[Federal Register Volume 60, Number 47 (Friday, March 10, 1995)]
[Rules and Regulations]
[Pages 13216-13285]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-5410]




[[Page 13215]]

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Part II





Department of Transportation





_______________________________________________________________________



National Highway Traffic Safety Administration



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49 CFR Part 571



Medium and Heavy Vehicles; Stability and Control During Braking, 
Stopping Distance Requirements for Vehicles Equipped With Air and 
Hydraulic Brake Systems; Final Rules



49 CFR Part 393



Antilock Brake Systems for Commercial Motor Vehicles; Proposed Rule

Federal Register / Vol. 60, No. 47 / Friday, March 10, 1995 / Rules 
and Regulations 
[[Page 13216]] 

DEPARTMENT OF TRANSPORTATION

National Highway Traffic Safety Administration

49 CFR Part 571

[Docket No. 92-29; Notice 5]
[Docket No. 93-69; Notice 2]
RIN 2127-AA00
RIN 2127-AE75


Federal Motor Vehicle Safety Standards; Stability and Control of 
Medium and Heavy Vehicles During Braking

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

ACTION: Final rule.

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SUMMARY: In response to the Intermodal Surface Transportation 
Efficiency Act (ISTEA) of 1991, this final rule amends Standard No. 
105, Hydraulic Brake Systems, and Standard No. 121, Air Brake Systems, 
to require medium and heavy vehicles to be equipped with an antilock 
brake system (ABS) to improve the directional stability and control of 
these vehicles during braking. For truck tractors, the ABS requirement 
is supplemented by a 30-mph braking-in-a-curve test on a low 
coefficient of friction surface using a full brake application. By 
improving directional stability and control, these requirements will 
significantly reduce deaths and injuries caused by jackknifing and 
other losses of directional stability and control during braking.
    In addition, this final rule requires all powered heavy vehicles to 
be equipped with an in-cab lamp to indicate ABS malfunctions. Truck 
tractors and other towing trucks are required to be equipped with two 
separate in-cab lamps: one indicating malfunctions in the towing truck 
ABS and the other indicating malfunctions in the towed trailer or dolly 
ABS. Trailers produced during an initial eight-year period must also be 
equipped with an external malfunction indicator that will be visible to 
the driver through the rearview mirror of the towing truck or tractor. 
More specifically, the external trailer indicator will indicate an ABS 
malfunction to the driver, if the trailer is being towed by an older 
vehicle that is not equipped with an in-cab lamp for trailer ABS 
malfunction indication. In general, the indicators will provide 
valuable information about ABS malfunctioning to the driver and to 
maintenance and Federal and State inspection personnel.

DATES: Effective Dates: The amendments to 49 CFR 571.105 become 
effective on March 1, 1999. The amendments to 49 CFR 571.121 become 
effective on March 1, 1997. Compliance to Sec. 571.121 with respect to 
air-braked trailers and single unit trucks and buses will be required 
as of March 1, 1998.
    Petitions for Reconsideration: Any petitions for reconsideration of 
this rule must be received by NHTSA no later than April 10, 1995.

ADDRESSES: Petitions for reconsideration of this rule should refer to 
Docket 92-29; Notice 5 and should be submitted to: Administrator, 
National Highway Traffic Safety Administration, 400 Seventh Street, 
S.W., Washington, D.C. 20590.

FOR FURTHER INFORMATION CONTACT: Mr. George Soodoo, Office of Crash 
Avoidance, National Highway Traffic Safety Administration, 400 Seventh 
Street, SW., Washington, D.C. 20590 (202) 366-5892.

SUPPLEMENTARY INFORMATION:
I. Overview
II. Background
    A. The Safety Problem: Loss of Control Crashes
    B. Braking Systems, Tires, Wheel Lockup, and Loss of Control 
Crashes
III. US and Foreign Activities Related to Stability and Control 
During Braking Performance
    A. Early US Regulatory History
    B. PACCAR Case
    C. US and Foreign Experience with ABS since PACCAR
IV. Advance Notice of Proposed Rulemaking (ANPRM)
V. Agency Proposal
VI. Comments on the Proposal
VII. Agency's Supplemental Proposal
VIII. Comments on the Supplemental Proposal
IX. Agency Decision
    A. Requirement for and Definition of ABS
    1. Legal Authority
    2. Elements of the Requirement/Definition for ABS
    3. Dynamic Versus Equipment Requirements
    B. Independent Wheel Control
    C. Braking-In-A-Curve Test
    1. General Considerations
    2. Test Surface
    3. Test Speed
    4. Type of Brake Application
    5. Number of Test Stops for Certification
    6. Test Weight
    7. Loading Conditions
    8. Initial Brake Temperature
    9. Transmission Position
    10. Summary of General Test Conditions
    D. Reliability and Maintenance
    E. Requirements for Durability, Reliability, and Maintainability
    F. Alleged Safety Problems
    G. ABS Malfunction Indicator Lamps
    1. Number and Location; Duration of Trailer Requirement
    2. Conditions for Activation
    3. Activation Protocol for Malfunction Indicators
    4. Signal Storage
    5. Disabling Switch
    6. ABS Failed System Requirements
    H. Power Source
    I. Applicability of Amendments
    1. Trailers with Hydraulic or Electric Brakes
    2. Hydraulically Braked Vehicles
    J. Implementation
    K. Intermediate and Final Stage Manufacturers/Trailer 
Manufacturers
    L. Benefits
    M. Costs
IX. Rulemaking Analyses and Notices
    A. Executive Order 12866 and DOT Regulatory Policies and 
Procedures
    B. Regulatory Flexibility Act
    C. National Environmental Policy Act
    D. Executive Order 12612 (Federalism)
    E. Civil Justice Reform

I. Overview

    As part of NHTSA's plans to improve the braking performance of 
medium and heavy vehicles,<SUP>1 this final rule amends the agency's 
two brake standards for those vehicles by adopting requirements to 
improve the directional stability and control characteristics of these 
vehicles while braking. The two Federal Motor Vehicle Safety Standards 
(FMVSSs) are Standard No. 105, Hydraulic Brake Systems, and Standard 
No. 121, Air Brake Systems. In formulating this final rule, NHTSA has 
relied on extensive fleet studies of tractor trailer combinations 
equipped with antilock systems, road testing of such vehicles at the 
agency's Vehicle Research Test Center (VRTC), review of its Fatal 
Accident Reporting Systems (FARS) data and other crash data, the 
positive experience with ABS-equipped heavy vehicles in Europe and 
throughout the world, comments to the public docket about this 
rulemaking, and other available information.

    \1\Hereinafter referred to as ``heavy vehicles.''
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    In order to fully understand the safety problem being addressed by 
this rulemaking, it is necessary to examine in detail the reasons for 
wheel lockup and the consequences of such lockup. Moreover, in order to 
fully understand the reasons for the agency's decision to require that 
heavy vehicles be equipped with a closed-loop ABS, it is necessary to 
understand the general characteristics of brake systems, the force-
generating characteristics of tires, and the interactions between brake 
systems and tires.
    To provide the reader with a means for gaining this understanding, 
NHTSA has included an Appendix in this document, which provides a 
discussion of basic service brake systems, loss-of-control crashes, and 
ABS characteristics. The Appendix discusses the types of heavy brake 
systems that [[Page 13217]] are currently in use, how brake systems 
work, and why lockup occurs. It also discusses the force-generating 
characteristics of tires and how they are affected by varying levels of 
wheel slip and the need to take these characteristics into account in 
addressing the problem of loss-of-control crashes. Finally, the 
Appendix discusses the need for ABS and describes their method of 
operation. Several terms, such as ``wheel slip'' that are used 
throughout this notice are discussed in detail and defined in the 
Appendix. When terms whose precise meaning affects the understanding of 
the agency's rationale are introduced, the reader could refer to the 
Appendix for a discussion of the term.
    Therefore, readers who lack a technical background and who desire a 
more complete understanding of this rulemaking may wish at this point 
to read the Appendix before moving on to the rest of the preamble.
    NHTSA has decided to require the installation of ``closed-
loop''<SUP>2 antilock systems on all heavy vehicles. The agency, in 
accordance with Supreme Court precedent that required the agency to 
consider mandating the installation of a particular type of automatic 
restraint system (i.e., ``airbags only'') for passenger cars,<SUP>3 is 
adopting a rule that defines antilock brake systems, in performance 
terms, as systems that ``automatically control the degree of rotational 
wheel slip<SUP>4 during braking'' through sensors and transmitters that 
measure, transmit, and generate signals concerning the rate of wheel 
angular rotation to controlling devices which adjust brake application 
pressure to prevent wheel lockup. In addition, for truck tractors, the 
rule prescribes a 30-mph braking-in-a-curve dynamic test on a low 
coefficient of friction surface.

    \2\A closed-loop (control) system is one which examines the 
output of the system and adjusts the input to the system in response 
to that output. This inclusion of the output (or some function of 
the output) as part of the input to such a system is referred to as 
feedback.
    \3\(Motor Vehicle Manufacturers' Association v. State Farm 
Insurance, 463 U.S. 29, (1983))
    \4\See the Appendix for a discussion of this term and 
directional stability.
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    Although some commenters characterized NHTSA's definition as an 
impermissible design standard, NHTSA has specifically sought to avoid 
imposing unnecessary design restrictions or impeding the future 
development of ABS, by adopting a definition that permits any antilock 
brake system that ensures feedback between what is actually happening 
at the tire-road surface interface and what the device is doing to 
respond to excessive wheel slip. To the extent that NHTSA's definition 
restricts design choices, e.g., by requiring a ``feedback'' system in 
which control devices must respond to signals that monitor wheel slip, 
the requirements are stated broadly and in performance terms. Such an 
approach is consistent with that adopted in numerous other Federal 
Motor Vehicle Safety Standards, including Standard No. 108 which 
requires vehicles to be equipped with specified lamps and reflective 
devices, Standard No. 111 which requires that vehicles be equipped with 
rearview mirrors, and Standard No. 208 which requires vehicles be 
equipped with safety belts.
    Moreover, the United States Court of Appeals for the Sixth Circuit 
has upheld a dimensional restriction on rectangular headlamps, 
reasoning that ``uniformity of headlamp size is an element of headlamp 
performance.''<SUP>5 Accordingly, NHTSA has decided to reject the 
conceptual objections to ``closed-loop'' ABS systems expressed by 
commenters whose economic self-interest militates against the 
requirement, including manufacturers of alternative, non-electronic 
braking systems that are incapable of sensing and adjusting braking 
pressures to control that wheel slip, and an association of fleet 
owners that may wish to avoid incurring the added expense of purchasing 
vehicles that are equipped with electronic ABS systems.

    \5\Chrysler Corp. v. DOT, 515 F.2d 1053, 1058-59 (1975).
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    Currently, all powered<SUP>6 heavy vehicles equipped with ABS are 
required to be equipped with an in-cab ABS malfunction indicator lamp 
indicating malfunctions in the powered vehicle's ABS. Today's final 
rule requires trucks (including truck tractors) equipped to tow another 
air-braked vehicle to be equipped with another, separate in-cab lamp 
indicating malfunctions in the ABS(s) of the towed vehicle(s). For an 
eight-year period, the amendment requires trailers to be equipped with 
an external ABS malfunction indicator that will be visible to the 
driver of the towing truck or truck tractor through the rearview 
mirror. In particular, the external trailer indicator lamp will provide 
information to the driver, if the trailer is being towed by an older 
vehicle that is not equipped with an in-cab lamp indicating trailer ABS 
malfunctions. In general, the indicators will provide valuable 
information about ABS malfunctioning to the driver and to maintenance 
and Federal and State inspection personnel.

    \6\By powered vehicle, the agency means a vehicle equipped with 
an engine that propels the vehicle. In contrast, a non-powered 
vehicle, such as a trailer, is towed by another vehicle.
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    In separate, related documents published elsewhere in today's 
Federal Register, NHTSA announces its decision to reinstate stopping 
distance requirements for air-braked heavy vehicles and to establish 
such requirements for hydraulically-braked heavy vehicles. In addition, 
to carry out the antilock requirement, the Federal Highway 
Administration (FHWA) is announcing its intent to require such systems 
on heavy vehicles to be operational.
    NHTSA is issuing this final rule on directional stability and 
control pursuant to the Motor Carrier Act of 1991, a part of the 
Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991. 
Section 4012 directs the Secretary of Transportation to initiate 
rulemaking concerning methods for improving braking performance of new 
commercial motor vehicles,<SUP>7 including truck tractors, trailers, 
and their dollies. Congress specifically directed that such a 
rulemaking examine antilock systems, means of improving brake 
compatibility, and methods of ensuring effectiveness of brake timing. 
The Act requires that the rulemaking be consistent with the Motor 
Carrier Safety Act of 1984 (49 U.S.C. Sec. 31147) and be carried out 
pursuant to, and in accordance with, the National Traffic and Motor 
Vehicle Safety Act of 1966 (Safety Act) (49 U.S.C. 30101 et seq.).

    \7\Vehicles with a gross vehicle weight rating (GVWR) of 26,001 
or more pounds.
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    NHTSA notes that, in the mid-1970's, Standard No. 121 was amended 
to include stringent stopping distance requirements, coupled with a 
``no lockup'' requirement, which had the effect of requiring heavy 
vehicles to be equipped with antilock brake systems. In response to a 
legal challenge, the U.S. Court of Appeals for the 9th Circuit 
invalidated the stopping distance and ``no lockup'' requirements in 
Standard No. 121, along with certain other provisions, holding that the 
standard was ``neither reasonable nor practicable at the time it was 
put into effect.''<SUP>8

    \8\PACCAR v. NHTSA, 573 F.2d 632 (9th Cir. 1978), cert. denied, 
439 U.S. 862 (1978)
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    As explained throughout this document, the underlying conditions 
related to equipping heavy vehicles with antilock brake systems differ 
markedly from 20 years ago when the petitioners challenged the agency 
in PACCAR. First, antilock brake technology has advanced dramatically 
since the mid-1970's, and antilock brake systems are now in widespread, 
everyday use, both in this country and [[Page 13218]] throughout the 
world. Second, NHTSA's extensive fleet study about heavy vehicle 
antilock systems demonstrates that these systems are reliable when 
placed in use. Third, the agency's testing of truck tractors equipped 
with antilock systems indicates that they provide significantly 
improved directional stability and control compared to vehicles without 
antilock systems. Fourth, while the antilock systems used in the mid-
1970s also incorporated significantly larger, more aggressive 
foundation brakes, which were sometimes incompatible with less 
aggressive systems on existing vehicles when the antilock system 
malfunctioned, the requirements being adopted today do not necessitate 
such aggressive brakes. Therefore, they do not have the potential for 
creating a more dangerous highway environment. Fifth, the performance 
requirements adopted in today's final rule do not raise practicability 
concerns. Based on these and other considerations discussed throughout 
this document, NHTSA believes that today's final rule satisfies the 
concerns raised by the PACCAR court.

II. Background

A. The Safety Problem: Loss of Control Crashes

    Crashes involving heavy vehicles result in a significant number of 
fatalities and injuries, and a significant amount of property damage 
each year. Based on available statistics, NHTSA has estimated the 
number of crashes in 1992 for several different groups of heavy 
vehicles. For heavy combination vehicles, the agency estimates that 
there were about 168,000 crashes. These crashes resulted in about 
13,600 injuries and 387 fatalities to the occupants of heavy 
combination vehicles and about 51,500 injuries and 2,452 fatalities to 
the occupants of the other vehicles involved. For truck tractors 
operating without a trailer, also known as ``bobtail'' truck tractors, 
the agency estimates that there were about 8,400 crashes, resulting in 
about 1,200 injuries and 39 fatalities to truck tractor occupants and 
about 2,600 injuries and 178 fatalities to occupants of other involved 
vehicles. For heavy single-unit trucks and school buses, the agency 
estimates that there were about 192,600 crashes, resulting in about 
15,700 injuries and 165 fatalities to truck and school bus occupants 
and about 48,300 injuries and 891 fatalities to occupants of other 
involved vehicles. For transit and intercity buses, the agency 
estimates that there were about 49,500 crashes, resulting in about 
19,500 injuries and 28 fatalities to bus occupants and about 9,100 
injuries and 230 fatalities to occupants of other involved vehicles.
    Based on analyses of both national and state accident data, NHTSA 
estimates that between 10 percent and 15 percent of the crashes 
involving heavy combination vehicles (including bobtail truck tractors) 
involved in a jackknife or other braking-induced instability or loss of 
control. For a more detailed discussion of the injury statistics, the 
reader should refer to the Final Economic Assessment (FEA) for this 
rulemaking.
    This rulemaking focuses on crashes involving loss-of-control. Such 
incidents result from braking-induced wheel lockup with subsequent loss 
of the ability of the vehicle's tires to generate ``stabilizing 
forces.''<SUP>9 This loss of tire stabilizing forces can result in 
either vehicle directional instability if it occurs at the vehicle's 
rear wheels or loss of steering control if it occurs at the vehicle's 
steering (front) wheels.

    \9\See the Appendix which defines and discusses this term.
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B. Braking Systems, Tires, Wheel Lockup, and Loss of Control Crashes

    When a vehicle driver makes a brake application that is too 
``hard'' for conditions, the driver is likely to lock some or all of 
the vehicle's wheels (i.e., the wheels will be ``sliding'' rather than 
``rolling''). Locking up wheels is more likely to occur under 
conditions where the maximum forces that can be generated by the 
vehicle's tires are reduced, i.e., when the vehicle is lightly loaded 
or empty and/or when the road is slippery. When wheel lockup occurs, 
vehicle loss-of-control can result. Incorporation of an ABS decreases 
the likelihood of wheel lockup, and increases the driver's ability to 
maintain control during severe braking maneuvers, that would otherwise 
lead to wheel lockup and resultant loss of directional stability and 
control, if the vehicle is not equipped with an ABS.

III. US and Foreign Activities Related to Stability and Control During 
Braking Performance

A. Early US Regulatory History

    NHTSA has been concerned about the safety of heavy vehicle braking 
systems since the agency's inception. On October 11, 1967, the 
predecessor of NHTSA, the FHWA's National Highway Safety Bureau, 
published a notice of its intention to promulgate brake standards for 
hydraulic and air-braked trucks and buses, and air-braked trailers. (32 
FR 14279.) The initial notice of proposed rulemaking (NPRM) for air-
braked systems proposed various requirements, including requiring 
vehicles equipped with such systems to stop within certain distances, 
from certain speeds, without leaving a 12-foot wide lane and without 
lockup of any wheel ``more than momentarily.'' (35 FR 10368, June 25, 
1970.) A companion NPRM for hydraulic brake systems proposed 
essentially identical performance requirements for heavy vehicles 
equipped with those systems. (35 FR 17345, November 11, 1970.) These 
notices proposed that heavy vehicles would have to stop from 60-mph 
within 216 feet on a surface with a skid number of 75.\10\ The ``no 
lockup'' provision was intended to minimize skidding, spinning, and 
jackknifing due to wheel lockup and loss of directional stability.

    \10\A skid number describes the friction properties of pavement. 
A skid number of 75 is representative of a dry surface with a 
relatively high coefficient of friction. See the Appendix for a 
discussion of this term.
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    In the final rule establishing Standard No. 121, the agency decided 
to increase the 60-mph stopping distance from 216 feet to 245 feet. (36 
FR 3817, February 27, 1971.) The final rule amending Standard No. 105 
to extend its applicability to heavy vehicles, also increased the 60-
mph stopping distance for those vehicles to 245 feet. (37 FR 17970, 
September 2, 1972.) The requirements for air-braked vehicles were to 
become effective on September 1, 1973, and those for hydraulic-braked 
vehicles, on September 1, 1974.
    Although neither standard specifically required antilock, NHTSA 
anticipated that manufacturers would equip heavy vehicles with antilock 
brake systems to comply with these requirements. The agency explained 
that the less stringent stopping distance was being required to reflect 
more accurately the vehicle performance given the test track road 
surface's friction characteristics.
    Since the required stopping distances were shorter than the 
stopping performance achieved by certain heavy vehicles, new, more 
aggressive foundation braking systems were necessary for those 
vehicles. In particular, vehicles with short wheelbases needed to have 
considerably more aggressive front axle brakes to meet the shorter 
stopping distance requirements. If not kept properly adjusted, these 
more aggressive front brakes might produce a brake ``pull'' to one 
side, which was disconcerting to drivers, particularly on vehicles 
without power steering. In addition, drivers were concerned about loss 
of steering control caused by wheel lockup on the 
[[Page 13219]] steering axle. At the time, most manufacturers equipped 
their vehicles with antilock devices because the standards required 
stops to be made without more than momentary lockup of the wheels. 
These devices served to prevent steering axle lockup problems as well, 
but there was concern that safety problems could result on short-
wheelbase, high-center-of-gravity vehicles, in the event that the 
antilock system should malfunction.
    NHTSA extended the effective dates for the stopping distance 
requirements in Standard No. 105 and Standard No. 121. (37 FR 3905, 
February 24, 1972; 38 FR 3047, February 1, 1973; 39 FR 17550, 17563, 
May 17, 1974.) Prior to the final effective date for Standard No. 105, 
the amendments pertaining to heavy vehicles were withdrawn, so the 
requirements for heavy hydraulic-braked trucks and buses never went 
into effect. (40 FR 18411, April 28, 1975.) Standard No. 121 became 
effective on January 1, 1975, for trailers, and on March 1, 1975, for 
trucks and buses. At that time, the 60-mph stopping distance 
requirement remained at 245 feet. However, after several revisions to 
the stopping distance requirements, NHTSA amended the standard by 
extending the 60-mph stopping distance requirement to 293 feet, as 
requested by Freightliner in a petition for reconsideration. (41 FR 
8783, March 1, 1976.)

B. PACCAR Case

    In January 1975, PACCAR (a truck manufacturer), the American 
Trucking Associations (ATA), and the Truck Equipment and Body 
Distributors Association (TEBDA) sued the agency, challenging the 
stopping distance requirements in Standard No. 121, which they believed 
required the use of antilock brake systems.
    Specifically, the petitioners challenged the 245-foot stopping 
distance. The subsequent increase to 293 feet, a distance that did not 
necessitate such aggressive front brakes, occurred after the suit was 
filed. The petitioners argued that the agency failed to demonstrate a 
safety need for the standard and that the testing procedures were not 
objective, impracticable, and unreasonable. TEBDA objected to the 
standard's certification requirements.
    In response to the suit, the stopping distance and ``no lockup'' 
requirements in Standard No. 121, along with certain other provisions, 
were invalidated by the United States Court of Appeals for the 9th 
Circuit in PACCAR. The court held that NHTSA was justified in 
promulgating a standard requiring improved air brake systems and 
stability mechanisms. However, after reviewing the record about 
reliability problems with antilock brake systems then in use, the court 
further held that the standard was ``neither reasonable nor practicable 
at the time it was put into effect.'' Id. at 640. Among the court's 
other findings were that the agency had a responsibility (1) to examine 
the results of its rulemakings by investigating more fully the safety 
of vehicles in use, (2) to assure that the new systems it requires are 
reliable when placed in use, and (3) to determine that its regulations 
do not produce a more dangerous highway environment than that which 
existed prior to government intervention. Based on these findings, the 
court stated that

    * * * those parts of the Standard requiring heavier axles and 
the antilock device should be suspended. The evidence indicates that 
this can be accomplished if we hold, as we do, that the stopping 
distance requirements from 60 mph are invalid * * * We hold only 
that more probative and convincing data evidencing the reliability 
and safety of vehicles that are equipped with antilock and in use 
must be available before the agency can enforce a standard requiring 
its installation.

Id. at 643.
    The court also ruled on the objectivity and practicability of the 
testing procedures in Standard No. 121. First, the court stated that 
road surface skid numbers used for testing certified vehicles were 
``ill-chosen'' where they assumed the use of a particular tire no 
longer in production. Id. at 644. Second, the skid number method of 
testing was not objective. Id. at 644. Third, the testing procedure was 
not practicable because fluctuations in skid numbers on a given road 
surface made it impracticable for manufacturers to conduct tests that 
assure that their vehicles will exactly meet the objective standard 
when tested by NHTSA. Id. at 644. Fourth, manufacturers are entitled to 
testing criteria that they can rely on with certainty. Id. at 644. 
Fifth, the standard failed to specify formal and reasonably specific 
testing criteria about the time intervals between tests, the duration 
of permissible wheel lockup during tests, and the amount of curving in 
testing track roadways. Id. at 645. Sixth, the agency's suggestions of 
alternative methods of satisfying the Safety Act's ``due care'' 
provision were inadequate since such alternatives were not set forth in 
the regulations. Id. at 645.
    The court remanded the matter to NHTSA to clarify certain 
provisions in Standard No. 121. In response to PACCAR, the agency 
issued several notices amending the standard to be consistent with the 
decision. (43 FR 39390, September 5, 1978; 43 FR 48646, October 19, 
1978; 43 FR 58820, December 18, 1978; 44 FR 46849, August 9, 1979.) In 
the September 1978 notice, the agency amended the standard to specify 
test procedures and conditions for frictional characteristics of the 
test track surface, duration of time intervals between road tests, 
duration of permissible wheel lockup during road tests, the amount of 
curving in the test track, and the means for establishing the 
frictional resistance of the road test surface. In the October 1978 
notice, the agency set forth its interpretation of PACCAR to guide 
continuing compliance with the standard. Specifically, the notice 
explained that the court had invalidated the ``no lockup'' provisions 
in S5.3.1 and S5.3.2 as they apply to trucks and trailers, along with 
the related stopping distances established for 60-mph stopping tests 
for heavy vehicles. That notice also amended the requirements to 
provide for ``due care certification.'' In the December 1978 notice, 
NHTSA responded to petitions for reconsideration of certain aspects of 
the September 1978 notice, including vehicle exclusions and road test 
procedures. The agency withdrew the changes to specification of initial 
brake temperatures, skid number ranges, and duration of wheel lockup 
that were made in the September notice. In the August 1979 notice, the 
agency further clarified its interpretation of certain findings of 
PACCAR.

C. US and Foreign Experience With ABS Since PACCAR

    As a result of the 1978 PACCAR decision, U.S. manufacturers chose 
to halt development and production of ABS for heavy vehicles. For 
instance, before the 1978 ruling, A-C Sparkplug, a domestic 
manufacturer of ABS, produced about 180,000 ABS units per year. By 
1984, it was producing only about 500 units annually.
    NHTSA continued to study the effectiveness of heavy truck antilock 
brake systems. Among other things, the agency studied the in-use 
experience with ABS in other countries, conducted performance testing 
of ABS equipped heavy vehicles, and conducted an extensive domestic 
fleet in-use test of ABS equipped heavy vehicles.
    In response to section 9107 of the Truck and Bus Regulatory Reform 
Act of 1988, NHTSA submitted a report to Congress titled ``Improved 
Brake Systems for Commercial Vehicles'' (Report No. DOT HS 807 706). 
(April 1991) After discussing crash data concerning heavy vehicle brake 
systems, the report examined factors related to braking effectiveness, 
stability and [[Page 13220]] control during braking, and braking system 
compatibility of heavy combination vehicles. Among other things, the 
report indicated that the stopping distances and directional stability 
of heavy vehicles could be improved by equipping those vehicles with 
ABS.
    With respect to the in-use experience with ABS in other countries, 
NHTSA conducted a study of the performance, reliability, and 
maintainability of in-service commercial air-braked vehicles equipped 
with ABS in Europe and Australia.\11\ At the time of the study in 1987, 
there were approximately 1.5 million ABS-equipped trucks and tractors, 
and 0.9 million ABS-equipped trailers in use in Western Europe, and 
92,000 trucks and tractors and 80,000 trailers in Australia. ABS market 
penetration, at that time, in Western Europe was estimated to be 4.5 
percent for trucks and tractors and 5.6 percent for trailers, while in 
Australia the comparable figures were 1.3 percent for trucks and 
tractors, and less than 1 percent for trailers. Based on data derived 
from interviews with fleets which were using ABS and surveys conducted 
by ABS and vehicle manufacturers, the reliability of ABS when equipped 
on European vehicles was estimated to be 1 to 2 ABS component failures 
per 1000 vehicles per month. Based on those data, it was predicted that 
between 4 and 20 malfunctions would occur with the 200 ABS-equipped 
truck tractors involved in the NHTSA-sponsored two-year in-service 
fleet study, which was subsequently performed between 1989-91. In fact, 
nineteen ABS components failed, which is within the range predicted by 
the European study.

