
emergency maneuver using just the natural reaction of steering in the 
intended direction.
    Keeping the vehicle on the road prevents single-vehicle crashes, 
which are the circumstances that lead to most rollovers. However, if 
the speed is simply too great for the available road traction, even a 
vehicle with ESC will unavoidably drift off the road (but not spin 
out). Furthermore, ESC cannot prevent road departures due to driver 
inattention or drowsiness rather than loss of control.
How ESC Prevents Loss of Vehicle Control
    The following explanation of ESC operation illustrates the basic 
principle of yaw stability control, but it does not attempt to explain 
advanced refinements of the yaw control strategy described below that 
use vehicle sideslip (lateral sliding that may not alter yaw rate) to 
optimize performance on slippery pavements.
    An ESC system maintains what is known as ``yaw'' (or heading) 
control by determining the driver's intended heading, measuring the 
vehicle's actual response, and automatically turning the vehicle if its 
response does not match the driver's intention. However, with ESC, 
turning is accomplished by applying a brake force at a single wheel 
rather than by steering input. (The uneven brake force from braking 
only one wheel creates a yaw torque or moment that rotates the vehicle 
around a vertical axis.)
    Speed and steering angle measurements are used to determine the 
driver's intended heading. The vehicle response is measured in terms of 
lateral acceleration and yaw rate by onboard sensors. If the vehicle is 
responding in a manner corresponding to driver input, the yaw rate will 
be in balance with the speed and lateral acceleration.
    The concept of ``yaw rate'' can be illustrated by imaging the view 
from above of a car following a large circle painted on a parking lot. 
One is looking at the top of the roof of the vehicle and seeing the 
circle. If the car starts in a heading pointed north and drives half 
way around circle, its new heading is south. Its yaw angle has changed 
180 degrees. If it takes 10 seconds to go half way around the circle, 
the ``yaw rate'' is 180 degrees per 10 seconds or 18 deg/sec. If the 
speed stays the same, the car is constantly rotating at a rate of 18 
deg/sec around a vertical axis that can be imagined as piercing its 
roof. If the speed is doubled, the yaw rate increases to 36 deg/sec.
    While driving in a circle, the driver notices that he must hold the 
steering wheel tightly to avoid sliding toward the passenger seat. The 
bracing force is necessary to overcome the lateral acceleration that is 
caused by the car following the curve. The lateral acceleration is also 
measured by the ESC system. When the speed is doubled the lateral 
acceleration increases by a factor of four if the vehicle follows the 
same circle. There is a fixed physical relationship between the car's 
speed, the radius of its circular path, and its lateral acceleration.
    The ESC system uses this information as follows: Since the ESC 
system measures the car's speed and its lateral acceleration, it can 
compute the radius of the circle. Since it then has the radius of the 
circle and the car's speed, the ESC system can compute the correct yaw 
rate for a car following the path. Of course, the system includes a yaw 
rate sensor, and it compares the actual measured yaw rate of the car to 
that computed for the path the car is following. If the computed and 
measured yaw rates begin to diverge as the car that is trying to follow 
the circle speeds up, it means the driver is beginning to lose control, 
even if the driver cannot yet sense it. Soon, an unassisted vehicle 
would have a heading significantly different from the desired path and 
would be out of control either by oversteering (spinning out) or 
understeering.
    When the ESC system detects an imbalance between the measured yaw 
rate of a vehicle and the path defined by the vehicle's steering wheel 
angle, speed, and lateral acceleration, the ESC system automatically 
intervenes to turn the vehicle. The automatic turning of the vehicle is 
accomplished by uneven brake application rather than by steering wheel 
movement. If only one wheel is braked, the uneven brake force will 
cause the vehicle's heading to change. Figure 1 shows the action of ESC 
using single wheel braking to correct the onset of oversteering or 
understeering. (Please note that all Figures discussed in this preamble 
may be found at the end of the preamble, immediately preceding the 
proposed regulatory text.)
     Oversteering. In Figure 1 (bottom panel), the vehicle has 
entered a left curve that is extreme for the speed it is traveling. The 
rear of the vehicle begins to slide which would lead to a vehicle 
without ESC turning sideways (or ``spinning out'') unless the driver 
expertly countersteers. 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). It momentarily applies the right front brake to turn the 
heading of the vehicle back to the correct path. It will also cut 
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 action happens quickly so that the 
driver does not perceive the need for steering corrections. Even if the 
driver brakes because the curve is sharper than anticipated, the system 
is still capable of generating uneven braking if necessary to correct 
the heading.
     Understeering. Figure 1 (top panel) shows a similar 
situation faced by a vehicle whose response as it nears the limits of 
road traction is to slide at the front (``plowing out'' or 
understeering) rather than oversteering. In this situation, the ESC 
system rapidly detects that the vehicle's heading is changing less 
quickly than appropriate for the driver's intended path (i.e., the yaw 
rate is too low). It momentarily applies the left rear brake to turn 
the heading of the vehicle back to the correct path. Again, it will 
also cut 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).
    While Figure 1 may suggest that particular vehicles go out of 
control as either vehicles prone to oversteer or vehicles prone to 
understeer, it is just as likely that a given vehicle could require 
both understeer and oversteer interventions during progressive phases

[[Page 17244]]

of a complex avoidance maneuver such as a double lane change.
    Although ESC cannot change the tire/road friction conditions the 
driver is confronted with in a critical situation, there are clear 
reasons to expect it to reduce loss-of-control crashes, as discussed 
below.
    In vehicles without ESC, the response of the vehicle to steering 
inputs changes as the vehicle nears the limits of road traction. All of 
the e