    \11\``European/Australian Experience with Antilock Braking 
Systems in Fleet Service,'' U.S. Department of Transportation, 
NHTSA, DOT HS 807 269, March 1988.
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    Among the study's other findings were that maintenance was done 
only when a malfunction indicator activated; malfunction indications 
did not cause drivers to disrupt their operations and stop en route; no 
special maintenance was performed on the ABS beyond routine periodic 
inspections; no problems with electronic and radio frequency 
interference (RFI) were reported; with proper maintenance, ABS life was 
expected to equal that of the vehicle; and carriers reported that 
drivers liked driving ABS-equipped vehicles. Although some problems 
were encountered with wiring and connector failures, ABS manufacturers 
believed that their systems were generally reliable and expected future 
improvements.
    Since the completion of NHTSA's study, several European countries 
have issued regulations requiring heavy vehicles to be equipped with 
antilock brake systems. Specifically, the Economic Commission for 
Europe\12\ (ECE) Regulation No. 13 includes technical requirements for 
antilock systems in Annex 13 of its regulation.\13\ Annex 13 sets forth 
definitions of antilock brake systems and component parts, various 
``types'' of antilock systems, and test procedures. ECE's Annex 13 
specifies a design requirement and dynamic performance requirements. 
The European Economic Community (EEC Common Market) directive has 
identical requirements. As a result, since October 1, 1991, all heavy 
trucks (with GVWR greater than 16 metric tons), interurban buses (with 
GVWR greater than 12 metric tons), and heavy trailers (with GVWR 
greater than 10 metric tons) submitted for new type approvals in 
European countries adopting the standard have been required to be 
equipped with ABS. Accordingly, ABS have been installed on tens of 
thousands of European heavy vehicles that have traveled millions of 
miles over the last few years. All vehicles for which ABS is mandatory 
under Annex 13 are required to have a Category 1 system. Such systems 
are essentially the same as those required by today's final rule.

    \12\The Economic Commission for Europe (ECE) is a United Nations 
organization comprised of European countries plus the United States 
and Canada, which establishes requirements applicable to the type 
approval of motor vehicles and other products for sale in those 
nations that choose to apply the requirements.
    \13\Annex 13 is titled ``Requirements Applicable to Tests for 
Braking Systems Equipped with Anti-Lock Devices (Wheel-Lock 
Preventers).'' It is Annex 13 of ECE Regulation No. 13, which is 
titled ``Uniform Provisions Concerning the Approval of Vehicles with 
Regard to Braking.'' Regulation No. 13 is Addendum 12 of the 
``United Nations Agreement Concerning the Adoption of Uniform 
Conditions of Approval and Reciprocal Recognition of Approval for 
Motor Vehicle Equipment and Parts,'' done at Geneva on March 20, 
1958, which is commonly known as the ``1958 Agreement.''
---------------------------------------------------------------------------

    With respect to performance testing, NHTSA has issued two reports 
on the stopping distance capability of several different types of heavy 
air-braked vehicles at various loading conditions.\14\ The agency also 
tested some vehicles equipped with ABS, thus allowing comparisons about 
stopping distances with and without these devices. At the beginning of 
each test series, these vehicles were equipped with new tires and with 
new original equipment brake system components to provide consistency 
in test results. At the beginning of each testing series, the tests 
were conducted on various vehicles (school buses, transit buses, single 
unit trucks, tractor trailers) at the loaded and empty conditions and 
with various equipment (with ABS activated and deactivated). All the 
tests were straight line stops from 60 mph on a dry concrete surface. 
The test results indicated that: (1) All stops made with ABS were 
stable, regardless of whether the vehicle was operating fully loaded or 
empty, and (2) stopping distance improvements with ABS (compared to no 
ABS) were greatest in the bobtail configuration (+47 percent in one 
case), were significant with an empty trailer (+29 percent in one case) 
and were smallest (+4 percent) in the fully loaded condition.\15\

    \14\``NHTSA Heavy Duty Vehicle Brake Research Program Report No. 
9, Stopping Distances of 1988 Heavy Vehicles,'' (DOT HS 807 531, 
February 1990)
    \15\DOT HS 807 531, Table 4, page 19; Table 5, page 23; Table 6, 
page 25)
---------------------------------------------------------------------------

    NHTSA's fleet testing program of ABS-equipped truck tractors 
evaluated the reliability, maintainability, and durability of 200 truck 
tractors equipped with ABS. The fleet study found that current 
generation ABSs are reliable and can be successfully installed on 
commercial motor vehicles.<SUP>16 The agency added trailers to the 
fleet study program in 1990-1991 and found similar results. A copy of 
that study has been submitted to the public docket.<SUP>17 The findings 
of the fleet testing program are discussed later in this preamble.

    \16\``An In-Service Evaluation of the Reliability, 
Maintainability, and Durability of Antilock Braking Systems (ABS) 
for Heavy Truck Tractors,'' (DOT HS 807 846, Final Report, March 
1992.)
    \17\``An In-Service Evaluation of the Performance, Reliability, 
Maintainability, and Durability of Antilock Braking Systems (ABSs) 
for Semitrailers'' (DOT HS 808 059, Final Report, October 1993.)
---------------------------------------------------------------------------

IV. Advance Notice of Proposed Rulemaking (ANPRM)

    On June 8, 1992, NHTSA responded to Congress' 1991 mandate in ISTEA 
by publishing an advance notice of proposed rulemaking (ANPRM) 
announcing the agency's interest in measures to improve the directional 
stability and control of heavy vehicles during braking. (57 FR 24212.) 
The advance notice stated the agency's tentative conclusion that ABS 
represents the best available and most reliable technology to reduce 
jackknifing and other loss-of-control crashes during braking. The 
notice posed questions about such matters as the occurrence of loss-of-
control crashes; the availability and performance of systems to improve 
directional stability and control under all conditions of braking and 
vehicle [[Page 13221]] load; potential regulatory approaches to improve 
the directional stability and control of heavy vehicles during braking, 
including anticipated performance requirements, test procedures, and 
equipment requirements; a schedule for implementing requirements; 
diagnostic equipment to ensure in-use functioning of the systems; and 
anticipated costs of such requirements.

V. Agency Proposal

    On September 28, 1993, NHTSA proposed to amend Standard No. 105 and 
Standard No. 121, to add requirements that would improve the 
directional stability and control of heavy vehicles during braking. (58 
FR 50738.) NHTSA decided to propose that each heavy vehicle must be 
equipped with an antilock braking system that satisfies the agency's 
proposed definition of ABS. In addition, as a verification of the 
performance of the ABS, the agency proposed that a heavy vehicle comply 
with a braking-in-a-curve test.
    NHTSA stated that, in proposing these amendments, its overriding 
goal was to ensure the directional stability and control of heavy 
vehicles during braking. The agency stated that, to ensure adequate ABS 
performance by means of dynamic test requirements, it would need to 
establish a broad array of performance requirements that would test the 
directional stability and control of vehicles under a number of loading 
conditions, travel speeds, and deceleration rates, and on a wide 
variety of road surfaces, including roads that are dry, wet, icy, and 
``split mu.'' In addition, to ensure that directional stability and 
control are not provided at the expense of stopping distance, each of 
these tests would need to require the vehicle to stop within a 
specified distance.
    NHTSA explained, however, that an approach that relied exclusively 
on dynamic test requirements would raise serious practicability 
concerns, given the inherent variability of stopping distance 
performance on low coefficient of friction surfaces and the costs 
associated with requiring such an extensive array of dynamic 
performance test requirements. NHTSA, therefore, focused its efforts on 
expressly requiring that heavy vehicles be equipped with ABS, and on 
supplementing that requirement with feasible and practicable dynamic 
tests that check the directional stability and control, and stopping 
distance of vehicles under a limited set of circumstances that may be 
experienced in the real world.
    The proposal that heavy vehicles be equipped with antilock systems 
would have required that the front axle and at least one rear axle of 
each heavy vehicle be equipped with an ABS that would automatically 
control rotational wheel slip during braking by (1) sensing the rate of 
angular rotation of the wheels, (2) transmitting signals regarding the 
rate of wheel angular rotation to one or more devices which interpret 
those signals and generate controlling output signals, and (3) 
transmitting those controlling signals to one or more devices which 
adjust brake actuating forces in response to those signals. The agency 
stated its belief that these characteristics, specified in the 
definition of ABS, would permit the installation of any antilock 
braking system, provided that it is a ``closed-loop'' system that 
ensures feedback between what is actually happening at the tire-road 
surface interface and what the device is doing to respond to excessive 
wheel slip. NHTSA tentatively concluded that these criteria were 
necessary to ensure the introduction of systems that control wheel slip 
and sustained wheel lockup under a wide variety of real world 
conditions and thus would significantly improve safety.
    In addition, the NPRM contained a detailed discussion of the 
braking-in-a-curve test, including the test track's configuration, lane 
width, and test surface, the vehicle's test speed, the type and number 
of brake applications, loading conditions, control trailer 
requirements, and the initial brake temperature.
    NHTSA also proposed requirements for the ABS malfunction lamps and 
the power source for trailer antilock systems. The agency also 
addressed such considerations as requirements for diagnostic systems, 
the types of vehicles to be covered by the rulemaking, the 
implementation schedule for the proposed requirements, the rulemaking's 
potential effects on intermediate and final stage manufacturers and 
trailer manufacturers, and its costs and benefits.

VI. Comments on the Proposal

    NHTSA received over 60 comments in response to the NPRM. Commenters 
included heavy vehicle manufacturers, brake manufacturers, safety 
advocacy groups, heavy vehicle users, trade associations, State 
entities, and other individuals.
    Most commenters agreed that the agency should issue requirements to 
improve the stability and control of heavy vehicles during braking, 
thereby reducing the number of loss-of-control crashes. Advocates for 
Highway and Auto Safety (Advocates), the Heavy Duty Brake Manufacturers 
Council (HDBMC), the Insurance Institute for Highway Safety (IIHS), and 
Rockwell WABCO generally supported the agency's proposal to require 
heavy vehicles to be equipped with an ABS. These commenters stated that 
ABS will improve vehicle safety by providing improved braking 
performance and vehicle stability and control during braking.
    The American Automobile Manufacturers Association (AAMA)\18\, the 
American Trucking Associations (ATA), and fleet operators expressed 
mixed support for the rulemaking. AAMA stated that it ``reluctantly 
accepts the design specific proposal,'' given its concerns about the 
proposed braking-in-a-curve test procedure. ATA stated that it supports 
the use of ABS, but is concerned that the proposed effective dates 
would require universal use of ABS too soon to assure safety and 
reliability. AAMA and ATA stated that they would fully support the 
rulemaking, if the agency revised various aspects of the proposals. 
AAMA was primarily concerned about the practicability of the braking-
in-a-curve test. ATA was primarily concerned about the ABS equipment 
requirement and alleged problems with the reliability of separate 
tractor-to-trailer electrical cables/connecters. The agency notes that 
some of ATA's requested revisions would be major departures from the 
original proposal.

    \18\AAMA submitted joint comments on behalf of eight major 
domestic manufacturers of heavy vehicles: Chrysler, Ford, 
Freightliner, General Motors (GM), Mack Trucks, Navistar, PACCAR, 
and Volvo-GM).
---------------------------------------------------------------------------

    The National Private Truck Council (NPTC), the National Truck 
Equipment Association (NTEA), the National Association of Fleet 
Administrators (NAFA), and the National Association of Trailer 
Manufacturers (NATM) opposed requiring heavy vehicles to be equipped 
with ABSs. These commenters were primarily concerned about the costs 
that an ABS requirement would impose on fleets, final stage 
manufacturers of vehicles produced in multiple stages, and small 
trailer manufacturers. NTEA stated that it would be impracticable for 
final stage manufacturers to certify compliance with the braking-in-a-
curve test.
    Commenters also addressed specific issues raised in the NPRM, 
including the proposal to require vehicles to be equipped with ABS, the 
type of and definition for ABS, the braking-in-a-curve test procedure, 
the implementation schedule for the [[Page 13222]] requirements, the 
malfunction indicator requirements, the power requirement, and the 
rulemaking's cost. A more specific discussion of the comments, and the 
agency's responses, are set forth below.

VII. Agency's Supplemental Proposal

    Based on its analysis of comments on the NPRM and other available 
information, NHTSA issued a supplemental notice of proposed rulemaking 
(SNPRM) proposing a modified implementation schedule for the 
requirements in the agency's September 1993 NPRM and a requirement for 
independent wheel control on at least one axle. (59 FR 17326, April 12, 
1994.)
    With respect to leadtime, the agency proposed concurrent effective 
dates for the heavy vehicle stability and control requirements and for 
the heavy vehicle stopping distance requirements. Specifically, the 
agency proposed the following implementation schedule for both sets of 
requirements:

Truck tractors--2 years after final rule (1996)
Trailers--3 years after final rule (1997)
Air-braked single unit Trucks and buses--3 years after final rule 
(1997) Hydraulic-braked single unit trucks and buses--4 years after 
final rule (1998)

    With respect to independent wheel control, NHTSA proposed to 
require heavy vehicles to be equipped with an ABS that controls the 
wheels on at least one front and one rear axle, and independently 
controls the wheels on at least one of these two axles. The agency 
tentatively concluded that this would provide a necessary level of 
stopping distance performance on low mu and split mu surfaces. The 
agency posed a number of questions about the need for independent wheel 
control.

VIII. Comments on the Supplemental Proposal

    NHTSA received comments from AAMA, other vehicle manufacturers, 
brake manufacturers, safety advocacy groups, ATA, and others.<SUP>19 
Aside from ATA, almost all the commenters favored the proposed 
implementation schedule. Several commenters, including AAMA, Ford, 
Bendix, and Midland-Grau were concerned that the proposed requirements 
addressing independent wheel control were unreasonably design 
restrictive.

    \19\Comments on the SNPRM will be specifically labeled as such. 
Other comments will be assumed to be in response to the NPRM.
---------------------------------------------------------------------------

    Among the other issues raised by commenters were whether the 
proposal is a performance requirement, alleged reliability and 
maintenance problems with ABS, alleged safety problems caused by ABS, 
the regulation's benefits and costs, its applicability to hydraulic 
systems, and the possible need for a phased-in implementation schedule 
and a separate power circuit for operating the ABS.

IX. Agency Decision

A. Requirement for and Definition of ABS<SUP>20

    \20\The reader may wish to review the Appendix which provides a 
technical explanation of how antilock brakes work, including various 
methods of wheel control.
---------------------------------------------------------------------------

    In developing the proposal for this rulemaking, NHTSA considered 
what requirements are necessary to ensure improved stability and 
control for heavy vehicles. Among other things, the agency considered 
whether adequate performance relating to stability and control could be 
ensured solely by means of dynamic vehicle performance test 
requirements.
    The agency stated in the NPRM its belief that, in order for an 
approach relying solely on dynamic tests to be successful, it would be 
necessary to establish a broad array of dynamic performance 
requirements that would test the directional stability and control of 
vehicles under a variety of loading conditions, travel speeds, and 
deceleration rates, and on a variety of road surfaces, including ones 
that have coefficients of friction that are low, high, and split mu. In 
addition, in order to ensure that stopping distance performance is not 
compromised in the attempt to improve directional stability and control 
during braking, it would be necessary for these performance 
requirements to specify maximum stopping distances.
    NHTSA explained, however, that the poor correlation between 
stopping distance performance and the peak friction coefficient<SUP>21 
(PFC) of low coefficient of friction surfaces, combined with the costs 
associated with such an extensive array of dynamic performance 
requirements, would, at this time, raise serious practicability 
concerns about any approach that included such an array of dynamic test 
requirements.<SUP>22 NHTSA therefore focused its efforts on a single 
provision expressly requiring that heavy vehicles be equipped with 
antilock systems, and on identifying feasible and practicable dynamic 
tests that could supplement that provision by directly assessing the 
directional stability, control and stopping distance of vehicles under 
some of the wide variety of circumstances that may be experienced in 
the real world.

    \21\See the Appendix for a discussion of this term.
    \22\``MVMA/NHTSA/SAE Round Robin Brake Test,'' Transportation 
Research Center of Ohio, Report No. 091194, August 26, 1991.
---------------------------------------------------------------------------

    This section discusses the proposed provision expressly requiring 
that heavy vehicles be equipped with antilock systems. More 
specifically, NHTSA proposed to require that each heavy vehicle be 
equipped with an ABS that satisfies the following definition:

    ``Antilock braking system'' means a portion of a service brake 
system that automatically controls the degree of rotational wheel 
slip during braking by:
    (1) sensing the rate of angular rotation of the wheels;
    (2) transmitting signals regarding the rate of wheel angular 
rotation to one or more devices which interpret those signals and 
generate responsive controlling output signals; and
    (3) transmitting those controlling signals to one or more 
devices which adjust brake actuating forces in response to those 
signals.

    In developing this definition, the agency specifically sought to 
avoid unnecessary design restrictions or impede the future development 
of ABS. NHTSA stated in the NPRM that it believed that the proposed 
requirement would permit any ABS, provided that it was a closed-loop 
system that ensures feedback between what is actually happening at the 
tire-road surface interface and what the device is doing to respond to 
changes in wheel slip.
    For a number of reasons discussed in the NPRM (and below), NHTSA 
tentatively concluded that a device that satisfies these criteria is 
necessary in order to prevent wheel lockup under a wide variety of real 
world conditions, thereby significantly improving safety.
    A number of commenters, including vehicle manufacturers and brake 
manufacturers, recognized the practicability problems currently 
associated with some dynamic performance requirements and accordingly 
supported the agency's proposal to require heavy vehicles to be 
equipped with ABSs. AAMA stated that despite its strong preference for 
what it termed ``performance requirements,'' it would accept an 
explicit ABS requirement, provided that the braking-in-a-curve test is 
not adopted and the effective date for the proposed stopping distance 
requirement is made concurrent with the other effective dates for this 
rulemaking.<SUP>23 That organization stated that, in general, 
manufacturers ``much prefer performance over design specifications 
because performance [[Page 13223]] requirements allow new, improved and 
more cost-efficient technological means to achieve desired safety 
ends.'' Nevertheless, AAMA indicated that it was willing to accept an 
ABS equipment requirement because it believes there are significant 
practicability problems associated with various dynamic tests that the 
agency has considered, including the braking-in-a-curve test.

    \23\AAMA's specific concerns about the braking-in-a-curve test 
are discussed in a later section of this document.
---------------------------------------------------------------------------

    Similarly, Rockwell WABCO stated that it ``reluctantly accepts the 
proposal for an ABS equipment standard rather than a performance 
standard.'' That commenter stated that it normally opposes equipment 
standards since they have the potential of restricting the 
implementation of new technology. However, it stated that, in this 
case, ``the current difficulty in formulating valid, repeatable 
performance criteria prohibit a true performance standard at this 
time.'' Rockwell WABCO concluded that ``the proposed combination of an 
equipment specification and a performance test is both understandable 
and acceptable'' for now.
    Advocates stated that it is convinced that:

    The agency's resolve to mandate a basic level of ABS as required 
equipment on all tractors, trucks, trailers, and buses with 
verification of desirable safety performance gained through a single 
major operating test, is the most appropriate way to ensure that the 
substantial safety benefits of heavy vehicle ABS are realized 
quickly.

    Midland-Grau stated that the characteristics specified in the 
proposed definition will permit any antilock brake system, provided 
that it is a ``closed-loop'' system that ensures feedback between what 
is actually happening at the tire-road surface interface and what the 
device is doing to respond to changes in wheel slip.
    Mr. John Kourik, a brake engineer, stated that the proposed 
definition:

1. Selects the proper technology to assure optimum stability and 
control, [and]
2. Supplements the intent of the original definition with a high 
degree of sophistication. This should eliminate the inferior 
mechanisms and devices that have been offered by `toying' with the 
brevity of the original definition while making representations and 
distorted claims to suggest equivalency to ABS. Thus, the new 
definition should end the ``smoke and mirrors'' promotions of 
alleged substitutes for ABS.

    According to Mr. Kourik, the proposed definition would preclude the 
use of unsophisticated equipment that does not sense changes in the 
wheel rotation rate, e.g., equipment such as mechanical devices, 
pneumatic dampeners, hydraulic dampeners, hydro/mechanical units, and 
electro/mechanical units.
    Other commenters strongly opposed the proposed ABS requirement. ATA 
argued that NHTSA had proposed a ``design standard for ABS'' that is 
``unlawful because it is contrary to the agency's statutory mandate to 
issue only performance standards.'' Citing the statutory definition of 
``motor vehicle safety standard,'' that organization stated that, under 
the Safety Act, the requirements in Federal motor vehicle safety 
standards must prescribe performance, not design obligations.
    ATA claimed that, despite the statutory mandate, much of the 
agency's proposal represents design requirements. Specifically, ATA 
stated that there were additional impermissible design aspects to the 
proposal, including the definition of ABS, and the requirements for 
trailer electrical power to be transmitted by a separate circuit 
specifically provided for that purpose and for warning systems to be 
electrical.
    ATA also argued that the proposed definition for ABSs is 
unnecessarily design-restrictive, and would stifle innovation and 
require continual updating of the standard. ATA stated that the 
requirements would preclude anything but electronic systems, thereby 
prohibiting mechanical systems. That organization also argued that the 
requirements would impair efforts to develop new electronic 
technologies.
    Several small companies which manufacture or sell brake products 
also argued that the proposed requirements are inappropriately design-
restrictive. They argued that NHTSA should change the proposed 
definition of ABS so that devices other than computerized ABS can be 
used to meet the requirements. Trade International Corporation (TIC) 
argued that the proposed definition for ABS is fundamentally flawed 
because it does not specify what the system is supposed to accomplish 
but rather specifies how the system is supposed to work. It argued that 
a system could satisfy the definition but not accomplish the desired 
function.
    After carefully considering the comments, NHTSA has decided to 
adopt the proposed requirement for and definition of ABS. The agency's 
response to the comments, including a more detailed discussion of some 
of the comments summarized above, is presented in the sections which 
follow.
1. Legal Authority
    NHTSA disagrees with ATA's allegation that the agency does not have 
the statutory authority to issue a ``design standard.'' NHTSA's 
longstanding position<SUP>24 on this subject, which is presented in the 
form of a hypothetical discussion concerning the agency's authority to 
regulate the width of motor vehicles, is set forth below:

    \24\This discussion has been presented in past NHTSA letters, 
including a May 2, 1979 letter to the Insurance Institute for 
Highway Safety.
---------------------------------------------------------------------------

    We believe that the National Traffic and Motor Vehicle Safety 
Act * * * would permit issuance of a safety standard that regulated 
or limited vehicle width, if it were found that such a regulation 
``meets the need for motor vehicle safety'' (Sec. 103(a), 15 U.S.C. 
1392(a)). As is true with every motor vehicle safety standard, 
however, it would be necessary to establish a reasonable, objective 
basis for the conclusion that this regulation can be justified by 
safety benefits obtainable, to avoid a judicial conclusion that the 
action is ``arbitrary, capricious, [or] an abuse of discretion.'' (5 
U.S.C. 706). The issue, in other words, would not be one of basic 
authority, but of justification.
    Although it may be argued that such a safety standard would be a 
regulation of ``design, and not performance'', for reasons set forth 
below we feel that this argument is insubstantial and reflects an 
inadequate understanding of the Act and the safety standards * * *.
    Section 102(2) of the Act (15 U.S.C. 1391) defines a motor 
vehicle safety standard as ``a minimum standard for motor vehicle 
performance, or motor vehicle equipment performance, which is 
practicable, which meets the need for motor vehicle safety and which 
provides objective criteria.'' Section 103(f) of the Act also 
requires the standards to be ``reasonable, practicable and 
appropriate for the particular type of motor vehicle * * * for which 
it is prescribed.''
    It has sometimes been suggested that the inclusion of the word 
``performance'' in this definition suggests the existence of a 
dichotomy between vehicle design and performance. We do not, 
however, consider that there is a dividing line between standards 
that regulate performance and standards that affect design. Senator 
Magnuson recognized the absence of any dichotomy when he said that 
some safety standards would necessarily determine the configuration 
of some vehicle components. (112 C.R. 20600 (Aug. 31, 1966.)). In 
fact, all safety standards have a strong effect on vehicle or 
equipment design, in spite of their being phrased in ``performance'' 
terms. This is necessarily so since the design of vehicles and 
equipment determines the quality of their performance. (Some 
confusion over ``design'' may arise from the common use of the word 
to mean appearance or shape. In our work, however, the word means 
the sum of all of the characteristics that a product is intended to 
have, e.g., size, weight, interrelationship of components, 
materials, and markings.)
    Each of our safety standards meets the need for motor vehicle 
safety by specifying requirements for the performance of a 
particular vehicle or item of equipment. Any design that will 
satisfy the requirements may be used for the system or item of 
equipment. The extent to which the choice of a design 
[[Page 13224]] is restricted by a particular standard is purely a 
matter of degree, depending on the specificity of the requirement. 
We try, in carrying out the congressional mandate, to make the 
requirements as broad as the safety need allows. We will probably 
never have to reach the level of a true ``design specification'' as 
an engineer would use the term, i.e., a detailed description of 
every significant aspect of a product including the materials and 
manufacturing processes used. This is true because the standards 
deal only with the safety-related characteristics of the regulated 
items, e.g., the height, width, and strength of a head restraint and 
the light output of a headlamp.
    In some cases, the configuration of a vehicle component or item 
of equipment is the characteristic that relates to safety. A good 
example of this is our standard on transmission shift levers (No. 
102), which standardizes the position of Park, Reverse, etc., on all 
our passenger cars today. There, standardization of at least some 
external aspects of the component is needed for safety's sake. A 
second example is our standard on control identification (No. 101), 
where again an enforced similarity in the words and symbols used to 
identify vehicle controls is the heart of the safety requirement * * 
*.
    Thus, if the width of a vehicle is, in fact, the characteristic 
that is found to require regulation for safety purposes (analogously 
to the spacing of headlamps in Standard 108 or the width of a head 
restraint in Standard 202), there should be no doubt of NHTSA's 
authority to regulate it.

    NHTSA's requirements for specified safety equipment are at the 
heart of many of the Federal motor vehicle safety standards. Indeed, 
thousands of the lives saved and the injuries reduced or prevented by 
Federally-mandated safety features are the direct result of 
requirements for specific types of equipment. Most prominent among 
these requirements is the 25-year-old requirement in Standard No. 208, 
Occupant Crash Protection, for the installation of specific types of 
safety belts. This is the most heavily judicially and Congressionally 
scrutinized safety standard, and no question has ever been raised about 
the agency's authority to issue such a standard.
    Equipment requirements are critical for helping to ensure that 
vehicles have many of the items necessary to guarantee safety. For 
example, it is critical for drivers to be able to see where they are 
going, and for their vehicle to be seen by other drivers. The safety 
standards therefore require items that are critical for driver 
visibility and vehicle conspicuity in the rain and at night. Standard 
No. 104 requires vehicles to have a windshield wiping system, Standard 
No. 108 requires vehicles to be equipped with specified lamps and 
reflective devices, Standard No. 111 requires that vehicles be equipped 
with rearview mirrors, and Standard No. 205 specifies the types of 
glazing which may be used in various locations.
    Many other safety standards, including the existing brake 
standards, specify equipment requirements that meet equally important 
safety needs. Thus, the extremely narrow reading of the word 
``performance'' advocated by ATA is inconsistent with the entire 
history of the Federal program for motor vehicle safety standards, and 
indeed with a majority of the existing standards.
    The case law addressing this issue has clearly upheld NHTSA's 
authority to issue safety standards that directly affect design. In 
Chrysler v. DOT, 515 F.2d 1053 (6th Cir. 1975), for example, the court 
upheld a dimensional restriction on rectangular headlamps. That court 
reasoned that:

    Uniformity of headlamp size is an element of headlamp 
performance. Design freedom would inhibit safety, and certainly the 
congressional purpose of encouraging safety-related competition 
among manufacturers is meaningless in this context.
    We conclude that the dimension restriction at issue here 
essentially serves to ensure proper headlamp performance and lies 
within the regulatory authority granted by Congress to the NHTSA.

515 F.2d at 1058, 1059.
    Moreover, in Motor Vehicle Manufacturers Association v. State Farm, 
463 U.S. 29 (1983), the United States Supreme Court held that, before 
rescinding a general requirement for automatic restraints because one 
type of automatic restraint (e.g., the detachable automatic safety 
belt) might be ineffective, NHTSA must consider establishing an airbag-
only requirement. The Court further stated that the agency could 
prohibit detachable automatic safety belts if the agency determined 
that they would not provide effective passenger protection. Therefore, 
the Supreme Court clearly recognized NHTSA's authority both to require 
specific safety equipment deemed to provide superior safety protection 
and to prohibit specific equipment that the agency deemed to provide 
inferior safety protection.
    NHTSA therefore rejects ATA's argument concerning the agency's 
authority to require specified safety equipment. However, as indicated 
above, the agency does, in carrying out its statutory mandate, attempt 
to make its safety requirements as broad as the safety need allows. The 
relevant issue for this rulemaking is thus not whether the agency 
proposed an unlawful ``design standard,'' but instead whether the 
proposed requirement/definition for ABS is unnecessarily design-
restrictive. For the reasons discussed below, NHTSA has concluded that 
each element of the proposed requirement/definition for ABS is 
necessary to meet the safety need for improved stability and control.
2. Elements of the Requirement/Definition for ABS
    Far from proposing a detailed ``design requirement,'' NHTSA simply 
proposed to require vehicles to be equipped with an ABS consistent with 
the generally understood meaning of that term among brake engineers. 
The agency used this approach precisely to avoid imposing unnecessary 
design restrictions or impeding the future development of ABS. As 
discussed in the NPRM, the definition is sufficiently broad to permit 
the installation of any antilock braking system, provided that it is a 
``closed-loop'' system that ensures feedback between what is actually 
happening at the tire-road surface interface and what the device is 
doing to respond to changes in wheel slip.
    In developing the proposed definition, the agency relied on the 
Society of Automotive Engineers<SUP>25 (SAE) J656 (Apr88) ``Automotive 
Brake Definitions and Nomenclature'' and the Economic Commission for 
Europe's Regulation 13, Annex 13 (1988). SAE J656 refers to ABSs as 
``wheel slip brake control systems'' that automatically control 
rotational wheel slip during braking. Among the terms related to ABS 
that are defined in SAE J656 are ``modulator'' and ``wheel slip 
sensor.'' These terms are used in SAE's test procedure for antilock 
systems, as specified in SAE J46 (JUN80) ``Wheel Slip Brake Control 
System Road Test Code.'' Similarly, Annex 13 of ECE Regulation 13 
refers to ``anti-lock devices'' as systems which automatically control 
the degree of slip, in the direction of rotation of the wheel(s). The 
Annex 13 definition of ABS also states that such devices include ``a 
sensor or sensors, a controller or controllers and actuating valves.'' 
The agency's proposed definition of ABS incorporated the terms set 
forth in SAE J656 and ECE Regulation 13 to reflect the attributes of 
antilock systems as commonly understood by the automotive engineering 
industry.

    \25\The Society of Automotive Engineers is a voluntary 
professional organization that establishes recommended practices 
related to various aspects of motor vehicles.
---------------------------------------------------------------------------

    The proposed equipment requirement specifies simply that vehicles 
must be equipped with an ABS which is defined [[Page 13225]] as a 
system that automatically controls the degree of rotational wheel slip 
during braking, by (1) sensing the rate of wheel rotation, (2) 
transmitting signals regarding the rate of wheel rotation to a device 
which interprets those signals and generates responsive controlling 
signals, and (3) transmitting those controlling signals to a device 
which adjusts brake actuating forces in response to those signals. For 
reasons discussed below, each of these elements is necessary to meet 
the need for safety. In addition, the definition only states the 
performance required of the ABS components, not how the components must 
detect wheel rotation, etc.
    As discussed earlier in this preamble, the safety problem being 
addressed by this rulemaking is that whenever the driver applies the 
brakes with too much force relative to extant tire and road conditions, 
sustained wheel lockup occurs. This usually results in loss of vehicle 
directional stability and/or steering control; i.e., a jackknife, spin-
out or skid, and often a crash. Such sustained lockup most often occurs 
when the road is slippery or when the vehicle is lightly loaded or has 
no cargo. This is because drivers are likely to make a hard brake 
application in a panic situation, and the resulting braking forces 
easily cause lockup when the road is slippery or when the vehicle is 
lightly loaded or empty. Moreover, drivers are unable to sense lockup 
quickly enough to control it.<SUP>26

    \26\``Improved Brake Systems for Commercial Motor Vehicles,'' 
DOT 807 706 Section 3.2.2; pages 3-5.
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    In order to address this safety problem, NHTSA has determined that 
it is necessary to prevent the brake system from generating forces that 
result in uncontrolled lockup. This need is addressed in part by the 
first element of the requirement/definition: each ABS must 
automatically control the degree of rotational wheel slip during 
braking.<SUP>27 Automatic control is necessary since drivers cannot 
control lockup in an emergency situation. By the time a driver can 
sense that lockup has occurred, it is often too late to prevent the 
sustained lockup that results in loss of directional stability or 
control.

    \27\As discussed in the Appendix, wheel slip refers to the 
proportional amount of wheel/tire skidding relative to vehicle 
forward motion, and lockup is simply the condition of 100 percent 
wheel slip.
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    The second element of the requirement/definition (sensing rate of 
wheel rotation and transmitting signals about the rate to a device that 
generates responsive control signals) is necessary to ensure that 
lockup will be prevented or controlled for all road surfaces and under 
all load conditions, and also to ensure that stability is not provided 
at the expense of stopping distance. The prevention of sustained 
lockup, and resulting loss of directional stability and control, should 
not be accomplished simply by putting weak brakes on the vehicle or 
lowering braking forces under all conditions. Thus, in addressing this 
safety problem, the agency must consider the twin goals of preventing/
controlling lockup and ensuring good stopping distance under all road 
surface and load conditions.
    In a braking situation, the more the driver depresses the brake 
pedal, and thereby increases braking forces, the more quickly the 
vehicle will stop, so long as the braking force is not so high that it 
causes wheel lockup. Thus, if stopping distances are to be minimized 
during braking, it is necessary to permit the hydraulic or air pressure 
to rise to a point just below the point where lockup would occur.
    Moreover, the amount of pressure that causes lockup will vary 
dramatically depending on the road surface and vehicle loading. In 
order to ensure that braking force rises to a point just below the 
point where lockup would occur, it is necessary for an ABS to sense 
either each of the factors on which lockup is dependent, i.e., road 
surface friction, vehicle loading, dynamic weight transfer during 
braking, condition of brake linings, etc., or the product of all of 
those factors, i.e., the rate of wheel rotation from which wheel slip 
can be determined. Since it may not be technologically feasible for an 
ABS to sense all of the factors which may lead to lockup, the 
definition specifies that an ABS must sense the product of those 
factors, i.e., the rate of wheel rotation.
    The rest of the second element of the definition is necessary to 
ensure that an ABS uses the relevant information, i.e., rate of wheel 
rotation, to control wheel slip and prevent lockup. The relevant 
information must be transmitted to a device which interprets the 
information and generates responsive controlling signals. Those 
controlling signals must then be transmitted to a device which adjusts 
brake actuating forces in response to those signals.
    NHTSA has determined, based on all available information, that a 
device that lacks any one of the elements specified in the definition 
could not meet the need for safety addressed by this rulemaking, since, 
for the reasons discussed above, its operation would not be dependent 
on factors that are relevant to the desired safety performance.
    The agency notes that while several commenters asserted that the 
proposed definition is unnecessarily design restrictive, none attempted 
to explain how a device not meeting one or more of the elements could 
ensure stability and control for heavy vehicles for a wide range of 
test surfaces and loading conditions.
    Most of the commenters arguing that the proposed definition is 
unnecessarily design restrictive were small companies which manufacture 
or sell brake products. In essence, they wished the agency to change 
the proposed definition of ABS so that their devices can be used to 
meet the requirements. These companies are, of course, free to develop 
and sell products that meet the definition. Also, to the extent that 
these companies produce products that do not meet the definition, they 
are free to sell them as supplemental equipment, so long as the 
products do not create compliance problems or contain safety defects. 
However, for the reasons discussed above, and expanded on below in the 
context of these comments, products which do not meet the definition 
would not prevent sustained wheel lockup.
    Strait-Stop, a company which manufactures what it calls a 
``noncomputerized ABS,'' argued that the proposed ABS definition is 
discriminatory and excessively design-restrictive because it 
necessitates the use of electronic computerized systems with wheel 
speed sensors. It argued that the agency's tests ``(do) not prove, 
conclusively, that the computerized ABS is the only alternative to 
accomplish stability and control.'' Strait-Stop also stated that 
NHTSA's fleet study indicated that computerized ABS activated very 
rarely, only 1.4 times per 10,000 brake applications or 1.1 times per 
10,000 miles driven, and that it is a tool with which drivers will not 
gain familiarity. In contrast, Strait-Stop stated that its device 
activates approximately 98 percent of the time that the driver applies 
the brakes, thereby enabling drivers to become familiar with the 
system. While Strait-Stop did not describe how its ``non- computerized 
ABS'' works or precisely what it does, that company stated that its 
device uses ``modulation but not reduction of braking pressure.'' 
Moreover, literature about its system indicates that the air flow from 
the foot (treadle) valve to the relay valve is interrupted through the 
Strait-Stop system and pulsates the brake chambers. The ``system 
intermittently repeats the on and off cycle at a pre-set rate.''
    Jenflo Brake-Aid (Jenflo) also argued that the proposed ABS 
definition is discriminatory, and that the definition should be revised 
to permit braking devices other than the ones tested by the 
[[Page 13226]] agency. Jenflo manufactures a device for air brake 
systems which causes a ``pulsing (or air pressure to) the brake 
actuators hundreds of times per minute, (that will) cause the tires to 
approach lock-up, then the brakes are off for a `small' fraction of a 
second and are just as rapidly reapplied.'' As a result, the air 
pressure is continually released and reapplied on all the controlled 
wheels during all but ``normal'' braking.
    Trade International Corporation (TIC) stated that the proposed ABS 
definition is unnecessarily narrow and could preclude the use of 
available, beneficial products and technologies, and also impede the 
development of other useful products and technologies. TIC argued that 
a system which continuously modulates the braking force applied to 
every wheel whenever braking force is applied would not satisfy the 
definition because it lacks the specified sensing and transmitting 
functions, regardless of its ability to prevent wheel lockup and/or 
enhance braking effectiveness.
    The devices referred to by Strait-Stop, Jenflo Brake-Aid, and TIC 
all ``pulse'' the air pressure for essentially all but normal brake 
applications. These commenters did not explain in detail how these 
products work. However, based on the available information, they 
provide the same ``pulsing'' of air pressure at a fixed pulsation rate 
for all brake applications above some braking or turning threshold. 
Regardless of how they work, however, the devices cannot ensure the 
twin goals of preventing/controlling lockup and ensuring good stopping 
distance under all road surface and load conditions, if they do not 
meet the proposed definition. This is because, for the reasons 
explained above, their operation would not be dependent on the factors 
that are relevant to the desired safety performance. Only by 
continuously sensing and responding to what is actually happening at 
the tire/road surface interface can an ABS system optimize the braking 
pressure so as to both prevent lockup and minimize stopping distances. 
As discussed in the ABS Wheel Slip Control Strategies section of the 
Appendix, one effect of varying road surface and vehicle load 
conditions on the operation of ABSs is the varying controlling 
frequencies that are needed to adapt to these varying conditions. The 
fact that these other devices incorporate a fixed pulsation rate 
demonstrates their lack of adaptability to varying road surface and 
vehicle load conditions. As shown in Figures 17 and 18 in the Appendix, 
the ABS controlling frequency needs to be relatively slow, between 1 
and 2 cycles per second, in order to prevent sustained excessive wheel 
slip on very low friction surfaces and needs to be much faster, 
approaching 10 cycles per second, in order to achieve very short 
stopping distances on high friction surfaces. The increase in stopping 
distance on high friction road surfaces that would result from a system 
which exhibited a slower than optimum ABS controlling frequency may not 
be great. However, the impact of a much faster than optimum ABS 
controlling frequency on a very low friction surface would be sustained 
and excessive wheel lockup. As shown in Figure 17 in the Appendix, 
wheel lockup can occur very rapidly. Figure 17 also shows that from the 
time that the ABS solenoid is activated to reduce brake pressure it 
takes about 0.25 seconds before the wheel even begins to spin up, about 
0.35 seconds for the wheel to reach one-half of the vehicle's speed and 
more than 0.6 seconds for the wheel to reach the vehicle's speed. If 
the devices referred to by Strait-Stop and Jenflo Brake-Aid pulse the 
brakes several times a second, the ``off'' portion of pulsation cycle 
would not be sufficiently long to allow the locked wheel to spin up 
prior to the next ``on'' portion of the cycle which would result in 
sustained wheel lockup.
    The basic problem with devices that do not incorporate feedback on 
what is happening at the tire/road surface interface (as required by 
the definition of ABS mandated by this amendment) such as those 
described by Strait-Stop, Jenflo and TIC, is that they are ``blind'' to 
the road and surface conditions on which the vehicle is operating and 
thus make the same response each time, regardless of whether that 
response is appropriate for the existing circumstances. In other words, 
the systems cannot appropriately adjust their cycle rate or the degree 
of pressure variation to compensate for the effects that load condition 
and road surface friction can have on the lockup and spinup times of a 
vehicle's wheels. This lack of ``adaptability'' to changes in load and 
road surface conditions results either in sustained wheel lockup (and 
resultant loss of stability and control) or in stopping distances that 
are much longer than the vehicle would otherwise be able to achieve 
under those conditions for which the system was not optimized. As a 
result, even if these systems enhanced vehicle stability on one type of 
surface, they would provide inferior braking on a different surface. 
For instance, the relatively high brake pressure required for short 
stopping distance on a high coefficient of friction surface would lock 
the wheels on a slippery surface because wheel lockup occurs when the 
braking force at the tire/road surface interface, needed to resist the 
torque generated by the brake, is greater than that which can be 
generated from the available surface friction. Because wet surfaces 
have lower friction levels, vehicles on these roads will lock up at 
lower levels of brake pressure. Conversely, if the pulsating mechanical 
system were designed so that brake pressure was reduced in a manner 
that ensured that lockup would not occur during hard braking on a 
slippery surface, stopping distances would be very long when braking on 
high coefficient of friction surfaces.
    NHTSA also notes that in order to optimize stopping distance and 
maintain vehicle stability, an antilock system must be capable of 
reducing, holding, and reapplying braking pressure to each controlled 
wheel. The wheel speed sensor monitors the rotational speed of the 
wheel. When a monitored wheel approaches a lockup condition, there is a 
sharp rise in peripheral wheel deceleration and in wheel slip. If this 
rise exceeds the designed threshold levels, the ECU sends signals to 
the modulator device to hold or reduce the build-up of wheel brake 
pressure until the danger of wheel lockup has passed. The brake 
pressure must then be increased again to ensure that the wheel is not 
underbraked for the road surface conditions. During automatic brake 
control, it is important for the wheel speed to be constantly monitored 
so that the maximum braking force for the conditions could be achieved 
by a succession of pressure-reduction, pressure-holding, and pressure-
reapplication phases. The agency notes that the systems described by 
Strait-Stop, Jenflo and TIC reduce and reapply pressure, without 
reference to road conditions, brake forces, or impending wheel lockup.
    With respect to Strait-Stop's argument that drivers will not gain 
familiarity with the kinds of ABS systems tested by NHTSA because the 
systems activate only rarely, the agency notes that no special 
familiarity is necessary to operate the system properly. ABS is a 
safety device which operates automatically in emergency situations.
    Strait-Stop also alleged that the system defined and tested by 
NHTSA does not prevent lockup. While that company did not explain this 
comment, the agency assumes that Strait-Stop is distinguishing between 
momentary lockup and sustained lockup. All of the systems tested by 
NHTSA prevent sustained lockup.
    Strait-Stop argued that the inference that the screened-out systems 
would not [[Page 13227]] meet the braking-in-a-curve test requirement 
is unsupported since the agency has not tested and, in some cases has 
refused to provide testing for them. As discussed above, it is possible 
that a system not meeting the proposed definition could be optimized to 
provide enhanced stability for a particular test on a particular test 
surface. However, such a system would provide inferior braking 
performance on other surfaces and/or under different test conditions.
    There is no requirement or reason for the agency to test every 
invention identified by commenters in a rulemaking proceeding. The 
agency can use its technical and engineering analysis to determine what 
performance attributes are necessary to meet the need for safety, and 
it can also often make determinations about whether particular devices 
would provide safety benefits by the same means.
    NHTSA has also analyzed another type of device, from Emergency 
Brake Technologies, described by Dr. Barry Wells. This is an emergency 
braking device that is manually activated by the driver through a dash-
mounted switch that activates arms that drop polyurethane wedges and 
rubber flaps under the vehicle's wheels. After the device is activated, 
the vehicle must be stopped and reversed so that the wedges can be 
removed from beneath the wheels. Emergency Brake Technologies claims 
that this device ``could stop a fully loaded vehicle in the same 
distance as an automobile and completely eliminate jackknifing.'' While 
NHTSA does not have any opinion concerning whether this device might 
provide benefits in some emergency stopping situations, the device 
would not meet the need for safety being addressed by this rulemaking, 
i.e., ensuring stability and control during braking. In fact, the 
dropping of polyurethane wedges and rubber flaps under the wheels would 
create essentially the same condition as fully-locked wheels, and 
therefore could result in a loss of control. Once the driver activated 
this system, the driver would be committed to a quick, sliding stop. 
The driver would have no capability to release the device once applied, 
and could also have difficulty steering around a problem. While such a 
device could provide short stopping distances under dry-road 
conditions, it would do so by sacrificing vehicle stability and 
control.
    ATA and Strait-Stop commented that the proposed definition would 
preclude anything but electronic systems, thereby prohibiting 
mechanical systems. NHTSA notes that this is incorrect, since the 
definition does not require electronics for the sensing of the wheel 
rotation, or transmission of wheel rotation or controlling signals. 
Such functions could be performed using pneumatic, hydraulic, optic, or 
other mechanical means. The agency notes that it is likely that 
electronic systems will be used, given currently available 
technologies. All ABSs currently marketed in the United States are 
electronic in nature.
    In the case of an ABS that does not require electrical power for 
operation, the only mandatory electrical requirement in this rulemaking 
(addressed later in this document) is for malfunction indicator lamps 
used to signal a problem in the ABS.
    ATA also argued that the requirements would impair efforts to 
develop new electronic technologies. ATA stated that the restrictions 
would limit engineers' abilities to develop electronic braking (brake-
by-wire) systems (EBS) by forcing the logic for such systems to be 
based on existing ABS designs. According to ATA, EBS is designed to 
handle all braking functions: compatibility, load sensing/brake 
proportioning, balance, timing, ABS, traction control, and failure 
control. ATA stated that successful development of these systems may 
require that designers not be tied to a rotational slip view of wheel 
lockup.
    NHTSA disagrees that the proposed ABS requirements will impair 
efforts to develop EBS. The agency notes that Robert Bosch GmbH 
currently markets the Bosch-ELB Electronically Controlled Commercial 
Vehicle Brake, in Europe. This system includes ABS, traction control, 
and electronic service braking (with pneumatic backup) functions, and 
uses the same wheel speed sensor arrangement as does Bosch's ABS sold 
without EBS. This indicates that EBS is fully compatible with current 
ABS technology, including wheel speed sensors. Furthermore, a 
combination-unit vehicle with good brake balance, compatibility, and 
timing may still be capable of being over-braked by the driver, 
especially when operated lightly-loaded or on slippery road surfaces, 
and such a vehicle would still require ABS to prevent wheel lockup when 
operated under these conditions. The development of the Bosch 
electronic braking system proves that the rotational slip view of wheel 
lockup does not hinder the development of successful EBS.
    ATA also stated that the requirements could ``hold back'' disc 
brake technology since disc brakes are ``virtually incompatible'' when 
used together with drum brakes on a combination vehicle. ATA appears to 
believe that because EBS can make the ``decisions'' to compensate for 
those major differences, it is needed for disc brake technology to come 
into general use. The agency notes that, according to product 
literature, the Bosch-ELB system measures wheel speeds and brake 
actuator pressures at each wheel position, and microcomputers in the 
electronic control unit store and process these data and transmit the 
correcting commands accordingly. This system could, therefore, 
compensate for incompatibilities in brake force balance on a vehicle, 
and would permit safe introduction of disc brakes on vehicles. This 
system incorporates ABS technology that complies with the agency's 
proposed ABS requirements, as well as ECE Regulation 13. Therefore, 
NHTSA disagrees with ATA's argument that ABS requirements will hold 
back disc brake technology.
    In a somewhat different vein, TIC argued that a system could 
satisfy the proposed definition but not accomplish the desired function 
of preventing lockup. As part of this argument, TIC stated that the 
proposed definition for ABS is fundamentally flawed because it does not 
specify what the system is supposed to accomplish but rather specifies 
how the system is supposed to work. TIC's comment in essence raises the 
issue of whether the definition is sufficient, by itself or with other 
requirements, to meet the need for safety.
    As indicated at the beginning of this section, the agency developed 
a broad definition precisely to avoid imposing unnecessary design 
restrictions or impeding the future development of ABS. The ABS 
definition is based on the premise that wheel lockup is the source of a 
vehicle's loss of directional stability and steering control during 
braking, and that any device designed to improve such stability during 
braking must control the source of that instability. Hence, the 
definition establishes a linkage between the input, signals that sense 
wheel lockup, and the output, modulated brake pressure to prevent wheel 
lockup. This is essentially the extent of the design constraints 
established by the agency, and it gives the industry considerable 
latitude to design and develop individual components, ranging from 
sensor design and placement, to the ECU control algorithm and to brake 
pressure modulation frequency.
    NHTSA rejects TIC's argument that the definition does not specify 
what the system is supposed to accomplish but rather how the system is 
supposed to work. Modulating brake pressure in [[Page 13228]] response 
to information about rate of angular rotation is part of what is 
supposed to be accomplished. As discussed above, the rate of angular 
rotation reflects what is happening at the tire/surface interface.
    NHTSA further concludes that the requirement/definition for ABS is 
sufficient at this time to meet the need for safety. In arguing that a 
system can satisfy the definition but not accomplish the desired 
function, TIC provided the following ``extreme example'':

    Consider the following system: (1) a set of angular rate of 
rotation sensors, one on every wheel; which (2) transmit signals 
whose level is proportional to the rate of angular wheel rotation to 
a device which compares the signals and generates control signals; 
and (3) transmits those control signals to devices which increase 
the braking force applied to any wheel which has an angular rotation 
rate higher than the wheel which has the lowest angular rotation 
rate. Such a system satisfies every element of the proposed 
definition, however, the result of implementing such a system would 
be that if any wheel locked up during braking all wheels would lock 
up!

    While TIC itself acknowledged that its example was ``extreme,'' 
NHTSA notes that its basic premise also is silly, since it assumes that 
a manufacturer would deliberately build a brake system that could not 
work. In considering the impacts of its standards, NHTSA must assess 
how manufacturers are likely to respond, not unrealistic hypothetical 
situations. The basic premise underlying this rulemaking is that 
manufacturers will respond to the definition/requirement for ABS by 
providing systems that will prevent wheel lockup. This view is 
confirmed by the comments of the vehicle and brake manufacturers. There 
is no evidence that manufacturers would respond by deliberately 
building systems that do not prevent lockup but instead cause lockup.
    Moreover, the definition for ABS does not stand in a theoretical 
vacuum. Manufacturers must design their brake systems to meet other 
safety requirements (including stopping distance requirements and, for 
some vehicles, the braking-in-a-curve test). It might not be possible 
to meet those requirements with systems that did not prevent lockup but 
instead caused lockup. Manufacturers are also subject to Federal 
requirements concerning safety-related defects. And, of course, 
manufacturers must ensure customer satisfaction.
    The agency also notes that there is absolutely no incentive for 
manufacturers to provide ABS systems that do not function as they 
intended. TIC's comment essentially raises the possibility that a 
manufacturer might spend all the money necessary to meet the definition 
of ABS and then include a faulty ECU control algorithm. However, there 
is no basis to believe that this would happen. The agency only 
addresses unreasonable safety risks in developing safety standards and 
need not address unrealistic hypothetical possibilities.
3. Dynamic Versus Equipment Requirements
    As discussed in the NPRM and above, NHTSA considered whether 
adequate performance relating to directional stability and control 
could be ensured solely by means of dynamic test requirements, but 
concluded that, at this time, there would be practicability problems 
associated with the broad array of dynamic test requirements that would 
be associated with such an approach. The agency therefore decided to 
propose a single provision expressly requiring that heavy vehicles be 
equipped with antilock systems, and on identifying feasible and 
practicable dynamic tests that could supplement that provision by 
directly assessing the directional stability, control and stopping 
distance of vehicles under some of the wide variety of circumstances 
that may be experienced in the real world.
    ATA commented that the desired result from mandating the 
installation of ABS is ensuring that a vehicle can be controlled during 
a stop, and asserted that the proposed braking-in-a-curve performance 
requirement, with certain changes, would accomplish this conceptually. 
However, ATA did not substantiate its assertion about the efficacy of 
such a requirement, standing by itself. ATA did not address the 
practicability problems of adopting a set of dynamic performance 
requirements, or even the practicability problems associated with 
applying the braking-in- a-curve requirement to all affected vehicles. 
ATA did, however, suggest that the agency initiate additional research 
and development for what it called ``true performance tests.''
    While NHTSA plans to continue research on dynamic performance tests 
for trucks, buses and trailers, it has concluded that the desired 
safety benefits of ABSs could be achieved now by means of a specific 
equipment requirement for ABS and (as discussed below) a dynamic 
performance test requirement applicable to truck tractors only. NHTSA 
is charged by the Safety Act with promulgating safety standards that 
meet the need for safety. Moreover, Congress was sufficiently concerned 
about the directional stability and control problems associated with 
heavy vehicles that it specifically required NHTSA to conduct a 
rulemaking that examines and could result in requiring the installation 
of ABSs in these vehicles. The agency has concluded that large safety 
benefits can be obtained by requiring ABSs on heavy vehicles, and has 
developed requirements that will ensure installation of this safety 
equipment.
    NHTSA disagrees with the suggestion that it delay implementation of 
this life-saving rule while it conducts further research in search of 
the type of rule ATA desires. The overall history of agency rulemaking 
is one of gradual progression, when and where practicable and 
beneficial to safety, toward increasingly sophisticated and 
increasingly more dynamic performance standards. However, relying 
exclusively on dynamic performance requirements has never been a 
statutorily mandated requirement. Were it so, there would be many fewer 
Federal motor vehicle safety standards today--and many thousands more 
deaths and injuries, occurring annually.

B. Independent Wheel Control

    In the NPRM, NHTSA proposed to require that the antilock brake 
system monitor and control the wheels of the front axle (i.e., steering 
axle) and the wheels of at least one rear axle. NHTSA believed that 
this would ensure that the wheels on the steering axle and the wheels 
on the selected rear axle were directly controlled by the ABS. By 
``directly controlled,'' the agency meant that the signal provided at 
the wheel or on the axle of the wheel would directly modulate the 
braking forces of that wheel or axle. The agency tentatively concluded 
that it is necessary to specify that the ABS directly control the 
steering axle because some ABSs control only a vehicle's drive-axle, 
which could result in the loss of steering control if the front wheels 
locked during braking.
    Several commenters addressed the need for front wheel control. ATA 
strongly opposed mandating ABS for the steering axle of single-unit 
trucks and suggested that the agency reconsider the requirement for 
tractors. In contrast, Rockwell, WABCO, Freightliner, AAMA, Advocates, 
and IIHS favored requiring that an ABS be installed on front axles. 
AAMA favored equipping each vehicle with an ABS that has at least one 
independent channel of control for the wheels on a front axle and at 
least one independent channel of control for the wheels on a rear axle. 
However, AAMA objected to mandating more than two independent channels 
of control. [[Page 13229]] 
    NHTSA did not specifically address the concept of independent 
control in the NPRM, but addressed it in the SNPRM by proposing that 
the wheels on at least one axle be independently controlled. The agency 
in today's final rule defines an ``independently controlled wheel'' to 
mean a directly controlled wheel for which the modulator device does 
not modulate the brake forces at any other wheel on the same axle. This 
means that a side-by-side control strategy on a tandem axle could have 
the wheels on the sensed axle of the tandem being independently 
controlled by a modulator, and the wheels of the other axle of the 
tandem being indirectly controlled by the modulator for the wheel on 
the sensed axle on the same side of the vehicle.
    Rockwell, Freightliner, Advocates, and IIHS commented that the 
regulatory language in the NPRM requiring each axle to be directly 
controlled by an ABS would allow select low<SUP>28 antilock systems on 
any axle. These commenters believed that an antilock system must 
provide independent control at each wheel of a heavy vehicle to ensure 
good, overall ABS performance in the areas of stability and stopping 
distance. Accordingly, they recommended that the equipment requirement 
include language that would require ``independent control of each 
wheel'' of the axles that are required to be ABS-controlled. They 
believed that the inclusion of such a requirement would prevent 
significant degradation in stopping performance, particularly on a 
split mu surface. Bosch recommended a minimum requirement of a four-
sensor, three- modulator-valve (which is referred to as a 4S/3M system) 
ABS. Freightliner favored requiring at least four independent channels 
of control, i.e., two for each axle, to allow independent control of 
each wheel on the front and a rear axle. Similarly, IIHS favored 
requiring the brakes for each wheel on the front axle and the brakes 
for each wheel on one rear axle to be independently controlled. 
Advocates recommended that the ABS be functional on all axles, not just 
one axle in each multiple axle set on a heavy vehicle.

     <SUP>28See the Appendix for a discussion of this term.
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    Based on its analysis of these comments and other available 
information, NHTSA issued an SNPRM proposing modifications to the NPRM 
to require heavy vehicles to be equipped with systems that 
independently control each wheel on at least one axle of a truck, a 
truck tractor, or a bus (i.e., 4S/3M systems). As explained in the 
SNPRM, the agency tentatively concluded that a minimum requirement that 
ABS provide independent wheel control on at least one axle would 
provide an acceptable level of stopping distance performance on low mu 
and split mu surfaces. The agency believed that a vehicle with 
independent ABS wheel control would stop in a shorter distance than 
either a vehicle equipped with an axle-by-axle ``select low'' control 
ABS, or a non-ABS equipped vehicle operated by a driver making his or 
her best efforts to minimize stopping distance through manually 
modulating the brake pedal. The agency also proposed to prohibit tandem 
control<SUP>29 by an ABS, by requiring that no more than two wheels be 
controlled by one modulator valve. NHTSA requested comments about its 
proposal for independent control of each wheel on at least one axle and 
about prohibiting tandem control by an antilock system.

     <SUP>29As explained in the appendix, tandem control refers to 
having two adjacent axles being controlled by the same modulator 
valve. Specifically, while each axle has its own wheel speed sensor, 
the brakes on two axles are controlled by one modulator valve.
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    In response to the SNPRM, NHTSA received comments from Ford, AAMA, 
Strait-Stop, GM, Navistar, White GMC, Bosch, PACCAR, Eaton, Midland-
Grau, Truck Trailer Manufacturers Association (TTMA), Advocates, and 
ATA about the proposal to require independent control on at least one 
axle. Aside from Freightliner, WABCO, Bosch, Advocates, and IIHS, most 
other commenters opposed the proposal claiming that requiring 
independent control would be unreasonably design-restrictive. Bosch 
stated that the proposal is appropriate since at least one of the axles 
that contributes most to vehicle deceleration in the loaded condition 
should have the ability to have its wheels individually controlled. 
Ford, AAMA, GM, Navistar, PACCAR, Eaton, and Midland-Grau stated that 
the agency should specify direct control as a minimum requirement but 
not require independent control. AAMA stated that the standard should 
permit any control system that provides stability without substantial 
degradation in stopping distance. Ford claimed that any requirement 
that ABS must employ more than two channels of control would not result 
in any safety advantage over its two-channel system, but would result 
in substantial and unnecessary incremental costs to Ford and might 
jeopardize its ability to meet early implementation dates. Midland-Grau 
strongly opposed the SNPRM's approach, claiming that it presented a 
major change in scope from performance requirements and minimal design 
requirements. Specifically, it complained that the SNPRM changed the 
rulemaking's focus from directional stability and control to stopping 
distance on split mu surfaces.
    Consistent with their comments on control philosophies, AAMA, GM, 
White GMC, PACCAR, and Midland-Grau also opposed the proposed 
definition of ``independently controlled wheels.''<SUP>30 AAMA and 
PACCAR claimed that the proposed definition does not accommodate widely 
used ABS algorithms and control technologies. It requested that the 
word ``only'' be omitted since its inclusion in the definition would 
inappropriately preclude antilock systems that ``rely on wheel speed 
information from both wheels on an axle to modulate brake pressure at 
each of the wheels.''

     <SUP>30The agency proposed to define ``Independently Controlled 
Wheel'' as a ``wheel at which the degree of rotational wheel slip is 
sensed and corresponding signals are transmitted to one controlling 
device that adjusts the brake actuating forces only at that wheel in 
response to those signals.''
---------------------------------------------------------------------------

    Ford, AAMA, GM, Navistar, White GMC, PACCAR, Eaton, and Midland-
Grau opposed prohibiting tandem control. TTMA requested that trailers 
equipped with more than three axles be excluded from the requirements, 
claiming that it would be very expensive to equip these vehicles, which 
account for only four percent of trailer production, with ABS.
    ATA and Strait-Stop opposed specifying the type of wheel control, 
claiming that doing so creates an impermissible design requirement. 
Strait-Stop stated that the proposed approach prohibits creativity in 
the development of other technology that may accomplish the performance 
standards more effectively with greater economic efficiency.
    Several commenters submitted test data about various ABS 
configurations. WABCO and Freightliner submitted simulated test data 
showing that 4S/2M systems on truck tractors provide very poor stopping 
distance performance on split mu surfaces, compared with 4S/4M systems. 
These commenters reported that the 4S/2M systems they tested took 
between 316 percent and 353 percent of the norm to stop on a split mu 
surface, with driver best effort being defined as the norm, or 100 
percent. Ford and Bendix submitted simulated data showing that 4S/2M 
systems incorporating the modified select high regulation (MSHR<SUP>31) 
wheel slip control strategy on truck tractors perform acceptably. 
Bendix also submitted vehicle test data showing that the stopping 
distance performance with [[Page 13230]] tandem control ABS 
incorporating the MSHR wheel slip control strategy (2S/1M) on trailers 
is comparable to the performance of a 2S/2M system.

     <SUP>31See the Appendix for a discussion of this term.
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    As explained above, in establishing the requirements applicable to 
the stability and control of heavy vehicles, NHTSA has decided that, at 
a minimum, wheels on the steering axle and at least one rear axle of a 
powered vehicle must be controlled by a closed-loop antilock system. 
Similarly, the wheels on at least one axle of a semitrailer and dolly, 
and the wheels of at least one front axle and one rear axle of a full 
trailer must be controlled by a closed-loop antilock system. The agency 
has decided that requiring a closed-loop antilock system is necessary 
to ensure the directional stability and control of heavy vehicles 
during braking.
    NHTSA emphasizes that requiring a closed-loop antilock system is a 
minimum requirement that the agency believes will ensure the safety of 
heavy vehicles. The agency has also decided to establish supplementary 
requirements beyond these minimum requirements that address the type of 
wheel control for various types of vehicles. In establishing these 
supplementary requirements, the agency has sought an approach that is 
responsive to the many and oftentimes disparate views of the commenters 
and that ensures safety performance objectives, while considering 
practicability, costs and, to the extent possible, stated industry 
practice.
    The supplementary equipment requirements, which specify the type of 
wheel control, are based on the philosophy that, for the reasons set 
forth below, an incrementally higher level of stability performance 
during braking is warranted for truck tractors compared to that which 
is appropriate and needed for trailers, single-unit trucks, and buses. 
First, truck tractors, when used in a combination vehicle, are 
articulated and therefore are more likely to lose control than single-
unit vehicles. Second, truck tractors typically have shorter wheelbases 
than single-unit trucks, trailers and buses and therefore are more 
susceptible to locked wheel-induced, unrecoverable loss of control than 
are any of these other vehicle types. This loss of control typically 
manifests itself as a jackknife when tractors are coupled to 
semitrailers. Third, truck tractors typically travel approximately five 
times more annual miles than single-unit trucks, three times more miles 
than trailers (since there are proportionally three times as many 
trailers in use than there are tractors which tow them), and 
approximately seven times as many miles as buses. This substantially 
larger use proportionally increases a truck tractor's exposure to risk. 
Fourth, truck tractors typically operate on roads (i.e., interstate 
highways and rural State and U.S. routes) that have comparatively 
higher posted speed limits and vehicle operating speeds than the roads 
on which single-unit trucks and many buses generally operate. A higher 
operating speed exacerbates the consequences of braking-induced wheel 
lockup and loss-of-control. This is a significant contributing factor 
to the high proportion of heavy vehicle braking instability-related 
crashes, fatalities and injuries that involve combination-unit trucks.
    Based on the above considerations, NHTSA has decided that the 
requirements for truck tractors must be more stringent than those for 
the other vehicle types. Specifically, on at least one of the truck 
tractors's axles, each wheel must be independently controlled by an ABS 
modulator. With respect to a given wheel, ``independently controlled'' 
means a wheel at which the degree of rotational wheel slip is sensed 
and corresponding signals are transmitted to a modulator that adjusts 
the brake actuating forces at that wheel on the axle or at other wheels 
on other axles. The agency has decided to revise the definition in 
response to AAMA's comment on the definition of independently 
controlled, since its inclusion might inadvertently prohibit acceptable 
systems. Requiring independent control ensures that a wheel provides 
optimal braking forces on all surfaces, enabling the vehicle to achieve 
near optimal braking on all surfaces, especially split mu ones.
    In most cases, the axle with independent wheel control will likely 
be the tractor's drive axle(s). Commenters, including AAMA, Midland-
Grau, and Bendix, submitted to the agency road testing data about how 
certain antilock systems improved the braking efficiency and 
directional control and stability of various vehicle configurations. 
Based on these data, the agency believes that independently controlling 
the drive axle(s) will result in incrementally better braking 
performance on split mu road surfaces than the other ABS equipment 
configurations that are permitted on the other vehicle types covered by 
this rule.
    Rockwell WABCO correctly stated that allowing select low ABS on all 
axles will result in substantially longer stopping distances on split 
mu surfaces, particularly when the differences between the coefficients 
of friction on the two surfaces is large. Notwithstanding this 
shortcoming, the agency believes that a select low system is 
appropriate for the front axle for the following reasons. First, since 
the front axle brakes typically provide about 25 percent of the braking 
on a truck tractor, the stopping distance degradation with select low 
on the front axle will be small. Second, having equal braking forces at 
each wheel alleviate steering wheel ``pull'' that would occur on a 
split mu surface with ABS independently controlled front brakes. Third, 
current antilock systems installed on the front axle of heavy vehicles 
tend to use SLR, MSHR, or MIR wheel slip control strategies.<SUP>32 No 
vehicle manufacturer uses a system in which front axle control is 
purely independent wheel control. Accordingly, the agency has 
determined that it would be inappropriate and impracticable to prohibit 
the use of select low control on front axles.

     <SUP>32SLR, MSHR, MIR and other wheel slip control strategies 
are discussed in the Appendix.
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    NHTSA has also decided that it is necessary to prohibit tandem 
control on tractors to further ensure the safe braking performance for 
tractor trailers. This decision is based on test data<SUP>33 which 
indicate that tandem control does not provide an acceptable level of 
stopping distance performance for truck tractors, even though it may 
ensure a heavy vehicle's stability and control.

     <SUP>33``Improved Brake Systems for Commercial Motor 
Vehicles,'' DOT HS 807 706, April 1991,
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    Notwithstanding its decision to prohibit tandem control on truck 
tractors, NHTSA has decided that tandem control is appropriate for 
vehicles other than truck tractors, such as trailers and single unit 
vehicles. Vehicle test data submitted by Ford, Bendix, and Midland 
showed comparable vehicle stopping distance performance, and in some 
cases superior performance, of tandem control (2S/1M) systems compared 
with side-by-side control (2S/2M) systems, without any difference in 
vehicle stability performance. Vehicle test data also showed comparable 
ABS performance with MSHR tandem control on trailer axles. Accordingly, 
today's requirements permit direct control 2S/1M systems for converter 
dollies, semitrailers, and the front axles of full trailers. The agency 
further notes that single unit vehicles equipped with 4S/2M systems 
have been approved for use in Europe as ``Category 1'' systems.

C. Braking-In-A-Curve Test

1. General Considerations
    As explained in the previous section on equipment requirements, 
NHTSA proposed requiring heavy vehicles to be [[Page 13231]] equipped 
with antilock systems, and supplementing that requirement with dynamic 
performance requirements to check the directional stability, control 
and stopping distance of such vehicles. The agency proposed only those 
dynamic performance requirements that it believed would be feasible and 
practicable for checking the directional stability of a vehicle when it 
is maximally braked. Specifically, in its September 1993 NPRM, the 
agency proposed a ``braking-in-a-curve requirement'' on a low 
coefficient of friction surface without a stopping distance 
requirement. Under this proposed requirement, heavy vehicles would have 
to be capable of stopping without loss of directional stability or 
control, while turning on a slippery surface during an aggressive or 
``hard'' stop. Separately, in its February 1993 NPRM, the agency 
proposed braking effectiveness requirements through the use of high 
speed (60 mph) stopping distance requirements on a high coefficient of 
friction road surface.
    NHTSA explained, in the September 1993 NPRM, its tentative 
conclusion that the braking-in-a-curve test on a low mu surface is an 
objective, repeatable, and practicable procedure for evaluating a heavy 
vehicle's directional stability and directional control. The agency 
further explained that the proposed braking-in-a-curve test is 
consistent with industry's views, since the Antilock Test Procedure 
Task Force of the Motor Vehicle Safety Research Advisory Committee 
(MVSRAC) recommended this procedure and the SAE has proposed it in 
Recommended Practice J1626, Braking, Stability, and Control Performance 
Test Procedures for Air-Brake-Equipped Truck Tractors.
    In response to the NPRM, Advocates stated that the agency's 
proposal to specify both an equipment and dynamic performance 
requirement was the most appropriate way to ensure that the substantial 
safety benefits of heavy vehicle ABS are realized quickly. Rockwell 
WABCO reluctantly supported the proposed combination of an equipment 
specification and a dynamic performance test, given the current 
difficulty in formulating valid additional, repeatable performance 
criteria. Midland-Grau favored this approach for truck tractors since 
it believed that merely issuing an ABS requirement, without an 
accompanying performance requirement, would allow ineffective systems 
in the marketplace.
    Allied Signal supported the braking-in-a-curve test for truck 
tractors, but opposed the test for other vehicles, stating that 
vehicles other than truck tractors have not been tested using this 
maneuver. Midland-Grau was also concerned that very little test data 
have been collected on vehicle types other than truck tractors. Volvo-
GM stated that the test is unsafe for many vehicles, and that a dynamic 
performance requirement is not necessary, given the provision requiring 
ABSs. AAMA stated that although it generally favors performance-based 
dynamic requirements for Federal Motor Vehicle Safety Standards, it 
opposes the braking-in-a-curve test given what it perceives as its 
``overwhelming practicability and objectivity problems.'' Among AAMA's 
concerns were that (1) there has been no test program by NHTSA to 
decide whether the test is suitable for single-unit trucks, buses, and 
trailers, (2) the braking-in-a-curve test alone cannot evaluate the 
effectiveness of an ABS, (3) there is a lack of repeatability of the 
braking-in-a-curve test procedure, and (4) no suitable test facilities 
exist for vehicle manufacturers to conduct compliance testing. Given 
these concerns, AAMA favored adopting, on an interim basis, an 
equipment requirement only.
    ATA, Strait-Stop, and several other commenters supported a dynamic 
performance-based requirement instead of an equipment requirement. They 
believed that this approach would encourage further development of 
antilock technology and would enable users to find the system that best 
suits their operation. ATA was concerned that an equipment requirement 
would preclude the development of more effective systems for different 
applications.
    TTMA believed that the braking-in-a-curve test is inappropriate for 
trailers. It stated that trailer manufacturers, many of which are small 
entities, do not have the financial resources or the facilities to 
conduct road testing.
    After reviewing the comments and other available information, NHTSA 
has decided to amend the Standard to include the braking-in-a-curve 
test for certain vehicles. The agency considered requiring surface 
transition tests (i.e., a test maneuver in which vehicle braking begins 
on a high coefficient of friction surface and then completes the stop 
on a low mu surface, and vice versa), a lane change test, and split mu 
or side-to-side differential coefficient of road surface friction 
tests, to achieve that objective. The tests would ideally be conducted 
at various speeds with different loading conditions and test surfaces. 
However, the agency has decided that it would be unnecessarily 
burdensome and costly to impose such an array of tests on heavy vehicle 
manufacturers. NHTSA has determined that the performance testing and 
equipment requirements imposed in today's final rules are the most 
appropriate method of ensuring directional control and stability.
    NHTSA has decided at this time to apply the braking-in-a-curve test 
to truck tractors, but not to other heavy vehicles. The agency believes 
that opposition by AAMA, Volvo-GM, and Midland Grau to the braking-in-
a-curve test requirement is based primarily on uncertainty about 
whether the test would also be required for single-unit vehicles, since 
the MVSRAC ABS Task Force developed the braking-in-a-curve test 
procedure for testing only truck tractors. Since neither the agency nor 
the Task Force included single-unit vehicles in the test program, NHTSA 
believes that AAMA and the others are concerned about whether the 
braking- in-a-curve test would appropriately evaluate directional 
stability and control of single-unit vehicles. Accordingly, NHTSA's 
decision to apply the braking-in-a-curve test at this time only to 
truck tractors should reduce the concerns of AAMA and other commenters 
that opposed this dynamic performance test.
    With respect to truck tractors, NHTSA has concluded that the road 
tests performed by the agency and the ABS Task Force provide sufficient 
justification to apply the braking-in-a-curve test to these vehicles. 
The agency notes that the industry, through the MVSRAC, previously 
endorsed and recommended to the agency, essentially the same dynamic 
performance test that is contained in this final rule. The Task Force 
test data and final report indicate that the braking-in-a-curve 
procedure is safe, practicable, and repeatable for truck tractors. 
Accordingly, the agency believes that this recommendation remains valid 
for tractor trailers.<SUP>34

     34TRC of Ohio, Report No. 091194, page 4, August 26, 1991.
---------------------------------------------------------------------------

    NHTSA has decided not to require single unit trucks, buses, and 
trailers to comply with the braking-in-a-curve test requirement at this 
time. The agency's limited testing of single unit trucks to the 
braking-in-a-curve maneuver revealed no specific safety problems. 
However, additional testing on a wider variety of trailers, dollies, 
and single-unit vehicles, including buses and trucks, would be 
appropriate to ensure that these vehicles could be safely tested to the 
braking-in-a-curve maneuver. Specifically, the agency is concerned that 
certain vehicles, especially ones with a high center of gravity, might 
be prone to roll over or otherwise lose control during such tests. 
NHTSA intends to develop performance test requirements equivalent to 
the braking- [[Page 13232]] in-a-curve test for the other vehicle types 
covered by this rule, assuming that future research indicates it 
possible to conduct the test in a safe fashion and to obtain 
meaningful, repeatable results. The agency anticipates conducting 
additional research and road tests to decide whether heavy vehicles 
other than truck tractors should be subject to this road test.
    Today's notice, including the agency's decision not to apply the 
braking-in-a-curve test to vehicles other than truck tractors, 
completes the comprehensive rulemaking to establish directional 
stability and control requirements that was initiated by the June 1992 
ANPRM. If NHTSA decides that it is in the interest of motor vehicle 
safety to apply the braking-in-a-curve test to single-unit vehicles or 
trailers, then it will issue a new proposal to initiate a subsequent 
rulemaking on this matter.
2. Test Surface
    In the NPRM, NHTSA proposed that the braking-in-a-curve test be 
conducted on a test surface with a peak friction coefficient (PFC) of 
0.5 to represent a low coefficient of friction surface. In formulating 
the proposal, NHTSA considered whether the proposed test surface 
specification raised practicability or objectivity concerns in light of 
PACCAR. The agency specifically requested comments on the proposed test 
surface specification.
    Three commenters addressed the test surface specification. Midland-
Grau stated that since maintaining a precise PFC value is not feasible, 
reasonable fluctuations of <plus-minus>10 percent are to be expected. 
Notwithstanding these inherent fluctuations, Midland-Grau commented 
that its testing shows that variability in the test surface PFC value 
of less than 10 percent does not affect the braking-in-a-curve test 
since no stopping distance is prescribed. AAMA stated that it is not 
possible to maintain a surface at a precise PFC. It further stated that 
it is not apparent whether it would be more conservative to conduct 
testing at a higher PFC than the proposed PFC. AAMA stated that the 
variability in the peak to slide ratio is significantly greater on wet 
surfaces than on dry surfaces, and that this ratio directly affects 
performance. Mr. Robert Crail, a brake engineer, stated without 
elaboration that using PFC rather than skid numbers will ensure that 
the test surfaces and test conditions will be reasonable and repeatable 
during actual vehicle testing.
    Before addressing the specific comments about the test surface, the 
following discussion summarizes the PACCAR decision's findings with 
respect to variability and how today's rulemaking responds to that 
ruling. As a result of that case, NHTSA has considered ways to better 
specify test surface adhesion. Prior to the Standard No. 135, Passenger 
Car Brake Systems, rulemaking, NHTSA defined road test surfaces by 
specifying skid numbers. A skid number is the frictional resistance of 
a pavement measured in accordance with a test procedure defined by the 
American Society for Testing and Materials (ASTM). However, given the 
fluctuations of skid numbers on a given surface, the PACCAR ruling 
invalidated certain aspects of Standard No. 121's reliance on this 
measure based on its potential impracticability. In the rulemaking 
proposing Standard No. 135, several commenters advocated specifying the 
peak friction coefficient as an alternative measure of a test surface's 
adhesion. The agency has concluded that PFC is more relevant for the 
stopping distance tests required by the standard because, unlike a skid 
number, the maximum attainable deceleration in a non-locked wheel stop 
is more directly related to PFC. As discussed in the Appendix, the skid 
number characterizes the slide (locked wheel) value of the coefficient 
of friction of a given road surface, and the PFC characterizes the peak 
(rolling wheel) value of the coefficient of friction of a given road 
surface. Since the agency's brake test procedures generally prohibit or 
limit wheel lockup during brake testing, specifying the peak friction 
coefficient is more relevant than specifying the skid number of the 
surface.
    NHTSA has also conducted ``Round Robin'' testing to understand 
further how fluctuations of PFC affect the stopping performance of 
heavy vehicles. Based on the above, NHTSA has decided that the braking-
in-a-curve test should be performed on a test surface with a PFC of 
0.5, which appropriately represents a typical low coefficient of 
friction road surface. Moreover, in today's companion rule adopting 
stopping distance requirements, the agency has decided it is 
appropriate to perform the primary 60 mph stopping distance tests on a 
test surface with a PFC of 0.9. Agency and industry testing indicate 
that a PFC of 0.9 represents a typical dry road surface.
    The requirement to specify test surfaces in terms of PFC rather 
than skid numbers also responds to PACCAR's concern about 
practicability problems caused by skid number fluctuations. Because the 
PFC values of surfaces measured may also indicate some fluctuation, the 
agency has considered whether the fluctuation significantly affects the 
requirement's objectivity. In an earlier rulemaking about Standard No. 
208, the agency explained that since some variability in any test 
procedure is inherent, the agency need only be concerned about 
preventing ``unreasonable'' or ``excessive'' variability to avoid 
causing manufacturers to ``overdesign'' vehicles to exceed the minimum 
levels of protection specified by the Federal safety standards. (49 FR 
20465, May 14, 1984; 49 FR 28962, July 17, 1984.) With respect to the 
braking-in-a-curve test, variability of the PFC value of the test 
surface will have a negligible impact on a vehicle's ability to comply 
with the requirements, which is to stay within the 12-foot lane. Since 
the test speed is set at the lesser of 30 mph or 75 percent of the 
maximum drive-through speed\35\ of the vehicle in the curve, any 
variability in the test surface will be compensated for by an increase 
or decrease of the maximum drive-through speed of the vehicle. If the 
maximum drive-through speed is less than 40 mph, this will result in a 
corresponding increase or decrease of the test speed, which cannot be 
higher than 30 mph. As a result, the variability of the test surface is 
not as critical an issue for the braking-in-a-curve test as it is for a 
stopping distance test on a high coefficient of friction surface, which 
includes a stopping distance measurement that is more affected by test 
surface variation. Based on these considerations, the agency has 
determined that the results of the braking-in-a-curve test will not be 
affected by minor variations in the test surface.

    \35\Maximum-drive-through-speed is defined as ``the highest 
possible constant speed that the vehicle can be driven through 200 
feet of a 500-foot radius curve arc without leaving the 12-foot 
lane.''
---------------------------------------------------------------------------

    The road surface requirements comply with PACCAR's holding that 
manufacturers are entitled to testing criteria that they can rely on 
with certainty, since they include objective terms and requirements, 
i.e., the test surface is at a PFC of 0.5. For the same reason, the 
requirements also comply with PACCAR's requirement that all methods to 
demonstrate compliance with the requirement be set forth in the 
regulation.
    In evaluating the requirement's practicability, NHTSA has 
considered possible difficulties with respect to building and 
maintaining test surfaces with a PFC of 0.5 for the braking-in-a-curve 
test and 0.9 for the high coefficient stopping test. (Those interested 
in building and maintaining a test surface should refer to NHTSA's 
[[Page 13233]] ``Manual for the Construction and Maintenance of Skid 
Surfaces,'' (DOT HS 800 814.) Variations in PFC for high coefficient of 
friction surfaces do not affect stopping distance test results 
appreciably. Moreover, while variations in PFC for low coefficient 
friction surfaces may affect the distance in which a vehicle stops, 
such variations are not relevant for the braking-in-a-curve test, which 
requires a vehicle to remain stable while it is stopped, not that it 
stop within a specified distance. After reviewing the comments and 
available information, NHTSA has concluded that specified test surfaces 
can be achieved and maintained. As explained above, recent ``Round 
Robin'' testing related to research about heavy vehicle braking by the 
agency and others on several test tracks indicates that the test 
surface specification does not raise practicability or objectivity 
concerns.\36\

    \36\TRC Report, August 21, 1991, page 6.
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    One of the PACCAR court's concerns was that the road surface skid 
numbers were based on an out-of-production tire. That concern is not 
relevant to today's final rule since it specifies a currently-produced 
tire. The requirements comply with PACCAR's concern about the testing 
method's objectivity because the peak coefficient of friction is an 
objective measure.
    NHTSA disagrees with AAMA's comment that it is not apparent whether 
it would be more conservative to conduct testing at a higher PFC than 
the proposed PFC. Data from the round-robin testing and other sources 
show that the stringency of a braking-in-a-curve test increases as the 
PFC of the test surface decreases, if the tests are conducted at the 
same vehicle speed. Since the requirement specifies a test speed based 
on the vehicle's maximum drive-through speed, which decreases as the 
test sequence PFC decreases, the resulting test speed will also be 
lower as the PFC decreases. Hence, the stringency of the braking-in-a-
curve test should not change with minor changes in the PFC of the test 
surface.
    NHTSA has decided that AAMA's other comments about the test surface 
requirement are without merit. That organization did not provide any 
data to substantiate its statements. Nor did it explain why it believes 
that ``variability in the peak to slide ratio'' is relevant. Similarly, 
AAMA's comment about ``simultaneously maintaining a given surface at a 
precise PFC and sliding coefficient (i.e., skid number) [being] 
completely infeasible'' is irrelevant to this rulemaking. The agency 
has never proposed a test surface requirement that specifies both the 
PFC and skid number values.
3. Test Speed
    In the NPRM, NHTSA proposed that the braking-in-a-curve test be 
conducted at 30 mph, unless the vehicle could not stay within the 12-
foot lane when driven through the curve at 30 mph. If the vehicle could 
not do so, the braking-in-a-curve test would be conducted at 75 percent 
of the maximum drive-through speed. NHTSA believed that the proposed 
vehicle test speed was sufficiently high to test ABS performance, but 
low enough so as not to pose an unsafe condition during the maneuver to 
the test driver of most vehicles, based on testing conducted by the 
agency\37\ and SAE J1626 Proposed Recommended Practice. The agency 
requested comments about the proposed test speed.

    \37\TRC Report, August 26, 1991.
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    Advocates opposed any reduction in the test speed below 30 mph. 
Specifically, it opposed permitting vehicles that cannot negotiate the 
curve at 30 mph to be tested at the 75 percent drive-through speed 
because it believed that this would be a ``free-floating criterion'' 
that could lead to ineffective antilock systems.
    Rockwell WABCO, Allied Signal, Midland-Grau, and AAMA requested 
that the test speed be clarified. Rockwell WABCO recommended that the 
vehicle test speed requirement be revised to read ``stopped from 30 mph 
or 75% of the maximum drive through speed, whichever is less.'' 
Similarly, Allied Signal suggested that the vehicle test speed be 
clarified to say that testing cannot exceed 30 mph. Midland-Grau 
recommended that the agency revise the requirement so that the test be 
conducted at only 75 percent of the maximum drive-through speed 
capability. It further stated that conducting the braking-in-a-curve 
test at speeds greater than 30 mph on a low mu surface could cause 
safety problems. AAMA stated that the NPRM incorrectly applied SAE 
J1626, which requires testing at 75 percent of drive-through speed to a 
maximum of 30 mph braking speed. It stated that under the proposal, a 
vehicle with a drive-through speed of 30 mph would be tested at 30 mph, 
while a vehicle with a drive-through speed of 29 mph would be tested at 
less than 22 mph. In opposing the proposed requirement, AAMA further 
stated that the determination of the drive-through speed is highly 
sensitive to driver skill, subtle vehicle maneuvers, and environmental 
conditions, and is therefore not repeatable.
    ATA recommended that NHTSA establish stopping or snubbing distance 
requirements for vehicles in a curve, using a braking speed which is 
between 95 and 100 percent of their maximum drive through speed.
    After reviewing the comments and available information, NHTSA has 
decided to specify that a vehicle's test speed for the braking-in-a-
curve test is ``30 mph or 75% of the maximum drive-through speed, 
whichever is less.'' This modification responds to the comments by 
Rockwell WABCO, Allied Signal, and Midland-Grau that the proposal was 
not consistent with SAE J1626. The agency believes that making the 
speed consistent with SAE 1626 will eliminate the possibility of 
discontinuities in the test's stringency for different vehicles. As 
AAMA correctly stated, the proposed test speed created an anomaly that 
benefitted vehicles with a maximum drive-through speed slightly below 
30 mph. For example, a vehicle with a maximum drive-through speed of 29 
mph would have been tested at 22 mph, while a vehicle with a maximum 
drive-through speed of 30 mph would have been tested at 30 mph. This 
would have meant that a 1 mph difference in maximum drive-through speed 
would have resulted in a 8 mph difference in test speed. This could 
have caused significant variations in test results for vehicles with 
slight differences in maximum drive-through speed. By establishing a 
test speed that is adjusted for differences in maximum drive-through 
speed and that would be more specific and distinct for each vehicle and 
test surface, the agency has minimized potential compliance testing 
problems that might occur due to variability in the test speeds for 
different vehicle and road test surface conditions.
    NHTSA notes that ATA's requested test speed and test conditions 
have not been tested by the agency or industry and therefore their 
adoption would not be appropriate at this time. The agency may evaluate 
ATA's proposal in future test programs.
    NHTSA believes that Advocates' opposition to permitting test speeds 
below 30 mph is unfounded. Similarly, the agency believes that AAMA's 
concern about the drive-through speed being unrepeatable is irrelevant. 
By allowing vehicles to be tested at 30 mph or 75 percent of maximum 
drive-through speed, whichever is less, the effects of test surface 
variation are eliminated.\38\

    \38\TRC Report, page 10. [[Page 13234]] 
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4. Type of Brake Application
    In the NPRM, NHTSA proposed that the stops be achieved through full 
brake applications in which the pressure at the treadle valve must 
reach 100 psi within 0.2 seconds after the application is initiated. 
The agency believed that these values properly represent full brake 
applications, in terms of both the application's degree of force and 
its duration. The agency stated that the stability and control 
requirements should evaluate worst case braking applications in an 
aggressive or ``hard'' stop and that full brake applications are more 
readily repeatable than the ``driver best effort'' applications.
    Midland-Grau agreed with the proposal to specify a full treadle 
application of 100 psi in 0.2 seconds for air braked vehicles. 
According to Midland-Grau's test data, full treadle applications at 100 
psi were achieved in 0.12 to 0.18 seconds, with the measurement taken 
at the treadle valve's primary output circuit located at the rear axle 
brakes. However, more time is needed to reach 100 psi at the secondary 
circuit located at the front axle brakes because its output supplies 
air to the quick release valves and then to the front axle brake 
chambers. Allied Signal stated that it is not possible to reach 100 psi 
within 0.2 seconds at the front axle output circuit of the treadle 
valve.
    After reviewing these comments, NHTSA has decided to revise the 
brake application requirement for air braked vehicles to require 100 
psi in at least one of the treadle valve's output circuits within 0.2 
seconds, thereby allaying Allied Signal's concern. This modification to 
the test condition should eliminate potential ambiguity concerning 
where the application pressure is to be measured.
5. Number of Test Stops for Certification
    In the NPRM, NHTSA proposed that a vehicle comply with the proposed 
braking-in-a-curve test in each of three consecutive stops for each 
combination of weight and road conditions. In contrast, the vehicle 
stopping performance tests in Standard No. 105 and Standard No. 121 
specify that the vehicle must meet the requirements at least once in 
six attempts through a best effort brake application. The agency 
tentatively concluded that six stops should not be needed to achieve 
the required performance in the braking-in-a-curve test, given the 
presence of an antilock brake system. The agency requested comments 
about the number of brake applications that should be required.
    Advocates, Midland-Grau, and Mr. Crail stated that three stops are 
sufficient for a vehicle with an antilock brake system to display 
compliance with the braking-in-a-curve test. They stated that without 
stopping distance requirements, this test procedure entails a simple 
performance test for the vehicle to maintain control in the 12-foot 
lane. Midland-Grau added that it uses three stops when conducting ABS 
performance tests, and that this number of brake applications is 
consistent with the SAE J1626 Recommended Practice and with the MVSRAC 
Antilock Brake System Task Force's final recommendations.
    AAMA argued that specifying three passes in three consecutive stops 
places an unrealistic burden on the driver to control the vehicle 
immediately with no opportunity to become familiar with the vehicle or 
test surface. AAMA recommended that manufacturers be given the option 
of conducting ten or more stops and certifying that the vehicle stayed 
within the 12-foot lane for any three consecutive stops.
    After reviewing the comments and the available information, NHTSA 
has decided that requiring compliance with the braking-in-a-curve 
requirements during three consecutive stops is appropriate. The agency 
notes that specifying three consecutive full treadle test stops is 
consistent with both the agency's own testing at VRTC and its testing 
in conjunction with the motor vehicle industry through the MVSRAC ABS 
Task Force. The use of full treadle brake applications to test an ABS-
equipped vehicle to the braking-in-a-curve maneuver requires less 
driver skill than the use of a driver's-best-effort modulated brake 
application (i.e., the type of application used in stopping distance 
performance tests) because the ABS automatically modulates the brakes. 
Further, more than three stops are unnecessary since the braking-in-a-
curve test requirement is not coupled with a stopping distance 
requirement. Therefore, NHTSA has decided not to adopt AAMA's 
suggestion that manufacturers be given the option of complying with 
only three of ten stops. Adopting that suggestion would make the 
braking-in-a-curve requirement unreasonably lenient.
6. Test Weight
    In the NPRM, NHTSA proposed that single unit trucks, buses and 
bobtail truck tractors be tested at their curb weight (including full 
fuel tanks) plus 500 pounds to account for the driver and 
instrumentation. The agency also proposed to allow a manufacturer to 
conduct the braking-in-a-curve test with a roll bar structure weighing 
up to an additional 1,000 pounds to protect the driver, based on a 
recommendation by the MVSRAC ABS Task Force. The agency requested 
comments about the appropriate unloaded test weight.
    Rockwell WABCO recommended that unloaded heavy vehicles be allowed 
to have less than 500 pounds added in the unloaded condition.
    After reviewing Rockwell WABCO's comment, NHTSA has decided to 
amend the test condition in the braking-in-a-curve test to specify the 
weight in the unloaded condition to be ``up to 500 pounds'' for driver 
and instrumentation.\39\ The agency notes that instrumentation hardware 
has been getting more compact and lightweight. Using the regulatory 
language ``up to 500 pounds'' will simplify the test condition since 
manufacturers will not have to add ballast to ensure that the weight is 
500 pounds. This change provides manufacturers with greater incentive 
to use the newer, lighter hardware. The agency believes that this 
modification will have no measurable effect on a vehicle's performance 
during the braking-in-a-curve test since a weight range of a few 
hundred pounds is of little significance in relation to a tractor's 
typical empty weight of more than 26,000 pounds.

    \39\The final rule also adopts the 1,000 pound allowance for a 
roll bar.
---------------------------------------------------------------------------

7. Loading Conditions
    In the NPRM, NHTSA proposed that braking-in-a-curve tests be 
performed in both the empty and loaded conditions, since a vehicle's 
braking performance varies depending on the amount of load that it is 
carrying. With respect to testing truck tractors in the loaded 
condition, the agency proposed two alternatives regarding the use of 
control trailers: (1) use a braked control trailer and (2) use an 
unbraked control trailer.
    Most commenters, including AAMA, Rockwell WABCO, and Midland-Grau, 
supported the unbraked control trailer alternative. These commenters 
believed that using an unbraked control trailer instead of a braked 
control trailer would eliminate many sources of variability and would 
provide more consistent and repeatable test data. AAMA stated that if 
the braked control trailer alternative were adopted, every aspect of 
the control trailer brake system would have to be precisely specified 
because the tractor's performance is directly affected by the 
performance of the control trailer. Midland-Grau stated that using an 
unbraked control trailer is consistent with SAE J1626 and the testing 
[[Page 13235]] performed by the MVSRAC ABS Task Force.
    Similarly, commenters on the February 1993 stopping distance NPRM 
strongly supported the unbraked control trailer alternative. Those 
commenters believed that the agency would have great difficulty 
defining the required performance of a braked control trailer and its 
ABS if the braked control trailer alternative were adopted.
    Mr. Crail and Strait-Stop stated that a truck tractor should be 
tested with an ABS-equipped control trailer because it is not normal 
for a combination vehicle to be operated with an unbraked control 
trailer. They believed that a braked control trailer would more closely 
reflect real world braking. Mr. Crail also stated that an unbraked 
control trailer could result in instability during testing.
    After reviewing the comments and other available information, NHTSA 
has decided to specify that truck tractors be tested with an unbraked 
control trailer for the braking-in-a-curve test. As the agency 
explained in the NPRM, the unbraked control trailer eliminates certain 
types of variability and provides more repeatable test data. Moreover, 
this approach eliminates the need for the agency to specify and vehicle 
manufacturers to comply with detailed foundation brake design 
requirements for the control trailer. Accordingly, the unbraked control 
trailer will provide more readily comparable test data among vehicles 
and more repeatable test parameters for manufacturers.
    NHTSA acknowledges that an unbraked control trailer does not 
represent a typical operating condition for a combination vehicle. As a 
result, real world combination vehicles will stop more effectively than 
a test combination vehicle that has brakes on its tractor but not on 
its trailer. Nevertheless, as most commenters stated, the unbraked 
control trailer provides significant benefits for testing a loaded 
truck tractor. Further, using the unbraked control trailer is 
consistent with SAE J1626 and the testing performed by the MVSRAC Task 
Force.
    As for Mr. Crail's concern about stability problems during testing, 
NHTSA does not agree that the use of an unbraked control trailer will 
result in such problems. It is true that using an unbraked control 
trailer will result in the kingpin receiving additional forces, since 
the trailer will still be pushing on the kingpin while the tractor is 
braking. However, the agency and industry conducted several braking-in-
a-curve tests with unbraked control trailers that indicated that these 
additional kingpin forces will not increase a vehicle's instability 
during testing.\40\

    \40\TRC Report #091194, page 4.
---------------------------------------------------------------------------

8. Initial Brake Temperature
    In invalidating parts of Standard No. 121, the court in PACCAR 
stated that the standard failed to specify formal and reasonably 
specific testing criteria about the time intervals between tests. The 
time interval between tests is important because it may affect brake 
temperature and thus brake lining performance. In response to PACCAR, 
the agency amended the standard to specify that the average brake 
lining temperature of the hottest axle be between 150 deg. and 200 
deg.F before performance tests could be conducted.
    In the February 1993 NPRM on stopping distance and the September 
1993 NPRM on stability during braking, NHTSA proposed that the average 
brake lining temperature of the hottest axle be between 250 deg. and 
300 deg. F before performance tests could be initiated. This range was 
based on testing conducted by VRTC<SUP>41. The agency believed that 
compared to current requirements, this provision would allow tests on 
heavy vehicles to be conducted within a shorter time between 
measurements at temperatures representative of in-service conditions, 
without affecting brake performance.

    \41\``Heavy Duty Vehicle Brake Research Program--Report No. 1,'' 
April 1985.
---------------------------------------------------------------------------

    Only Advocates commented on the proposal in the stability and 
control NPRM to increase the initial brake temperature from 150-200 
deg.F to 250-300  deg.F. Advocates supported the higher temperature 
range, stating that it is reasonable and representative of in-service 
temperature conditions. However, NHTSA received numerous comments about 
this issue in response to the stopping distance NPRMs. All commenters 
addressing the issue of initial brake temperature in those rulemakings 
strongly opposed the proposed change in temperature from 150-200  deg.F 
to 250-300  deg.F. Lucas argued that the higher initial brake 
temperature would be detrimental to drum brake performance. Lucas, 
HDBMC, and Rockwell WABCO stated that the proposed initial brake 
temperature would invalidate the vehicle manufacturer's data bank from 
Standard No. 121 testing at 150-200  deg.F, which has been accumulating 
since the 1970s. Midland-Grau commented that, among other things, the 
higher initial brake temperature would lead to more aggressive lining 
materials and vehicle compatibility problems.
    Abex, AAMA, and HDBMC stated that the proposed higher initial brake 
temperature would shorten testing time between 5 and 10 hours. However, 
they believed that problems associated with brake fade resulting from 
the higher initial brake temperature would far outweigh the nominal 
cost savings obtained by having a shorter test time. Test data provided 
by AAMA showed that while the higher initial brake temperature has a 
slight adverse effect (a 7-28 foot increase) on full service brake 
stopping distance, it has a significant adverse effect (a 25-98 foot 
increase) on emergency brake stopping distance.
    Rockwell WABCO stated that the perceived benefits of the higher 
initial brake temperature do not justify the increased vehicle testing 
and redesign that would be required to meet the proposed initial brake 
temperature.
    After reviewing the comments, the test data, and other available 
information, NHTSA has decided that an initial brake temperature in the 
150  deg.F to 200  deg.F range is more appropriate than the proposed 
temperature range. As the commenters stated, testing using the 150 
deg.F to 200  deg.F temperature range is more repeatable and results in 
less variation between runs, compared to testing conducted using an 
initial brake temperature of 250  deg.F to 300  deg.F, particularly for 
the emergency brake stops. The agency further notes that an initial 
brake temperature of 150  deg.F to 200  deg.F is within the 150  deg.F 
to 300  deg.F range recommended by the VRTC test report. The agency is 
aware that the lower temperature range increases the total test time by 
5 to 10 hours. Nevertheless, because the other advantages to the lower 
temperature range outweigh this concern, NHTSA has decided not to 
change the specification that the initial brake temperature be between 
150 to 200  deg.F.
9. Transmission Position
    In the NPRM, NHTSA proposed that the transmission be in neutral or 
the clutch pedal be depressed (clutch disengaged).
    ATA commented that, in real world panic stops, drivers will neither 
put the transmission in neutral nor depress the clutch pedal before 
making a brake application. Nevertheless, ATA acknowledged that 
retardation by the drivetrain could cause vehicle instabilities that 
would necessitate testing at speeds lower than the drive through speed.
    NHTSA has concluded that testing with the transmission in neutral 
or the clutch disengaged is appropriate to ensure that engine 
retardation does not affect a test which is intended to 
[[Page 13236]] evaluate the influence of brake systems on vehicle 
dynamic stability. Engine and drivetrain retardation forces vary from 
vehicle to vehicle and can affect vehicle stability on low coefficient 
of friction surfaces. Nevertheless, this is not the purpose of this 
test. By requiring that the transmission be placed in neutral for brake 
testing, the standard attempts to reduce these drive-train related 
braking influences on the service brake performance. Therefore, testing 
with the transmission in neutral or the clutch disengaged will 
eliminate influences that engine or drivetrain retardation would have 
on braking performance. This test condition therefore helps to ensure 
test repeatability and reproducibility.
10. Summary of General Test Conditions
    For the convenience of the reader, this section summarizes the 
general test conditions being adopted in this notice, as follows:
    <bullet> Vehicle Position--Centered in the test lane at the 
initiation of braking.
    <bullet> Steering--Driver to steer as necessary during braking to 
maintain vehicle control.
    <bullet> Initial Brake Temperature--The average brake lining 
temperature of the hottest axle between 150 to 200  deg.F.
    <bullet> Transmission--Neutral (or clutch pedal depressed).
    <bullet> Loading for Truck Tractors
    Empty (Bobtail): Curb Weight (including full fuel tanks) plus up to 
500 pounds for driver and instrumentation, and, at the manufacturer's 
option, a roll bar weighing up to 1,000 pounds.
    Loaded: Tractor is loaded with an unbraked control trailer, loaded 
above the kingpin only, so that the tractor is at GVWR and the trailer 
axle is at 4500 pounds. Tractor weight is distributed in accordance 
with the Gross Axle Weight Ratings (GAWRs). If the tractor's fifth 
wheel is fixed, preventing such loading, then the trailer is loaded 
until any one tractor axle reaches its GAWR.
    <bullet> Brake Burnish--Follow procedures in S6.1.8(b) of Standard 
No. 121.

Low Mu Braking-In-A-Curve Test

    <bullet> Run vehicle, empty and loaded.
    <bullet> Test Surface--PFC of 0.5, as determined with the ASTM 
E1136 SRTT tire on ASTM traction trailer using ASTM E1337-90 procedure.
    <bullet> Track Configuration--500 foot radius at lane center line.
    <bullet> Test Speed--30 mph or 75 percent of the maximum drive-
through speed, whichever is less. Maximum drive-through speed is the 
highest constant speed at which the vehicle can be driven through 200 
feet of curve arc without any part of the vehicle leaving the 12-foot 
lane.
    <bullet> Brake Application--Three full-treadle applications (i.e., 
air pressure of 100 psi at any treadle valve output circuit within 0.2 
second) for each loading condition.
    <bullet> Test Failure Condition--Vehicle must stay within the 12- 
foot lane during all three stops in order to comply with requirement.

D. Reliability and Maintenance

    In response to the SNPRM, ATA, United Parcel Service (UPS), and 
Tramec expressed concern about the durability, reliability, and 
maintenance of ABSs. ATA stated that the rule, if adopted, would result 
in significant maintenance problems, especially with respect to 
failures of electrical circuits and of the power source. It claimed 
that ABS components fail too often and that real world failure rates 
are higher than those in NHTSA's demonstration program. ATA further 
stated that it is inappropriate to compare the failure rates of ABS 
components that are not subject to wear with the rates for components, 
like brake linings and tires, that are subject to wear. ATA stated that 
existing connectors fail in large numbers and that what it mistakenly 
termed a ``separate connector requirement'' would double the failure 
rate, resulting in unreasonable costs.<SUP>42 It also stated that there 
have been many problems resulting from inadequate installation of ABSs, 
since malfunctions are frequently due to design problems, faulty 
installation, and lack of knowledge about ABS maintenance. ATA also 
stated that NHTSA did not take seriously enough malfunctions noted 
during the agency-sponsored in-service fleet study, which were 
rectified with only the expenditure of labor, namely corrections that 
involved inspections or minor adjustments.

    \42\The agency notes that it is requiring powering through a 
separate circuit, not a separate connector.
---------------------------------------------------------------------------

    ATA and UPS stated that new ABS equipped heavy vehicles have a high 
percentage of ``direct from factory'' ABS failures. UPS stated that 
``these systems are still plagued by incidents of failure that far 
exceed the normal level of problems encountered with other components 
of heavy duty trucks.'' ATA also stated that NHTSA did not take labor 
only failures (i.e., malfunctions that can be fully corrected through 
the use of labor without the need for new parts) seriously enough. ATA 
believes that they are a costly and serious problem that takes vehicles 
out of service.
    To evaluate the reliability of current-generation ABSs, NHTSA has 
conducted extensive field studies of ABS-equipped heavy truck tractors 
and semitrailers in developing this final rule. In response to the 
PACCAR decision, these studies were structured to assess whether 
current-generation heavy vehicle antilock brake systems were reliable 
and fail-safe, whether they inordinately increased vehicle maintenance 
costs, and whether they could be successfully maintained and would 
remain functioning in typical U.S. heavy truck operating environments.
    Between 1988 and 1993, NHTSA tracked the maintenance performance 
histories of 200 truck tractors and 50 semitrailers equipped with ABS, 
as well as the histories of a comparison group of 88 truck tractors and 
35 semitrailers not equipped with ABS, to determine the incremental 
maintenance costs and patterns associated with installing ABS on these 
heavy vehicles. Additionally, special on-board vehicle recorders were 
used to monitor the functioning and performance of the ABSs. Finally, 
drivers and mechanics at the participating test fleets were 
periodically interviewed to ascertain their views about the ABS test 
vehicles' performance and ease of maintenance. This multimillion dollar 
program was the largest of its kind that has ever been conducted by the 
agency or throughout the world. The study's authors concluded that, 
based on the data collected during the fleet study, currently available 
antilock braking systems are reliable, durable and maintainable.
    While ABS is not a zero-cost maintenance item, its presence on a 
vehicle did not substantially increase maintenance costs (less than 1 
percent for tractors, less than 2 percent for trailers) or decrease 
vehicle operational availability. Specifically, ABS use does not 
involve appreciably more intensive maintenance than present brake 
systems. The agency finds that the average annualized increase in 
lifetime maintenance costs ($3.47-$27.49 per vehicle) occasioned by the 
use of ABS, as indicated in the Final Economic Assessment (FEA) for 
this rulemaking, is a reasonable amount of additional maintenance. 
Further, the agency notes that a significant portion of the costs noted 
during the fleet study (i.e., those attributed to intermittent 
malfunction warning indications for which no problem was found and the 
system was simply reset or a simple adjustment was made) are likely to 
be reduced or eliminated as the algorithms inside the ECU that trigger 
ABS malfunction warnings are further refined to make them more 
discriminating, and as [[Page 13237]] quality control and installation 
skill improve.
    NHTSA further emphasizes that system malfunctions do not render the 
vehicle's braking system unsafe, since the brake system merely reverts 
to one without an ABS; in other words, foundation brakes are unchanged 
when ABS is added. The few incidents noted during the test program in 
which an ABS malfunction did compromise the vehicle's underlying brake 
system performance involved defective components.
    In both the tractor and the trailer studies, some test vehicles 
either arrived in the test fleets with faulty ABS or had ABS 
malfunction indications shortly thereafter, as a result of what was 
termed installation or pre-production design related problems. In 
general, these problems were easily remedied. Many were corrected by 
adjustments or minor repairs. Most were at least partially attributable 
to the prototype nature of many of the installations accomplished for 
this test program.
    The following examples illustrate the relatively minor nature of 
correcting most of the problems. (The agency notes that none of the 
problems listed affected vehicle braking.)
    <bullet> The electrical power source for the ABS ECU on a group of 
four trucks was incorrectly wired, at the time of installation, through 
the starter solenoid. These four trucks had to be rewired to make the 
ABS function properly.
    <bullet> Intermittent failure warnings were noted on three trucks 
from the beginning of their operation. Upon inspection, the trucks were 
found to have an incompletely assembled connector in the wiring 
harness. When this problem was corrected, the failure warnings ceased.
    <bullet> A group of 23 tractors had to be rewired to provide a 
separate electrical power source for the dash-mounted failure warning 
lamp so that it could function properly. The miswiring occurred during 
installation.
    <bullet> The ABS modulator valves on a group of 12 tractors had to 
be relocated on the vehicles' frame rails to eliminate an inadvertent 
physical interference problem with the vehicles' driveshafts. This 
problem occurred as a result of an oversight during installation.
    <bullet> On one truck, a sensor cable needed to be rerouted and 
resecured because of an interference/pinching problem with the wire and 
the steering gear.
    NHTSA emphasizes that these problems and others like them do not 
reflect inherent design flaws with ABS's principal components (i.e., 
the ECU, modulators, and wheel speed sensing hardware). Instead, they 
involve wiring and installation problems. This highlights the 
importance of using high quality wiring components and paying close 
attention to installation details. The agency anticipates that the 
frequency of these problems will be lower than that experienced during 
the agency's test program once ABS production/installations increase to 
a level high enough to enable the quality control programs typically 
utilized by suppliers and truck manufacturers to take effect.
    An average of 1.35 labor hours and $106.46 in replacement component 
parts costs per test truck tractor were necessary to rectify these 
installation/pre-production design related problems. Comparable figures 
for semitrailers were 1.9 labor hours and $65.36 in parts costs. All 
these costs are usually recovered by fleets under the terms of typical 
warranties offered by ABS suppliers and/or truck manufacturers. NHTSA 
notes that the start-up or installation/pre-production design related 
problems that the test fleets experienced are similar to the 
experiences that fleets were reported to have had with other devices 
such as electronically-controlled engines when they were first 
introduced on heavy trucks in the mid-1980's.
    During the two-year period in which the reliability of these 
systems was evaluated, 200 ABS-equipped test tractors accumulated 
39,818,659 miles of travel. During that time period, 126 trucks (63 
percent) needed ABS-related maintenance that could best be attributed 
to normal service wear factors rather than installation or pre-
production design related problems. A total of 421 incidents of this 
type occurred with the 125 trucks, the majority (321 or 76 percent) of 
which involved inspections/adjustments. The remainder (100 or 24 
percent) involved repairs/replacements. All brands of the ABSs involved 
in the test program experienced incidents of this type at one time or 
another during their in-service operation.
    Forty vehicles experienced more than one failure warning, 
interspersed over time, with two vehicles experiencing 35 and 31 
separate indications (23 percent of the total resets), respectively, 
without the source of the problem being uncovered. Two other trucks 
experienced 12 and 10 separate indications respectively. These four 
vehicles (4.5 percent of the trucks experiencing this problem) 
accounted for 30 percent of the total intermittent failure warning 
indications and resets.
    All five ABS suppliers' systems experienced intermittent failure 
indications with at least one of their forty test trucks involved in 
the test program. In each case, the ABS was either manually reset or 
the warning light did not reactivate when the truck's ignition was 
turned off and subsequently turned on again at some later time. 
However, 61 percent of the total failure warning indications of this 
type, and 34 percent of the vehicles experiencing intermittent failure 
indications, were attributable to one supplier's ABS. Another 
supplier's system accounted for another 18 percent of total failure 
warning indications and an additional 28 percent of the total vehicles 
involved. Since the time of the agency's test, both suppliers' systems 
have been modified to reduce the number of these false-positive 
malfunction indications.
    The table shown below indicates the maintenance related to in-
service wear that was required during the tractor portion of the 
program on each of the ABS components. Data are displayed by 
maintenance category (adjustments/inspections and repairs/
replacements). Inspections and ECU resets associated with intermittent 
failure warning indications were the principal occurrence. In general, 
most of the work did not involve parts replacements. Parts replacement 
incidents totaled 40, with 55 percent of these (22) involving failure 
warning lamp bulbs or fuses. The total average number of in-service 
wear related maintenance incidents, including all inspections, 
adjustments, repairs and replacements was 2.11 incidents per truck over 
the two-year period of the test.

                                                                                                                
[[Page 13238]]                                                                                                  
   ABS In-Service Wear Related Maintenance Incidents Over the Two-Year  
          Period of the Test, by System Component Needing Work          
------------------------------------------------------------------------
                                             Number of                  
                                              trucks         Number of  
                                             requiring        trucks    
              ABS component                inspections,      requiring  
                                           adjustments,   replacement of
                                           or repairs on  this component
                                          this component                
------------------------------------------------------------------------
Wiring Cables...........................              26               4
Wiring Connectors.......................              19               2
Sensors and Related Parts...............              22               3
Modulator Valves and Related Parts......               3               2
ECUs....................................              19               7
Fuses and Lamps.........................               7              18
System Resets...........................              84               0
                                         -------------------------------
      Total No. of Trucks per Column....             118              32
                                         -------------------------------
Overall No. of Trucks Involved in the In-                               
 Service Related Incidents..............                                
(1)125                                                                  
------------------------------------------------------------------------
Note: Columns are not additive.                                         

    Replacing the 19 faulty major ABS components, and performing all 
the other inspections, adjustments and repairs that were in-service 
wear related, resulted in approximately 403 hours of labor expenditure 
and $4,068 for parts replacements. At a standard hourly rate of $35 per 
hour, the total cost of $18,173 for labor and parts amounts to 0.046 
cent-per-mile (based on 39,818,659 total miles of travel) for the cost 
of maintaining the ABSs over the two-year period.
    Inspections/ECU resets, which only involved labor expenditure, 
accounted for 45 percent of these total costs. Even though they 
occurred infrequently, ECU replacements tend to be costly, accounting 
as they did for 21 percent of the in-service wear related maintenance 
costs.
    Similar findings were noted for the 50 ABS-equipped semitrailers 
that also were evaluated. The test vehicles accumulated 4,001,369 miles 
of in-service use during almost two years of operation during the 
program. During that time period, 23 semitrailers (46 percent) needed 
ABS-related maintenance that could best be attributed to normal service 
factors, rather than installation or pre-production design related 
problems. This compares favorably to the 63 percent of tractors 
requiring ABS service during the tractor program. A total of 44 
incidents of this type occurred with the semitrailers, with the 
majority (29, or 66 percent) involving inspections or adjustments. The 
remainder (15, or 34 percent) involved repairs or replacements. These 
percentages are similar to the 76 percent for adjustments and 
inspections and 24 percent for repairs and replacements seen during the 
tractor program.
    The following table shows in-service trailer maintenance that was 
required during the program for each category of ABS components. 
Inspections and ECU resets associated with failure warning indications 
were the principal occurrence. Parts replacement incidents totaled six, 
with three of these being status light bulbs and three speed sensors. 
In general most of the work did not involve parts replacement.
    The average number of in-service maintenance incidents, including 
all inspections, adjustments, repairs, and replacements was 0.88 
incidents per semitrailer over the two-year test period. This compares 
well with the 2.11 incidents per tractor seen during the tractor 
portion of this program.
    Replacing six faulty ABS components, plus performing all other 
inspections, adjustments, and repairs that were in-service related, 
resulted in about 44 man-hours of labor expenditure and $234 for parts 
replacements. At a standardized hourly rate of $35 per hour, the total 
cost of maintaining the ABSs, for labor and parts, over two years 
($1774) amounts to 0.044 cents-per-mile (based on 4,001,369 total miles 
of travel). The inspections and ECU resets (which only involved labor 
expenditure) accounted for 35 percent of the total costs. Comparable 
tractor figures are 0.046 cents-per-mile for total costs and 45 percent 
of the total costs for inspection and ECU reset, indicating that 
semitrailers performed very much like tractors.

ABS In-Service Wear Related Maintenance Incidents Over the Two-Year Test
                 Period by System Component Needing Work                
------------------------------------------------------------------------
                                             Number of                  
                                           semitrailers      Number of  
                                             requiring     semitrailers 
              ABS component                inspections,      requiring  
                                          adjustments or   replacements 
                                            repairs on        of this   
                                          this component     component  
------------------------------------------------------------------------
Wiring Cables...........................               4               0
Wiring Connectors.......................               2               0
Sensors and Related Parts...............              10               3
Inspection, with No Problem Found (NPF).              12               0
ECUs....................................               4               0
Fuses and Lamps.........................               3               3
                                         -------------------------------
[[Page 13239]]                                                          
                                                                        
      Total No. of Semitrailers per                                     
       Column...........................              23               6
                                         -------------------------------
Overall No. Semitrailers Involved in the                                
 In-Service Related Incidents...........                                
(1)23                                                                   
------------------------------------------------------------------------
Note: Columns are not additive.                                         

    At the completion of the overall 5-year test program, NHTSA 
conducted a final follow-up survey among the participating fleets. 
Among the 13 fleets that were continuing to maintain the ABS on the 
original test tractors, 97 percent of those tractors had functioning 
ABS. On the other hand, ABSs were not functioning on two-thirds of the 
original test tractors in the three fleets surveyed that chose not to 
continue maintaining the systems. This demonstrates that fleets must be 
willing to maintain the ABS if it is to be kept operational. An analogy 
can be drawn between the need to periodically inflate tires and the 
need to periodically perform minor, routine maintenance of ABS systems. 
Even though neither is time-consuming or costly, this type of 
maintenance is necessary if anticipated performance is to be achieved.
    ATA commented on the SNPRM that the ABS repair/replacement rate 
(14-33 incidents per 100 vehicles per year) indicated in the agency's 
fleet study significantly understated the actual rate, citing the 
experience of one of its member carriers which recorded six to thirteen 
times as many ``repair incidents.''
    Although NHTSA has not had the opportunity of reviewing the records 
ATA cited, the agency is inclined to believe that the difference in 
rates may be attributable to a difference in the definition of a 
``repair incident.'' The agency fleet study data cited by the ATA 
(i.e., 14-33 incidents per 100 vehicles per year) were for ``repairs/
replacements'' of ABS components. They did not include instances in 
which ``inspections'' or ``adjustments'' were made. For instance, 
adjustments of wheel speed sensors are not included in this total. This 
exclusion was necessary because comparable inspection/adjustment data 
were not available for the other vehicle components whose maintenance 
histories were being compared in the fleet study to that for the ABSs.
    The above discussion accounts for all the in-service maintenance 
activity that was performed on the test ABSs. The ``monitoring'' to 
which ATA refers did not in any way contribute to or detract from the 
reliability data for the ABSs under evaluation. That monitoring was 
intended to ensure that all the maintenance work that was performed was 
recorded, so that a complete picture could be portrayed of the extent 
and nature of maintenance work that could be expected if U.S. heavy 
trucks were equipped with ABSs. Based on those data, the agency 
concludes that, overall neither unreasonable amounts or excessively 
costly additional maintenance will be imposed on U.S. heavy truck 
operators in order to maintain ABS. Thus, the agency disagrees with 
ATA's assessment that significant maintenance problems will arise ``* * 
* when the equipment is used outside the close monitoring it received 
in the NHTSA demonstration program.''
    ATA further stated that ABSs are ``* * * not yet as durable as they 
must be for successful operation * * * in the U.S.'' That organization 
cited the fact that, as described above, three of the original 
participating fleets which ceased participating in the test program had 
appreciable proportions of non-functioning ABSs on their original test 
vehicles because they no longer maintained the systems.
    NHTSA notes that this outcome could be anticipated with many other 
components besides ABS, that are installed on motor vehicles, for 
example, tires, engines, etc. All such components require periodic, and 
occasionally non-periodic, non-scheduled maintenance, in order to 
remain functional. Notwithstanding, the agency believes that the data 
contained in the two fleet study reports indicate that equipping 
vehicles with ABS is appropriate. Taken in total, those data indicate 
that, while ABS is not a zero-maintenance component, it is neither 
difficult nor unduly expensive to maintain. The fleet test results 
indicate that the level of maintenance attention needed to keep ABS 
functional is reasonable relative to the safety benefits that are 
estimated to result from use of these systems.
    ATA also disagreed with the comparisons that were made in the 
agency's fleet study of repair and malfunction rates of ABS compared to 
other components on the vehicle that were susceptible to wear-related 
replacement. In the fleet study, comparisons were made between the 
maintenance histories of ABS and comparable histories for wheels/hubs, 
foundation brake components, pneumatic brake components, electrical 
system components, and tires.<SUP>43 These items were chosen because 
the agency believed that the maintenance patterns and costs of only 
these components could have been affected by the presence of ABS on the 
vehicle. The agency decided that it would be inappropriate to compare 
ABS maintenance results to items, such as engines and other drivetrain 
components, whose maintenance histories and costs would be unaffected 
by the presence of ABS.

    \43\DOT HS 8070846, pages 3-24; DOT HS 808-059, pages 3-19, 3-
20.
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    ATA also questioned whether maintenance problems could have been 
underreported by a factor of 2.5 because the on-board recorders used 
during the trailer fleet study recorded less miles of travel (1.6 
million vehicle miles of travel) than were accumulated by all the test 
trailers (4 million miles) during the test program. NHTSA notes that 
the maintenance history and cost data reported in the two studies were 
not affected by this discrepancy. The recorders were primarily used to 
obtain statistical information on the relative frequency of ABS 
activations per mile of travel. While their secondary purpose was to 
monitor ABS functioning, this was done only as a backup to the standard 
maintenance reporting and [[Page 13240]] record-keeping activities of 
the participating fleets. The ABS maintenance histories that are 
reported in the fleet studies were derived from those maintenance 
records and are known to be thorough and complete.
    ATA further believed that NHTSA's fleet studies underreported ABS 
maintenance problems. That organization cited incidents in which 
drivers failed to couple the second tractor-to-trailer electrical 
connector that was installed to power the ABS and instances in which 
drivers drove for an extended time period without reporting an ABS 
malfunction.
    NHTSA believes that ATA's additional concerns about maintenance 
problems with ABSs are without merit. With regard to the first point, 
even though a limited number of drivers did not, in some instances, 
couple the separate tractor-to-trailer electrical connector, this fact 
does not affect whether those trailers' antilock systems received 
electrical power. The trailer ABSs in question were all wired 
redundantly to accept backup power from the stop lamp circuit on the 
other tractor-to- trailer electrical connector that the drivers did 
connect. Therefore, the ABSs on these trailers were functioning 
throughout the test using backup power from the standard tractor-to-
trailer electrical connector, and were exposed to the possibility of 
malfunctioning just as much as the other test trailers in the study 
were.
    As to ATA's claim that some drivers did not report a malfunction 
for an extended period of time, there were only a few instances of 
drivers driving for a time with non-functioning ABSs. The functional 
status of ABSs on test vehicles was checked, no less than monthly, by 
test study personnel, and often more frequently by fleet maintenance 
personnel. Therefore, in each case, the existence of a nonfunctioning 
ABS was detected after only a limited number of trips were made under 
that condition.
    ATA attached to its comments letters from some of its members, 
including Consolidated Freightways, Inc. (Consolidated), UPS, and Ruan 
Transportation Management Systems (Ruan). ATA characterized these 
letters as indicating that ABS ``* * * failures are still happening and 
that other things are going wrong also''. Consolidated's submittal 
contained a sample listing of maintenance shop orders describing 
various repairs performed on ABS installed on its vehicles.
    NHTSA could not ascertain the statistical prevalence of these 
incidents in Consolidated's fleet, given the way Consolidated presented 
its data. Thus, these incidents have only anecdotal value. 
Nevertheless, the nature and description of these incidents parallels 
those experienced and recorded during the agency's fleet study. For 
instance, several incidents cited by Consolidated involved faulty wheel 
bearings that knocked wheel speed sensors out of adjustment. NHTSA 
believes that these incidents should not be viewed as ABS failures. 
Further, other carriers have suggested that the ABS' ability to detect 
faulty wheel bearing conditions, which fail regardless of whether a 
vehicle is equipped with ABS, is a safety and maintenance benefit, not 
a detriment. The majority of other incidents cited by Consolidated 
involved minor wiring/connector problems that can be readily solved by 
tractor manufacturers' use of higher quality wiring/connector 
components or better attention to installation quality control. 
Carriers may address such situations through traditional warranty and 
customer complaint channels and, if necessary, through buying vehicles 
from manufacturers with higher overall product quality ratings.
    UPS cited data indicating that the ABS malfunction warning light on 
40 percent of a sample of ABS-equipped vehicles received from the 
factory since 1990 was activated when the vehicles were delivered. UPS 
did not provide detailed information listing the causes of these 
malfunction indications. Further, UPS did not explain whether the 
problems were remedied by simple adjustments of the same sort that are 
typically done during ``dealer preparation,'' prior to a dealer's 
delivering a vehicle to the customer. The agency notes that many large 
fleets such as UPS assume the dealership role when they receive large 
orders of vehicles directly from the factory. As a result, they assume 
responsibility for making this type of minor ``make-ready'' 
adjustments.
    UPS also cited high proportions of ABS ``hard repairs or 
replacements,'' but did not define what constituted a ``hard repair.'' 
Thus, it is not possible for NHTSA to determine whether some of these 
might have been considered ``inspections/adjustments'' under the 
reporting scheme used in the agency fleet study or to put any of these 
figures in context or interpret them relative to the study's findings.
    Ruan indicated that it was having difficulty getting an ABS 
supplier to respond to its requests for problem-solving help. Ruan 
listed a series of problems, similar to those noted in the agency's 
fleet study and cited by other carriers. Ruan's comments were anecdotal 
in nature and did not include any statistical information that would 
help portray the extent to which this affected their overall 
maintenance activities or costs. Nevertheless, all of the ABS suppliers 
and the major truck manufacturers have indicated, in the discussions 
they held with the agency on May 3, 4, and 19, 1994<SUP>44, that they 
are committed to providing field service support staff, training, 
maintenance information, and other help to remedy the problems cited by 
Ruan and others. NHTSA has repeatedly stated that manufacturers must 
make service support available to fleets to ensure the success of this 
rulemaking effort. The agency anticipates that the ABS suppliers and 
major truck manufacturers will provide this support, given their 
statements in response to the NPRM that they are prepared to and are 
now doing so.

    \44\Memos about these meetings have been placed in the public 
docket.
---------------------------------------------------------------------------

    In response to ATA's comment about the occurrence of ABS 
malfunctions due to out of adjustment wheel speed sensors, NHTSA 
believes that there are several reasons other than faulty ABS design 
for this phenomenon. Among the most common reasons observed during the 
agency's fleet study were sensor misadjustment during initial 
installation; faulty sensor retaining clips; sensor wires being 
installed with too little slack, resulting in the sensor's being 
partially pulled out from its mounting block when the vehicle's 
steering gear or suspension moved; faulty or improperly installed wheel 
bearings; or failure to readjust the sensor after performing 
maintenance work in the wheel end area that results in the sensor being 
knocked out of adjustment. NHTSA emphasizes that the relative frequency 
of these types of incidents was not high. Five of the two hundred test 
trucks experienced problems of this type before being, or shortly after 
being placed in service. In addition, twenty-two of the trucks 
experienced problems of this type over the two year, 40 million mile 
test program. With the exception of the faulty clip problem, which has 
been permanently rectified, all the remaining reasons for the 
occurrence of this condition are the result of installation quality 
control lapses, faults with other components, or misinformed 
maintenance practices. The failures were not caused by faulty sensor 
design. The agency anticipates that the rate of incidence of even these 
few events will decrease as quality control efforts and mechanics' 
awareness and skill in maintaining ABS improves. [[Page 13241]] 
    In response to ATA's comment that mechanics will have difficulty 
installing and maintaining ABS, NHTSA recognizes that mechanic training 
will be necessary to ensure the long term viability of ABS systems. 
However, based on the agency's fleet test results, the agency finds 
that, once trained, mechanics can successfully maintain the systems. 
The study's results indicate that those fleets committed to providing 
mechanics the support needed to deal with ABSs can keep the systems 
operational with relative ease and efficiency and at reasonable cost. 
ABS suppliers and truck manufacturers have indicated a commitment to 
providing field service support for the systems. If fleets begin 
utilizing these services now, mechanics will be capable of maintaining 
the systems as more ABS-equipped vehicles are introduced into fleet 
service.
    Based on its anecdotal experience with electronic engines, ATA 
stated that truck manufacturers will not correct the wiring and 
installation related problems evidenced in the test. Specifically, ATA 
stated that ``* * * none of the OEM's yet follow the engine 
manufacturer's guidelines on how wiring/sensors are to be placed and no 
two of them do it the same way''.
    NHTSA believes that ATA's comparison between electronic engines and 
ABS is not relevant. That organization's comparison fails to portray 
the extent of problems that were reported to have occurred with 
electronic engines when they were first introduced in the mid to late 
1980's. The lower malfunction rates now being experienced with 
electronic engines are the result of having worked through initial 
design and installation problems, a pattern the agency notes is now 
repeating with ABS, as it becomes more widely installed and used. In 
addition, ATA's comments about wiring/sensor placement on electronic 
engines appear to imply that the lack of uniformity in this regard adds 
complexity to the task of maintaining these engines, rather than 
implying that truck manufacturers are improperly or inadequately 
installing engines in vehicles they produce. Unless there is some 
compelling reason or requirement for manufacturers to install a given 
component in a single way, the fact that they do it differently is to 
be expected, given the need and desire for design flexibility. The same 
flexibility is likely to be true with ABS installations. Electronic 
engines are in widespread use within the trucking industry today. It is 
therefore reasonable to infer that truck manufacturers are installing 
them properly. Based on the data collected in its two fleet studies, 
the agency believes that the carriers can and will be able to 
successfully maintain ABS as well.
    ATA further stated that the agency's thinking was ``* * * seriously 
flawed * * *'' because the agency-supported fleet study contained 
listings of ABS malfunctions that were remedied with only the 
expenditure of labor and did not require repair or replacement of a 
component part, with added parts-associated costs. ATA claimed that the 
report's inclusion of these type malfunctions implied ``* * * some 
lesser class of failure''. ATA's reference in this regard was to 
instances in which a false- positive ABS malfunction indication 
occurred which necessitated an inspection and system reset, with no 
other problem being found or remedy needed.
    NHTSA disagrees. Rather than minimizing the consequences of these 
occurrences, the inclusion of them in the two reports highlighted the 
agency's concern about such events. During the tractor portion of the 
study, they occurred comparatively frequently with 88 of the 200 test 
tractors experiencing a total of 290 intermittent malfunction warning 
indications.<SUP>45 The situation improved markedly, however, in the 
later trailer portion of the study. Here, 12 of the 50 test trailers 
experienced a total of 15 of these false-positive malfunction 
warnings.<SUP>46 The cost impact of these occurrences is noted in the 
fleet study reports. The reports further noted that such malfunctions 
accounted for 45 percent of the total in-service maintenance costs for 
tractors and 35 percent for trailers. Notwithstanding these findings, 
the fact that a significant reduction in the frequency of these 
occurrences was noted between the time of the tractor and trailer 
portions of the study, indicates that the reliability of the components 
greatly improved.

    \45\DOT HS 807-846, page 3-17.
    \46\DOT HS 808 059, page 3-14.
---------------------------------------------------------------------------

    ATA further implied that these types of failures resulted in lost 
vehicle productivity, because an affected vehicle would have to be 
taken out of service to remedy the situation. Contrary to ATA's 
assertion, none of the test vehicles were pulled out of operational 
service by the fleets as a result of these malfunction indications. 
Instead, corrections were made when the vehicle returned to its 
dispatch point and before it was next dispatched. Further, no dispatch 
opportunities were missed because of these incidents.
    NHTSA notes that the agency's fleet study summarized the cost 
impact of ``false-positive'' ABS malfunctions. Specifically, these 
incidents accounted for 45 percent of the total in-service maintenance 
costs for tractors and 35 percent for trailers. The agency's fleet 
study report summarized the cost impacts as follows: In the case of 
tractors, those costs were $0.00021 per mile, while for trailers the 
figure was $0.00015 per mile. These figures are reasonable, given that 
it costs $1.38-$1.54 per mile to operate a truck with a driver.<SUP>47 
Moreover, based on the trailer fleet study, NHTSA expects these costs 
to decrease significantly over time, since many of them were associated 
with ECU malfunction warning algorithms that ABS suppliers have since 
modified to make them less prone to inappropriate activation.

    \47\Modern Bulk Transport Magazine, June 1994, page 84.
---------------------------------------------------------------------------

    Based on the above considerations, NHTSA concludes that there is no 
basis for accepting ATA's position that more leadtime beyond that 
specified in this final rule is needed to successfully implement ABS 
use in heavy vehicles. NHTSA further concludes that maintenance costs 
associated with ABS are neither excessive nor unreasonable compared to 
other maintenance costs and that these costs will not be significantly 
reduced if the implementation dates of this rule are further delayed.

E. Requirements for Durability, Reliability, and Maintainability

    ATA requested that the Standard include requirements to address the 
durability, reliability, and maintainability of ABSs. ATA was concerned 
that premature degradation of ABS performance would create a safety 
risk associated with loss of ABS. Specifically, that organization 
requested requirements addressing corrosion resistance and 
electromagnetic susceptibility. It stated that such requirements are 
``necessary to assure that the equipment provided to meet the stability 
and control requirements proposed in this standard can do so 
repeatedly.<SUP>48''

    \48\NHTSA responds to the issue of the alleged safety risk in 
the next section.
---------------------------------------------------------------------------

    NHTSA concludes that separate requirements addressing the 
durability, reliability, and maintainability of ABS are not needed at 
this time. As detailed above, the ABS fleet evaluation conducted by the 
agency on 200 tractors and 50 trailers demonstrated that current 
generation ABSs are durable, reliable, and maintainable. Based on the 
fleet study and comments by manufacturers, NHTSA concludes that 
[[Page 13242]] separate component tests are not necessary.

F. Alleged Safety Problems

    ATA contends that current-generation ABSs can fail ``unsafe,'' 
i.e., ABS malfunction can result in the foundation brakes becoming 
inoperative. That organization states that this is a ``significant * * 
* safety problem'' and cites five incidents, two of which occurred 
during the agency's fleet studies, as corroboration for this 
suggestion. No other commenter alleged that current-generation ABSs 
fail in an unsafe manner.
    The issue raised by ATA concerns the likelihood of ABS malfunctions 
that would either reduce brake system performance or render a vehicle's 
underlying brake system completely inoperative. Based on the data 
collected during the NHTSA's in-service fleet evaluation of ABS, the 
agency finds that the likelihood of such occurrences is negligible. 
Therefore, NHTSA concludes that ATA's concern is unwarranted and 
unsubstantiated.
    During the two-year evaluation of 200 ABS-equipped truck tractors, 
a total of 421 incidents were recorded involving in-service wear 
related ABS malfunctions. The vast majority (99.8 percent) of these 
malfunctions were benign. When the ABS became inoperative, the vehicle 
reverted to a normally-braked vehicle without ABS protection and 
remained fully operational until the malfunction was remedied. 
Similarly, during the two-year evaluation of 50 ABS-equipped 
semitrailers, 44 such incidents were noted. All (100 percent) were 
benign.
    Only one ABS malfunction incident occurred during the tractor fleet 
study that resulted in the vehicle having reduced, braking performance. 
Even this incident, which involved a manufacturing defect in the 
surface coating of a piston slide valve in the modulator section of a 
drive-axle-only ABS on one tractor, did not totally compromise the 
brake performance. When the ABS supplier involved found the cause of 
this failure, a design change was made to rectify the problem and all 
the other test units in the fleet study were retrofitted with the 
improved design. Despite making this change, the ABS supplier involved 
subsequently chose not to produce this system. The agency emphasizes 
that this failure did not result in the complete loss of braking power 
on the vehicle. When the failure occurred, the vehicle experienced 
reduced braking capability on two of its five axles. The driver was 
able to maintain control of the vehicle and stop it. Despite the fact 
that it took longer than usual for the vehicle to stop, there were no 
adverse consequences as a result of this incident.
    As ATA acknowledged in its comments, failures such as this are 
rare. In this case, the failure was the result of a manufacturing 
defect, an atypical situation. This incident is not indicative of a 
general flaw in presently designed ABS systems of the type that would 
support the contention that ABSs typically fail unsafely.
    By comparison, during the same time period, the fleet studies 
reported 580 incidents involving the tractors, and 170 incidents 
involving the trailers, in which repairs or replacements were made to 
brake system components that were not related to the ABS.<SUP>49 These 
malfunctions could have compromised the brake system performance of the 
affected vehicles. Included among these were repairs or replacements of 
leaking or faulty relay or quick release valves, leaking or worn brake 
chambers or air hoses, and other miscellaneous repairs of leaking 
fittings. The agency notes that, despite their potential gravity, these 
failures went unheralded, and were simply repaired when detected. Fleet 
maintenance personnel expressed no special concern about this type of 
malfunction, treating them as routine occurrences.

    \49\DOT HS 808 059, page 3-18; DOT 807 846, page 3-23.
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    NHTSA's fleet study experience parallels the experience found 
during roadside inspections of heavy vehicles. FHWA's Office of Motor 
Carriers<SUP>50, reports that in 1992, 1,655,668 heavy vehicles were 
inspected by state and federal officials under the Motor Carrier Safety 
Assistance Program (MCSAP), and 461,715 (28 percent) of these were 
placed out-of-service for mechanical defects that were deemed 
significantly hazardous enough to warrant repairs at that location 
before the vehicle was operated again. A total of 908,184 out-of-
service defects were noted, 54 percent (487,238) of which were brake 
system related. The majority of these (68 percent) involved out-of-
adjustment brakes, but the remainder (157,717) involved defects in 
either the foundation or pneumatic portions of the system (e.g., 
cracked brake drums, chafed or worn air hoses, leaking brake chamber 
diaphragms, etc.), all of which could significantly compromise brake 
system performance in a severe braking maneuver. These data indicate 
that, on average, nearly one of every ten in-use heavy vehicles is 
operating with at least one significant non-adjustment related brake 
system defect, that, for whatever reason, goes unnoticed and/or is not 
repaired by fleet personnel, until the condition is discovered in an 
inspection. The National Transportation Safety Board<SUP>51, among 
others, has concluded that this situation is already serious enough to 
warrant more ``* * * consistent attention to brake system 
maintenance.''

    \50\Annual Report on Program Quality and Effectiveness, Fiscal 
Year 1992, U.S. Federal Highway Administration, Office of Motor 
Carriers, June 1993
    \51\Heavy Vehicle Air Brake Performance, National Transportation 
Safety Board Report No. SS-92/01, April, 1992.
---------------------------------------------------------------------------

    Problems associated with the foundation brakes appear to far exceed 
those caused by a potential malfunction to the ABS. Moreover, neither 
the frequency of ABS malfunctions nor their consequences, as noted in 
the fleet study, indicate that adding ABS will worsen this situation. 
In fact, the agency concludes that adding ABS will significantly 
contribute to improving it by partially compensating for brake system 
force imbalances that result from poorly performing or inoperative 
individual brakes on a vehicle. Ordinarily, under lightly loaded or 
empty operating conditions, the operative/properly performing brakes 
attempt to compensate for the reduced braking power absent from the 
inoperative/poorly performing brake(s). As a result, they over-brake 
and tend to lock up as increasing levels of brake pressure are applied 
in an effort to stop the vehicle. Although ABS is not a substitute for 
proper maintenance, under these conditions, its addition to a vehicle's 
braking system will be beneficial, since it will prevent lockup.
    NHTSA emphasizes that the one isolated incident identified in its 
fleet study that involved an ABS malfunction that compromised the 
vehicle's braking performance is markedly different from those 
described in PACCAR. In that case, it was argued that when an ABS 
failed, the vehicle's underlying brake system was unsafe. The 
circumstances that gave rise to such concerns are very different from 
those of today. ABS technology for motor vehicles was very new in the 
1970s. In response to aggressive stopping distance requirements and a 
prohibition against wheel lockup, manufacturers equipped their vehicles 
with ABSs and extensively redesigned the pneumatic and foundation brake 
portions of their braking systems. The new foundation brakes in many 
cases incorporated highly aggressive brake linings. When malfunctions 
occurred with a vehicle's ABS, the vehicle was left with a much more 
aggressive and powerful foundation brake system than the brake 
[[Page 13243]] systems that had been in general use. Additionally, 
since the pneumatic portion of the system was different from what had 
been in use, brake application and release timing on vehicles with 
malfunctioning ABSs were also different. Thus, for example, if the ABS 
on an ABS equipped tractor became inoperative, and the tractor was 
coupled to a non-ABS-equipped trailer, the tractor's brakes still 
functioned but were extremely incompatible with those of the trailer. 
The tractor's brakes applied and released differently and were much 
more aggressive. These differences led to braking force imbalance 
problems that were very disconcerting to drivers. While situations such 
as this did not constitute brake failures per se, drivers nevertheless 
perceived the performance of their vehicles to be very unacceptable and 
termed these situations brake system failures.
    In the 1970s, there were several highly publicized incidents in 
which radio frequency interference (RFI) problems caused the ABS to 
cycle continuously during a brake application, thereby greatly 
diminishing braking power by venting brake system air pressure. The 
agency notes that manufacturers have completely eliminated the 
potential for RFI problems since current generation ABSs have been 
designed with shielded wiring systems and more sophisticated 
electronics that are better able to recognize spurious signals. No RFI 
problems have been reported with current-generation ABSs.
    The numerous complaints of brake system malfunctions reported by 
drivers prompted the PACCAR court to find that the agency had a 
responsibility to determine that its regulations do not produce a more 
dangerous highway environment than that which existed prior to 
government intervention.
    NHTSA has determined that today's final rule requiring heavy 
vehicles to be equipped with ABSs will result in a significantly safer 
highway environment than if no regulation were issued. Unlike 20 years 
ago, the manufacturers will not need to significantly redesign their 
braking system or use aggressive brake linings to meet stopping 
distance requirements. Further, ABS is no longer an immature 
technology. It has undergone 20 more years of development, been 
installed on tens of thousands of European vehicles pursuant to the 
1991 ECE requirement, and been fleet tested extensively in this country 
by NHTSA and the industry.
    NHTSA is aware of no consistent pattern of incidents in this 
country in which current generation antilock systems have experienced 
malfunctions like those that concerned the PACCAR court. As for the 
incidents cited by ATA alleging that an ABS malfunction resulted in an 
unsafe condition, the first one involving a manufacturing defect is 
discussed above. The second incident involved leaking air in the relay 
valve portion of a combined relay valve/ABS modulator valve on the 
steer axle of one truck involved in the agency's fleet study. Strictly 
speaking, this is not an ABS malfunction, since the air leak that 
occurred involved the service brake portion of this combined ABS/relay 
valve. The leakage was caused by oily sludge in the air system, which 
clogged the relay valve, thereby allowing service brake air pressure to 
vent, rather than being directed to the brake chamber controlled by 
that relay valve. The vehicle was equipped with an aftercooler type air 
cleaner/dryer. Such a leak would result in reduced braking performance, 
not total loss of the vehicle's brakes. This type of failure is similar 
to the non ABS related malfunctions that are described above and which 
were noted in both the fleet study and during roadside MCSAP 
inspections.
    ATA's comments implied that the ABS suppliers' recommended solution 
for this problem (i.e., that tractors be equipped with desiccant style 
air cleaners, in order to provide cleaner air), was unacceptable and 
that to use such cleaner/dryers demonstrates that ABS require a higher 
level of maintenance. NHTSA believes that it is reasonable to expect 
that fleets will use desiccant air dryers, or another type of 
comparably performing air cleaning system, since such systems will 
enhance the durability and safety of tractor and trailer braking 
systems by keeping the pneumatic portion of the brake system cleaner. 
The marketplace appears to have recognized this fact and is responding 
accordingly. Air cleaning/drying systems are now being installed on 
more than 80 percent of all new air brake-equipped powered heavy 
vehicles, with more than 90 percent of these being the desiccant type. 
Based on current usage, the agency anticipates that air cleaning/drying 
systems will be in almost universal use within the next few years.
    ATA provided few details about the third incident cited in its 
comments. That incident involved an ABS equipped tractor trailer 
combination participating in an ATA test program. That organization 
stated that the vehicle was ``* * * generating consistent stopping 
distance results when, in the middle of one run, there was a loss of 
braking which significantly increased the stopping distance.'' ATA 
offered no explanation or reason for this outcome, except to indicate 
that ``* * * no indication of an ABS failure by either the tractor or 
trailer ABS warning lamps * * *'' was noted. Since ABS malfunction was 
not indicated as the reason for the unexplained increase in stopping 
distance that occurred during the test of one of its fleet member's 
trucks, there is no reason to believe that this incident is indicative 
of an ABS problem.
    The fourth incident ATA cited involved a vehicle that was 
retrofitted with an ABS by the carrier and experienced reduced braking 
effectiveness during a test stop. Agency discussions with ATA staff and 
with the ABS supplier indicate that the vehicle was a truck tractor 
that was tested after the tractor had been equipped with an upgraded 
ABS. The ABS supplier subsequently concluded that a soldered connection 
had broken in the ECU and that this may have caused intermittent 
activation of one of the four modulators controlled by the ECU. Based 
on its investigation of the ECU in question, and its knowledge of how 
the ABS was configured, the ABS supplier believed that the truck had 
experienced a reduction in braking, but not a total loss of braking 
power. NHTSA emphasizes that this incident is atypical and not 
indicative of normal ABS performance, since the fleet study identified 
no similar incident.
    The fifth and final incident described by ATA is reminiscent of the 
``phantom failures'' that were reported to have occurred with early 
1970's vintage ABSs. The causes of most of those ``failures'' were 
neither fully explained nor linked to ABS flaws. In this incident, the 
accident report simply claimed that ``* * * the vehicle would not 
stop.'' ATA's account of this incident indicates that no problems were 
found in either the tractor's or the trailer's braking system after 
this incident.
    NHTSA notes that other factors such as slippery road conditions or 
improperly adjusted brakes are just as likely as ABS malfunction to 
have caused the driver to believe that the vehicle would not stop or 
that it was stopping too slowly. Without additional information, it is 
not possible for the agency to assess the cause of this incident, or 
respond to the implication that the incident is somehow indicative of 
an inherent ABS flaw.
    Contrary to ATA's allegations that existing ABSs have significant 
safety problems, most commenters, including vehicle and brake 
manufacturers, appear to agree with NHTSA's assessment that current 
generation ABSs are safe and [[Page 13244]] reliable. Unlike the 1970's 
when several vehicle and brake manufacturers objected to the 
rulemaking, and ATA, TEBDA, and PACCAR challenged the antilock standard 
in court, comments to the September 1993 NPRM indicate that vehicle and 
brake manufacturers now generally believe that the proposal was 
appropriate and today's antilock systems provide significant safety 
benefits. Along with the safety advocacy groups, HDBMC, AAMA, GM, 
Rockwell WABCO, Midland-Grau, and Bendix generally supported the 
agency's September 1993 proposal to require heavy vehicles to be 
equipped with an antilock brake system. No vehicle or brake 
manufacturer opposed the rulemaking, aside from objecting to details in 
the proposal. These commenters stated that ABS will improve vehicle 
safety by providing improved braking and vehicle stability and control. 
Specifically, such systems will prevent wheel lockup, thereby 
preventing jackknifing and other loss of control accidents. Neither the 
vehicle nor brake manufacturers expressed concern that today's ABSs 
would fail in such a way as to compromise basic braking performance, as 
ATA alleges.
    Strait-Stop stated that computerized ABSs will not prevent brake 
fade since these systems do not avoid or minimize heat build up. As a 
result, it alleged that computerized ABSs will not avert accidents 
related to runaway trucks. In contrast, it stated that its system 
results in cooler and therefore better brakes. The agency is not in a 
position to respond to Strait-Stop's claim that its product minimizes 
brake heat build up. Strait-Stop did not submit any data to 
substantiate its claim and the agency has no data of its own on this 
issue.
    NHTSA emphasizes that Strait-Stop has not suggested that an ABS 
will contribute to brake heat build-up, but merely stated that it will 
not reduce brake heating. Reducing brake heating, and thus the 
potential for brake fade, is not one of the design goals of an ABS, nor 
is it the focus of this rulemaking. ABS is intended to prevent wheel 
lockup. Brake fade is most typically caused by one or more of the 
brakes on a vehicle being out of adjustment, thereby causing the other 
properly adjusted brakes to have to absorb a disproportionate share of 
the kinetic energy that needs to be dissipated when a fully loaded 
heavy truck attempts to descend a grade. In this situation, the 
properly adjusted brakes are overworked, causing them to overheat and 
fade. This in turn results in a loss of braking power. Equipping a 
vehicle with either ABS or the Strait-Stop product will not rectify 
brake maladjustment(s). Likewise, equipping a vehicle with ABS will not 
decrease the motor carriers' existing need to properly adjust their 
vehicles' brakes in order to avoid brake overheating and fade on 
downgrades.

G. ABS Malfunction Indicator Lamps

    Since the discussion on ABS malfunction indicator lamps is lengthy, 
NHTSA first summarizes its decisions regarding this subject and then 
addresses the details of each decision. In today's final rule, NHTSA is 
amending Standard No. 105 and Standard No. 121 to require all powered 
heavy vehicles to be equipped with an in-cab lamp for indicating a 
malfunction of the ABS on that vehicle. In addition, the final rule 
requires truck tractors and other trucks that are equipped to tow 
trailer(s) to be equipped with a second in-cab lamp. The purpose of the 
second lamp is to indicate malfunctions in the trailer ABS. Finally, 
trailers manufactured during an interim eight-year period are required 
to be equipped with an external malfunction indicator lamp.
    Each of these ABS malfunction indicator lamps is required to 
activate whenever there is a malfunction affecting the generation or 
transmission of response or control signals in the ABS that it is 
monitoring. In addition, the lamp is required to store information 
about a malfunction in that ABS until the next start up. Vehicle 
manufacturers are prohibited from equipping their vehicles with a 
device to disable any malfunction indicator lamp.
    NHTSA also has amended the failed ABS system requirements to 
prohibit any change in brake timing in the event of an ABS malfunction 
that affects the generation or transmission of response or control 
signals.
1. Number and Location; Duration of Trailer Requirement
    Standard No. 121 now requires that each tractor, truck, and bus be 
equipped with an in-cab lamp that indicates malfunctioning in the ABS 
of that vehicle. In the NPRM, the agency proposed that truck tractors 
be equipped with a second in-cab lamp that would indicate malfunctions 
in the trailer ABS. The agency proposed further that the in-cab lamps 
be required to be ``mounted in front of and in clear view of the 
driver.'' The agency noted that this requirement is essentially the 
same as the current requirements in Standard No. 105 and Standard No. 
121. These existing provisions require a continuous message to a driver 
when the ignition is in the ``on'' or ``run'' position.
    NHTSA has decided to adopt its proposal that each truck tractor and 
single unit vehicle be equipped with an in-cab lamp to indicate 
malfunctions in the ABS of that vehicle. The agency believes that it is 
essential that a driver be notified about an ABS malfunction, so that 
the problem can be corrected. The commenters, including vehicle 
manufacturers and brake manufacturers, generally supported the proposal 
for an in-cab malfunction indicator. Only Strait-Stop opposed this 
proposal, stating that it would necessitate the use of an electrical 
ABS.
    NHTSA proposed to require that each trailer equipped with ABS be 
capable of sending a signal about a malfunction in the trailer ABS to a 
towing vehicle, and that all powered towing vehicles equipped with ABS 
have an in-cab lamp that would be activated when the towing vehicle 
receives signals indicating malfunctions in a trailer ABS. In addition, 
the agency proposed to require the installation of an external ABS 
malfunction lamp on trailers and dollies manufactured during the eight-
year period after trailers are first required to be equipped with 
ABS.<SUP>52 The agency believed that the external lamp would not be 
necessary on new trailers manufactured after the end of that period 
because, by that time, a significant majority of tractors in the heavy 
vehicle fleet, which would be responsible for the vast majority of 
miles driven by tractors, would be manufactured in compliance with the 
requirement for an in-cab lamp capable of receiving a malfunction 
signal from a trailer.

    \52\The eight-year time period for this interim proposal was 
intended to represent the average lifespan of a truck tractor.
---------------------------------------------------------------------------

    Commenters offered mixed views about requiring each towing vehicle 
to have a separate in-cab lamp to indicate a malfunction in a trailer 
ABS. Bosch, Midland-Grau and several other commenters supported the 
agency's proposal for requiring tractors to have two separate in-cab 
ABS malfunction indicator lamps: one indicating malfunctions in the 
tractor ABS, and the other, malfunctions in the trailer ABS. They 
stated that a driver would be able to respond to and possibly alter 
braking actions in the event of an ABS malfunction during emergency 
situations if the driver knew whether the malfunction was in the 
tractor ABS or in the trailer ABS. Midland-Grau strongly opposed having 
a single indicator, claiming that the tractor lamp sequence would 
camouflage the situation in which the trailer ABS lacked power. 
Midland-Grau further [[Page 13245]] stated that a single lamp would 
make it difficult to identify which vehicle had a malfunction without 
using separate diagnostic equipment.
    ATA, Allied Signal and fleet operators opposed the proposal that 
tractors have a separate in-cab malfunction lamp for the trailer ABS, 
claiming that these indicators were ``neither needed nor practicable at 
this time.'' AAMA supported a single in-cab malfunction lamp for each 
tractor to indicate an ABS malfunction on either the tractor or the 
trailer. It believed that there is no safety need for the driver to 
know immediately whether the ABS malfunction is in the tractor or the 
trailer. While AAMA stated that separate indicators would cause 
needless complexity to the instrument panel, it did not state that such 
a requirement would be impracticable.
    After reviewing the comments and other available information, NHTSA 
has decided to require each powered towing vehicle to have one in-cab 
malfunction lamp for the towing vehicle's ABS and another in-cab lamp 
for the trailer ABS. The agency believes that the ABS trailer fleet 
study final report<SUP>53 indicated that drivers are more likely to 
observe an in-cab malfunction indicator for a trailer than a 
malfunction indicator lamp on the front of the trailer, particularly if 
the trailer ABS is powered through the stoplamp circuit. This is so 
because the stoplamp circuit only activates when the brake is applied, 
a time when the driver will be paying more attention to the traffic 
conditions ahead. The report also indicated that ABS malfunctions were 
present on some vehicles for a long time, but were not reported, 
primarily because the drivers ``spent very little time looking in their 
mirrors while stopping'' and did not notice that the trailer ABS 
malfunction lamp was lighted.

    \53\``An In-Service Evaluation of the Performance, Reliability, 
Maintainability, and Durability of Antilock Braking Systems for 
Semitrailers,'' (October 1993),
---------------------------------------------------------------------------

    NHTSA does not agree with AAMA's recommendation for a single in-cab 
malfunction lamp for both the tractor and trailer antilock systems. As 
Midland-Grau stated, a driver would not be able to identify which 
vehicle in a combination was experiencing an ABS malfunction if only a 
single in-cab malfunction indicator lamp were required, since a single 
in-cab lamp would result in some trailer ABS malfunctions being 
camouflaged. Further, notwithstanding comments by AAMA and ATA that 
separate in-cab lamps add unnecessary complexity, combination vehicles 
in Europe have been equipped with such indicators for several years.
    NHTSA believes that it is appropriate also to require an external 
malfunction lamp on trailers and dollies for the eight-year period 
during which some non-ABS-equipped tractors are likely to be towing 
ABS-equipped trailers. The external lamp will indicate trailer ABS 
malfunctions to the driver of a non-ABS tractor, and will also assist 
Federal or State inspectors in determining the operational status of a 
trailer's antilock system. Nevertheless, notwithstanding Midland-Grau's 
recommendation to require the external trailer lamp permanently, the 
agency has decided not to do so, since after the transition period, the 
vast majority of trailer malfunctions would be expected to be indicated 
in-cab.
    In response to the SNPRM, TTMA stated that instead of locating the 
trailer lamp on the ``roadside nose of trailer, it should be located 
near the electronic control unit where the driver can check it during 
his walk-around inspection of the tractor trailer combination.'' It 
stated that some ABS may require that the trailer be moved at a low 
speed (less than 5 mph) to activate the check function (i.e., some 
antilock systems check the status of wheel speed sensors by looking for 
proper signals as the vehicle goes from 0 to 8 mph). TTMA also 
commented that it is not practical to mount an ABS malfunction lamp on 
converter dollies in a location in which the lamp will be visible in a 
driver's rearview mirror, yet not be susceptible to damage.
    While NHTSA recognizes the possibility of some susceptibility to 
damage, placing the external malfunction lamp in a different location 
on dollies would largely negate its benefits, because it would not be 
visible to the driver. For that reason, the agency has decided that the 
requirement will apply to dollies as well as other trailers.
    NHTSA is revising Standard No. 101, Controls and Displays, to 
clarify that the malfunction indicator lamp must be labeled with the 
words ``ABS'' or ``Antilock'' for trucks and truck tractors with air 
brakes. The agency notes that Table 2 in Standard No. 101 currently 
refers to Standard No. 105, but makes no reference to Standard No. 121. 
For the in-cab trailer ABS malfunction indicator, NHTSA is adopting the 
identification of controls in Standard No. 101 (i.e., ``Trailer ABS'' 
or ``Trailer Antilock'') as proposed in the NPRM.
2. Conditions for Activation
    Before this amendment, S5.1.6 of Standard No. 121 required the ABS 
warning signal to activate ``in the event of total electrical 
failure.'' In the NPRM, NHTSA proposed that the malfunction indicator 
lamp activate ``in the event of any malfunction in the system.'' The 
agency tentatively concluded that a driver needs to be informed about 
any malfunction because every ABS malfunction could affect the way in 
which drivers respond to a safety problem. The agency invited comments 
about when and in what situations the malfunction lamp should be 
required to activate.
    Fleet operators, AAMA, Rockwell WABCO, HDBMC, and Midland-Grau 
stated that the proposal to require the ABS malfunction lamp to 
activate upon ``any'' malfunction in the antilock system is 
impracticable, unreasonably costly, and overly broad. These commenters 
believed that it is only practicable and realistic for current 
technology to detect certain types of electrical malfunctions, namely 
those involving electrical discontinuities or electronic malfunctions, 
not mechanical failures of ABS components. AAMA and HDBMC stated that 
it would be unreasonably costly to provide continuous monitoring of all 
ABS malfunctions because many possible malfunctions are temporary in 
nature or may not directly affect ABS performance.
    Commenters suggested various ways to narrow the requirement. 
Rockwell WABCO recommended that the ABS malfunction indicator activate 
whenever a ``malfunction occurs affecting the generation and/or 
transmission of response and control signals.'' It stated that this 
should be a minimum requirement applicable to electrical faults in 
sensors, control valves and associated wiring. ATA, Allied Signal and 
fleet operators stated that a more practicable requirement for the ABS 
malfunction indicator would be to require activation in the event of 
(1) failure to sense angular rotation, (2) failure of the controlling 
device to generate controlling output signals, and (3) failure to 
transmit controlling signals to devices that modulate brake actuating 
forces.
    Based on the comments and other available information, NHTSA has 
decided to require ABS malfunction indicator lamps to activate for any 
malfunction that affects the generation or transmission of response or 
control signals in the vehicle's antilock brake system. The requirement 
does not apply to malfunctions such as sticking solenoid valves, small 
air leaks in the solenoid valve, or mechanical binding of a valve. The 
agency agrees with the commenters' arguments that the malfunction 
indicator requirement [[Page 13246]] should be modified because 
requiring activation in the case of ``any'' malfunction might have been 
impracticable. Under the modified requirement, only those malfunctions 
that are directly related to the antilock brake system must be 
indicated. Applying the indicator requirement to the ``generation'' of 
response and control signals serves to cover the components in the ABS 
that produce these signals. These components include wheel speed 
sensors which produce response signals for the control unit, and the 
control unit which produces control signals for input into the valves 
that modulate brake pressure. Applying the indicator requirement to the 
``transmission'' of response and control signals serves to cover the 
components in the ABS through which the generated signals are 
transmitted. These components include wiring, connectors, belts used in 
mechanical systems, and all components through which a generated signal 
can be transmitted.
    NHTSA notes that the generation and transmission of signals in ABSs 
are typically electrical in nature. Nevertheless, the agency has 
decided not to include the term ``electrical'' in the requirement so 
that the malfunction indicator requirements are applicable to non-
electrical, i.e., mechanical, ABSs as well. Accordingly, mechanical 
ABSs will have to comply with the malfunction indicator requirements.
3. Activation Protocol for Malfunction Indicators
    In the NPRM, NHTSA proposed standardizing the ABS malfunction 
indicator lamp system so that trucks and trailers would have the same 
activation pattern\54\ and same colored lamps to indicate an ABS 
malfunction. The agency believed that such a common indicator pattern 
would reduce ambiguity and confusion and expedite Federal and state 
inspections. The agency proposed that each ABS malfunction indicator 
lamp be yellow and activate when a problem exists but not activate when 
the system is functioning properly. In addition, the proposal would 
have required that whenever the ABS receives electrical power, the 
indicator lamp would provide a continuous visible indication until a 
function check of the ABS was completed. Under the proposal, the check 
function would have to be completed and the lamp extinguished (assuming 
that there was no underlying condition that warranted activating the 
lamp) before the vehicle was driven.

    \54\By pattern, the agency meant a common way that an indicator 
would react in response to a malfunction. Specifically, upon a 
failure, the indicator would activate and provide a continuous 
yellow signal.
---------------------------------------------------------------------------

    Rockwell WABCO stated that both the existing format in which a 
continuous signal is activated upon the ABS's total electrical failure 
and the proposed format for the ABS malfunction lamp are acceptable 
approaches. That company strongly recommended that the agency adopt a 
single approach for all heavy vehicles. Midland-Grau accepted the 
agency's proposal to require the lamp to extinguish before the vehicle 
is driven, even though it was concerned about an incomplete sensor 
check function.
    AAMA stated that the agency ``should allow the ABS malfunction 
indicator to be either illuminated or extinguished during low speed 
drive away after key-on.'' That organization requested that the agency 
affirm its view that the proposed language did not require the ABS 
indicator to be either illuminated or extinguished during low-speed 
driveaway after key-on. That organization was concerned that the 
proposal might prohibit certain existing systems that have an 
illuminated indicator until the vehicle reaches a speed of five to 
seven mph after key-on.
    Bosch recommended that an ``on-off-on'' blink sequence be used to 
indicate an ABS malfunction when the ignition is turned to the ``on'' 
or ``run'' position. It believed that this pattern would inform a 
relief driver of the presence of a malfunction and would assist Federal 
and State inspectors in determining the operational status of the 
vehicle's ABS.
    After reviewing the comments and other available information, NHTSA 
has decided to require the malfunction indicator lamp to activate when 
a problem exists and not activate when the system is functioning 
properly. Under this requirement, the indicator lamp is required to 
provide a continuous indication until a function check of the ABS is 
completed. The agency believes that this ABS malfunction lamp format, 
together with the requirement that the system stores malfunctions until 
the next key-on, is necessary to enable Federal and State inspectors to 
determine the operational status of an ABS without moving the vehicle. 
Elsewhere in today's Federal Register, the FHWA's Office of Motor 
Carrier Standards is issuing a notice explaining its intent to issue a 
companion regulation requiring that the ABSs on heavy vehicles be 
operational.
    NHTSA further notes that all vehicles will be required to have a 
continuously burning lamp in response to a malfunction. Accordingly, 
this requirement will standardize the activation format for all 
vehicles. Under that format, the ABS malfunction lamp extinguishes 
after a function check, and before the vehicle is driven. Since light 
vehicle ABSs currently use this format, the agency believes that heavy 
vehicle drivers will find it easier to understand the heavy vehicle ABS 
malfunction indicator if the same format is used. Furthermore, the 
adopted format is also consistent with the ECE requirement and 
therefore is consistent with the goal of international harmonization.
    NHTSA has concluded that the ``on-off-on'' blink sequence 
recommended by Bosch to indicate a malfunction during vehicle start-up 
would place an unwarranted burden on the driver, who would have to pay 
close attention to the malfunction lamp to observe the blink sequence 
during vehicle start-up and drive-away. Therefore, the agency rejects 
this recommendation.
4. Signal Storage
    In the NPRM, NHTSA proposed that the ABS indicator lamp system be 
capable of storing information regarding any malfunction that existed 
when the ignition was last turned to the ``off'' position. For 
instance, if the wheel speed sensors were malfunctioning before the 
vehicle was turned ``off,'' the system would be required to store a 
signal for that malfunction. As a result, the malfunction would be 
displayed when the vehicle was turned ``on'' again, as part of the 
function check.
    AAMA, Midland-Grau, Rockwell WABCO and several other commenters 
opposed the proposal to require the storage of ABS malfunctions that 
exist when the ignition is turned to the ``off'' position. AAMA stated 
that it is not appropriate to mandate this capability, claiming that 
many error messages are spurious or represent transient conditions, and 
therefore do not warrant automatic reactivation the next time the key 
is turned to the ``on'' position. It further stated that if a 
malfunction is non-transient, then the warning will reappear and that 
therefore it need not be stored. Midland-Grau believed that the 
proposal was design restrictive and would eliminate systems that do not 
have non-volatile memory (i.e., a system that remembers malfunctions 
when the system is shut down). Rockwell WABCO stated that this area 
does not need to be regulated, even though it acknowledged that all 
current electronic ABS have non-volatile memories to store and 
communicate current and past malfunctions. After reviewing the comments 
and other available information, NHTSA has decided that the malfunction 
storage requirement is necessary to ensure that relief drivers 
[[Page 13247]] and Federal and State inspectors are advised about any 
malfunctions in a vehicle's ABS without having to move the vehicle. 
This capability is important since inspectors would need to determine 
the operational status of the vehicle's ABS without moving the vehicle. 
Moreover, this capability is necessary since the agency has decided to 
require that the ABS malfunction indicator lamp extinguish before the 
vehicle is driven, provided that there is no existing ABS malfunction 
that warrants activation of the indicator.
    NHTSA disagrees with AAMA's claim that nontransient malfunctions 
will always reappear at the next key-on and therefore do not need to be 
``stored.'' A nontransient malfunction of the wheel sensor, which 
involves the generation of a wheel speed signal, is typically detected 
only when the vehicle is moving at a speed exceeding 8 to 10 mph, since 
a signal is only produced when the wheel rotates at some threshold 
wheel speed. Therefore, no signal is generated and hence no sensor 
malfunction is indicated if the vehicle is stationary. As explained in 
the NPRM and in the previous paragraph, one reason for requiring 
malfunctions to be stored is to ensure that preexisting malfunctions 
involving sensors are indicated before the vehicle is driven.
5. Disabling Switch
    NHTSA, in response to a rulemaking petition from ATA, proposed in a 
separate NPRM to allow a switch that a driver could use to turn ``off'' 
and ``on'' the in-cab malfunction lamp for a vehicle's ABS. (58 FR 
50732, September 28, 1993.)
    Advocates and vehicle and brake manufacturers strongly opposed the 
proposal. AAMA, Bosch, and Midland-Grau believed that such a switch 
would encourage drivers to disable the malfunction indicator of an 
important safety system, and thus set an undesirable precedent for 
allowing mechanisms that would disable other vehicle safety systems. 
These commenters stated that a constant reminder of a malfunction is 
the best way to inform drivers of a malfunction condition and encourage 
them to seek a repair of an ABS malfunction. In addition, they claimed 
that if the switch were used to turn off the malfunction lamp and the 
ignition remained ``on,'' a relief driver would not necessarily be 
informed of an ABS malfunction unless the relief driver used the switch 
to reactivate the malfunction indicator.
    ATA, Allied Signal, and fleet operators supported the proposal to 
allow an optional switch for turning the ABS malfunction indicator off, 
claiming it would enable the driver to prevent the malfunction 
indicator from being a distraction, especially at night when the amber 
light can appear to be excessively bright.
    NHTSA recognizes that some drivers view the malfunction indicator 
as an annoyance and thus might favor having a switch to turn it off. 
The agency is also aware of isolated cases in the truck tractor ABS 
fleet study in which malfunction indicators were disabled or taped 
over. Nevertheless, NHTSA agrees with AAMA and the brake manufacturers 
that permitting a disabling switch is inconsistent with motor vehicle 
safety. The information about a malfunction of an important safety 
system such as an antilock brake system should be communicated to the 
driver and should not be disregarded. Allowing drivers to turn off the 
ABS malfunction indicator would reduce the likelihood that a 
malfunction would be reported and corrected in a timely fashion. Use of 
such a switch might mask a potential safety problem, since an ABS 
malfunction could go undetected by the driver, if the disabling switch 
were activated. Allowing such a switch would also implicitly condone 
actions by some drivers that disable the malfunction indicator, since 
the agency would be allowing a disabling switch based on the argument 
that without a disabling switch drivers would defeat the switch. 
Moreover, allowing a malfunction indicator to be turned off would be 
inconsistent with Standard No. 101. Based on the above considerations, 
NHTSA has decided not to permit an optional disabling switch.
    NHTSA notes that ATA's concern about driver distraction may be 
reduced if the antilock malfunction indicator is dimmed at night. In 
specifying requirements for the illumination of various controls and 
displays including the ABS malfunction indicator, Section S5.3.4(b) of 
Standard No. 101 states that

    The means for providing the required visibility may be 
adjustable manually or automatically, except that the telltales and 
identification for brakes, highbeams, turn signals, and safety belts 
may not be adjustable under any driving condition to a level that is 
invisible.

Under this provision, an ABS malfunction lamp may be manually or 
automatically dimmed, provided that it is still visible to the driver. 
Nevertheless, the agency emphasizes that a malfunction indicator that 
is not visible to the driver would be prohibited.
6. ABS Failed System Requirements
    Section S5.5.1 of Standard No. 121 currently requires that the 
application and release times of the service brakes not increase when 
there is an electrical failure in the ABS. In the NPRM, NHTSA proposed 
removing the word ``electrical.'' That change would prohibit any 
malfunction in an ABS, whether or not electrical, from increasing the 
application and release times of the service brakes. The change would 
also make the requirement applicable to nonelectronic ABSs.
    ATA stated that the proposed requirement in Standard No. 121 for 
failed ABSs would be difficult to meet. It further stated that the 
failed ABS requirement for heavy vehicles in Standard No. 105 is more 
reasonable than the proposed requirements in Standard No. 121,\55\ 
since some types of ABS malfunctions in a vehicle with air brakes, such 
as a leaky valve, could result in an increase in service brake 
actuation and release times.

    \55\ Section S5.5.2 of Standard No. 105 requires that in the 
event of any failure in the antilock system, the vehicle must be 
capable of meeting the stopping distance requirement of 613 feet, as 
specified for a service brake system partial failure.
---------------------------------------------------------------------------

    NHTSA acknowledges that the proposed failed ABS requirement for 
heavy vehicles in Standard No. 121 is more stringent than the 
requirement in Standard No. 105. The agency could resolve this 
difference by making Standard No. 105 more stringent by deleting the 
word ``electrical'' or by amending Standard No. 121 to prohibit any 
change in brake timing in the event of certain, but not all, ABS 
malfunctions.
    After reviewing the alternatives, NHTSA has decided to revise 
Standard No. 121 to prohibit any change in brake timing in the event of 
those ABS malfunctions that affect the generation or transmission of 
response or control signals. The agency believes that this modification 
will ensure that the brake system reverts to normal braking without 
antilock control, in the event of such a malfunction in the antilock 
system. NHTSA notes that this modification parallels the change the 
agency made to the requirements governing the types of malfunctions 
that must be indicated by the malfunction lamp. This requirement will 
not apply to mechanical ABS malfunctions such as sticky valves. While 
mechanical malfunctions do happen, electrical malfunctions are far more 
prevalent. The agency believes that simply deleting the word 
``electrical'' would have made the requirement too broad and 
potentially impracticable, while [[Page 13248]] leaving the word in 
without additional changes would make the requirement too narrow.
    NHTSA notes that Standard No. 105's stringency cannot be increased 
in this final rule because the agency did not propose amending that 
Standard's failed ABS requirements. Nevertheless, the agency may 
conduct future rulemaking to make Standard No. 105's ABS failed systems 
requirements more consistent with the requirements in Standard No. 121 
and proposed Standard No. 135.

H. Power Source

    Section S5.5.2 currently permits the power source for trailers 
equipped with ABSs to be either the stop lamp circuit or a separate 
electrical circuit specifically provided to power the trailer ABS. In 
the NPRM, NHTSA proposed that ABSs be required to receive full-time 
power through a separate circuit, and to have backup powering through 
the stop lamp circuit. The agency tentatively decided that a full-time 
power source would be necessary to ensure that adequate power for the 
trailer's ABS is available, particularly for doubles and triples, and 
that a driver is aware of any ABS malfunction related to the trailer, 
since the stop lamp circuit is powered only when brakes are applied.
    The commenters had mixed views about whether full-time power for 
trailer ABSs should be provided through a separate circuit. AAMA, ABS 
suppliers, TTMA, and Advocates believed that the agency's proposed 
approach is appropriate and that the industry will be able to develop 
appropriate voluntary standards through the SAE for electrical circuits 
or connectors. Upon standardizing with one approach, uniformity would 
be ensured. Midland-Grau stated that it ``strongly supports'' the 
agency's proposal for full-time powering for the following reasons:
    1. The antilock systems being produced today are very reliable, but 
only as reliable as the power supply circuit which is supplying power 
to the antilock system.
    2. Having continuous power to the trailer ABS will allow for full-
time diagnostics continually updating the driver of the status of the 
trailer antilock system, and not just during braking.
    3. A separate electrical circuit is needed to have adequate and 
reliable power available should all the solenoids in the control valves 
be activated in double and triple combinations.
    4. To provide incentive to the industry (SAE, TTMA, TMC, etc.) to 
develop a ``common'' circuit for ABS on trailers, which may or may not 
ultimately involve a separate connector.
    5. To facilitate the use of higher capability trailer antilock 
systems, along with other electronic systems such as low air pressure, 
height sensing, and electronic braking.

Midland-Grau further stated that ``Because of cost, most fleets would 
prefer to power through the stop lamp switch not realizing that they 
are asking for the ABS reliability problems of the late 1970s to 
reappear again.''
    ATA and fleet operators opposed requiring full-time power for 
trailer ABSs. ATA stated that this requirement is an untested, 
unnecessary, and costly burden that NHTSA did not justify on a safety 
basis. ATA is concerned that a full-time power requirement would result 
in significant maintenance and reliability problems, basing its claims 
on the agency's fleet study. ATA also stated that requiring full-time 
power is premature since the industry is working on multiplexing 
systems,\56\ which, when fully developed and proven, would provide many 
opportunities for powering accessories on trailers.

    \56\Multiplexing is the process of combining several 
measurements for transmission over the same signal path.
---------------------------------------------------------------------------

    In response to the SNPRM, ATA elaborated on its initial comments 
opposing a requirement that trailer ABSs be electrically powered using 
a separate electrical circuit. ATA alleged that the requirement could 
not be justified and that no practicable method had been demonstrated 
for providing this separate source of power. Specifically, it stated 
that NHTSA's fleet study did not identify a single electrical powering 
system that performed in a reliable manner in the test. ATA further 
stated that it is impermissible for the agency to require a separate 
dedicated circuit after it had permitted stop signal powering as an 
option. (57 FR 30911, July 13, 1992.) It claimed that the agency has 
not justified what it terms a ``proposed rescission of the prior 
rulemaking decision to allow power through the stop lamp circuit.''
    NHTSA has decided to adopt the proposed full-time power requirement 
for trailer ABSs. The wording of the standard has also been amended to 
clarify that towing vehicles must have a corresponding separate 
circuit. By requiring a separate circuit, the agency will ensure the 
strongest possible source of electrical power from the tractor to 
ensure the functioning of all the ECUs and modulators that are employed 
in the antilock brake system, or systems, on single trailers, or 
multiple trailers and converter dollies in multitrailer combinations. 
Another important safety justification is that a separate circuit will 
ensure a continuous malfunction indication whenever a malfunction 
exists. As noted above, an ABS malfunction indicator powered by a stop 
lamp circuit would function only when the driver is applying the 
brakes. During braking, a driver would most likely be concentrating on 
traffic conditions ahead, and would therefore be less likely to see an 
ABS malfunction indication on the trailer. However, a driver is more 
likely to be aware of a trailer ABS malfunction, if the tractor has an 
in-cab malfunction indicator for the trailer ABS, since a continuous 
malfunction indication could be more noticeable.
    Typically, shared circuits that power other electrical devices 
besides the trailer ABS, such as stoplamps, cannot provide as much 
electrical power to the ABS as can a separate circuit dedicated to 
powering only the trailer ABS. This was demonstrated during the 
agency's trailer fleet study\57\ in which all the alternative 
approaches that utilized a separate dedicated electrical circuit to 
power the ABS, (except one approach involving the trailer battery 
approach, which has been abandoned by the ABS supplier that suggested 
it), provided higher voltage levels than did the shared stoplamp 
circuit system approach. The data shown in the table cited in Footnote 
33\58\ were for single semitrailer combinations. Voltage levels would 
have been even lower had doubles or triples combinations been part of 
the fleet study.

    \57\Reference Table 3.4, DOT Report No. HS 808 059.
    \58\DOT HS 808 059, Table 3.4, page 3-27.
---------------------------------------------------------------------------

    If electrical voltage levels drop below 7-10 volts, an ECU cannot 
function properly and will automatically shut down. The system will 
automatically reset itself if sufficient power is once again provided. 
However, during periods of low power, the ABS will not operate. The 
likelihood of power dropping below the point at which the trailer ABS 
shuts down increases as the number of additional stoplamps, or other 
power draining devices, such as ABS ECUs and modulators, increases.
    Trailer ABS systems on a single semitrailer typically consist of 
one ECU and one or two modulators. A two-trailer combination (i.e., a 
double) would utilize 3 ECUs and 3 to 6 modulators, while a three-
trailer combination (i.e., a triple) would utilize 5 ECUs and 5 to 10 
modulators. While the electrical current draw of ECUs is minimal, 
modulators typically draw 2-2.5 amps each. Depending on a system's 
configuration, the ABS on a single semitrailer could draw 2-5 amps, 
that [[Page 13249]] on a doubles combination could draw 6-15 amps, and 
that on a triple combination 10-25 amps. If a stoplamp circuit of the 
existing 7-pin cable connector/plug system were used to power the 
trailer ABS, the current draw of the stop lamp bulbs, added to that of 
the ABS, would create an overall current draw that could exceed 45 amps 
on a triples combination. Under such levels of current draw, there is a 
greatly increased likelihood that the ABS will no longer function on 
the second and third trailers in a triples combination.
    At present, standard industry practice throughout the trucking 
industry is to provide electrical power for a trailer from the tractor 
through a cable and connector/plug assembly, the SAE J560 connector. 
This connector uses a 7-pin configuration, with six power circuits and 
one common ground. All six power pins are now utilized for one 
electrical function or another.
    Although never directly stated, ATA's comments appear to be based 
on the premise that NHTSA's proposed requirement for a separate circuit 
is a directive that a second separate tractor- to-trailer cable and 
connector/plug system be used. Such a requirement would preclude the 
continued exclusive use of a single SAE J560 connector. However, the 
agency wishes to clarify that a second separate connector is not 
required. Accordingly, the agency has not specified a set method for 
providing the separate circuit. The agency intentionally left this 
choice to the industry in an effort to provide design latitude.
    NHTSA notes that there are many alternative ways of providing a 
separate circuit to power ABS. During the trailer fleet study, the 
agency evaluated several alternative methods of providing electrical 
power. To provide a baseline for comparisons with other approaches, the 
stoplamp circuit of the standard tractor-to-trailer electrical cable/
connector supplied power to the trailer ABSs for two of the five 
participating fleets. For these systems, the ABS received power every 
time the stoplamps were activated, but received no power when the 
brakes were not being applied.
    In addition, NHTSA evaluated three distinct methods of supplying a 
constant source of electrical power to trailer ABSs. First, one fleet 
used a 15-pin ``halo'' cable/connector/plug system (supplied by the 
Cole Hersee Company,\59\ which completely replaced the SAE J560 cable/
connector/plug. Two of the additional 8 pins (one for power, the other 
for a separate ground as well) were used to power the trailer ABSs. 
Second, another fleet used a second 6-pin connector/plug/cable, with 
backup power provided by the stoplamp circuit of the SAE J560 
connector. Third, another fleet used an auxiliary battery which was 
mounted on the semitrailer and was charged by electrical power from the 
semitrailer's refrigeration unit.

    \59\Herein after referred to as the 15-pin plug.
---------------------------------------------------------------------------

    NHTSA is studying the SAE J560 stoplamp-circuit-powered approach 
further, using ABS-equipped LCV combinations (known as Rocky Mountain 
doubles and triples). This study is part of the joint NHTSA/FHWA 
operational test program being conducted in response to Section 4007(d) 
of ISTEA. The basis for wiring these combinations in this manner was 
not, as ATA suggested in its comments, a decision by the agency that 
``* * * there is no safety need for separate new requirements related 
to the ABSs electrical system.'' Instead, the agency's decision was 
based on the need to determine the ability of the redundant stoplamp-
circuit to provide sufficient electrical power to operate the ABSs on 
all the trailers and dollies of a triples combination. In this test, 
the stoplamp circuit was wired in parallel with additional heavy duty 
wiring to the ABS, in an effort to maximize the possibility of success.
    NHTSA evaluated two aspects of the separate connector powering for 
trailer ABS in its in-service fleet studies: (1) the ability of each 
approach to provide a robust source of electrical power, through a 
separate dedicated circuit, to the trailer ABS, and; (2) the 
durability, reliability, and maintainability of these secondary 
powering approaches as well as the incremental costs associated with 
using any of those approaches. With respect to the first point, the 
data contained in Table 3.4, DOT Report No. HS 808 059, page 3-27 
indicate that all but one of the separate connector/separate circuit 
approaches provided higher voltage levels than did the shared stoplamp-
circuit-system approach. The exception was the battery approach which, 
as previously stated, has been abandoned. NHTSA has concluded that 
these data justify the requirement for separate circuit powering of 
ABS.
    NHTSA has also concluded that providing a separate source of power 
to trailers can be done practicably and economically. Regardless of 
whether a separate circuit or a shared circuit is used to power trailer 
ABS, ATA and other truck users have stated their preference for only 
one electrical cable/connector/plug system between tractors and 
trailers. The principal reason for wanting only one cable/connector is 
cost. All else being equal, utilizing two connectors would double the 
truck-user's replacement maintenance costs for these items, regardless 
of (and separate from) any costs associated with maintaining trailer 
ABSs by themselves. UPS commented that, on average, it already replaces 
two entire SAE J560 cable/connectors for each of their 15,791 vehicles 
each year. TNT Red Star Express fared somewhat better in this regard, 
reporting that it replaces 1.2 of these connectors per vehicle per 
year.
    In comparison, in NHTSA's fleet study of electrical system 
maintenance, the agency found that 0.4 SAE J560 cable/connector 
repairs/replacements were made per vehicle per year. This is a level 
substantially better than either UPS or TNT reported but, nevertheless 
twice the repair/replacement rate noted for ABS components (0.2 per 
vehicle per year). Since the cost of these cables/connectors is less 
than ABS component part costs, repair/replacement costs were less for 
these SAE J560 cable/connectors ($0.0002 per mile) than the overall 
repair replacement costs for all the ABS components ($0.00044 per 
mile).
    ATA commented that the overall cost of ABS-related maintenance 
would be on the order of 50 percent higher than indicated in the fleet 
study (i.e., $0.0002 + $0.00044 = $0.00064 per mile), if trailer ABS 
use necessitated a second tractor-to- trailer cable/connector/plug.
    As NHTSA has stated repeatedly, although today's final rule 
requires a separate circuit, it in no way mandates that a second cable/
connector be used. The agency has left the decision to the industry 
about what approach to use. Moreover, even if the industry decides to 
use two connectors temporarily or permanently, the agency believes the 
associated incremental maintenance costs associated with doing so are 
reasonable.
    NHTSA expects that one of four approaches will be chosen with 
respect to trailer ABS powering. First, the industry, through the SAE 
committees that are now considering this issue, could voluntarily 
settle on a new pin/circuit assignment scheme for the existing SAE J560 
connector, thereby ``freeing up'' a dedicated power circuit for the 
ABS. This approach could involve multiplexing of some signals. Second, 
the industry could develop and standardize a variant of the SAE J560 
connector that is compatible with the existing connector but which 
provides additional pins/circuits. Third, the industry could develop a 
totally new connector that will handle present and future tractor-to-
trailer powering and signalling/communication needs, and a transition 
could be made away from the [[Page 13250]] SAE J560 connector to this 
new connector. Fourth, the industry could decide to use a separate 
connector in addition to the existing SAE J560 connector.
    NHTSA is aware that the industry, through the SAE and the ATA's 
Maintenance Council, is actively considering the first three of these 
alternatives and that prototypes and, in some cases, production 
versions representing each alternative are currently available and 
being evaluated. A connector for the fourth approach has been 
standardized by the International Organization for Standardization 
(ISO). This connector (ISO 7638) is mandated for ABS connections in 
Europe, and thus is commercially available and in widespread use. The 
agency does not wish to hinder industry options in this regard or limit 
the design development process. Therefore, the agency has not specified 
the exact method for providing a separate circuit to trailer ABSs. 
NHTSA notes that hardware for one of these approaches is currently 
commercially available, and hardware for the other three may evolve 
within the time period between now and the effective date for 
implementing trailer ABS. Thus, practicable methods for achieving the 
separate circuit requirement are currently available, and either market 
forces or industry consensus is all that is needed to determine which 
will be the standardized method.
    Advocates were concerned that allowing the industry to develop a 
connector without government regulation could result in several 
connectors being available, which in turn would lead to incompatibility 
between tractors and trailers. AAMA stated that it was developing 
appropriate standards for trailer ABS power supply in cooperation with 
trailer manufacturers. In addition, SAE is interested in standardizing 
the ABS power supply.
    Based on the available information, NHTSA believes that the 
industry will decide on an appropriate electrical circuit and 
standardized connector to meet the proposed full-time power and in-cab 
malfunction lamp requirements, without the need for a detailed 
requirement. The agency emphasizes that it is important that the 
industry standardize on only one approach, to ensure compatibility 
between towing vehicles and their trailers. If the industry cannot 
voluntarily agree on a single approach, additional rulemaking may be 
necessary.
    NHTSA is aware that the industry is also working on multiplexing 
for tractor trailer electrical circuits, which could reduce the number 
of electrical wires needed for the various systems on the trailer. 
Nevertheless, multiplexing for combination vehicles is still in the 
developmental stage for most tractor trailer applications. The agency 
further notes that requiring that trailer ABSs receive full-time power 
will not prohibit multiplexing. Therefore, the agency believes that 
ATA's comments about multiplexing are not relevant.
    NHTSA further notes that ATA has misinterpreted the agency's 
previous 1992 rule to permit powering through either the stop lamp 
circuit or through a separate circuit. That rulemaking responded to a 
petition for rulemaking from WABCO, a brake manufacturer, to amend 
Standard No. 121 to eliminate a design restriction. Specifically, while 
trailer ABS was required to be powered by the stop lamp signal circuit 
prior to the amendment, the amendment permitted trailer ABS powering 
through either the stop lamp signal circuit or a separate circuit. The 
agency was concerned that the pre-amendment requirement might inhibit 
the use of some state-of-the-art trailer ABS that have more performance 
features, but also have higher power requirements. Therefore, contrary 
to ATA's statements that the agency was acting prematurely thereby 
preventing the development of multiplexing, the 1992 amendment 
broadened the flexibility afforded to manufacturers rather than limited 
it. In the notice adopting that amendment, NHTSA stated that the 
approach it adopted to remove the design restriction

will provide truck and trailer manufacturers and operators the 
flexibility needed to develop and use new trailer ABS systems. By 
providing such flexibility, the agency anticipates that more vehicle 
operators will decide to purchase ABS-equipped trailers. This is 
consistent with the agency's attempt [at that time] to foster 
voluntary adoption of trailer ABS by avoiding the specification of 
costly regulations that would act as disincentives for voluntarily 
equipping trailers and converter dollies with ABS. 57 FR at 30914.

Moreover, in the September 1993 NPRM proposing a full-time power 
requirement, NHTSA emphasized that the 1992 amendment was issued to 
``provide regulatory relief to manufacturers in developing new trailer 
ABS designs, at a time when trailer ABS was optional'' and that ``the 
agency would revisit the issue of trailer ABS powering in the context 
of rulemaking in which trailer ABS would be required.''
    Today's final rule culminates precisely the type of rulemaking 
envisioned in the 1992 notice. In today's final rule mandating that 
heavy vehicles be equipped with ABSs, the agency is addressing an 
entirely different situation from the one it was considering in 1992. 
NHTSA is analyzing how best to ensure safety through a mandatory 
requirement, not how to encourage the use of an optional safety device.

I. Applicability of Amendments

    In the NPRM, NHTSA proposed applying the ABS requirements to all 
vehicles with GVWRs exceeding 10,000 pounds. The agency explained that 
this proposal went beyond ISTEA's statutory directive for the agency to 
initiate rulemaking concerning methods for improving braking 
performance of ``new commercial motor vehicles,'' which are defined as 
vehicles with a GVWR of 26,001 or more pounds, including truck 
tractors, trailers, and their dollies.
1. Trailers With Hydraulic or Electric Brakes
    Manufacturers of trailers with electric or hydraulic brakes 
commented that they could not comply with the requirement because ABSs 
are not available for these types of vehicles.
    NHTSA wishes to clarify that the equipment requirement in today's 
final rule applies to powered heavy vehicles and to air-braked trailers 
and dollies, but not to trailers equipped with hydraulic or electric 
brakes. NHTSA notes that no FMVSS addresses vehicles equipped with 
electric brakes and that Standard No. 105 applies ``to passenger cars, 
multipurpose passenger vehicles, trucks and buses with hydraulic 
service brake systems.'' (see S3 ``Application.'') Since electric 
brakes are not covered by any FMVSS and Standard No. 105 does not cover 
trailers equipped with hydraulic brakes, today's amendment is not 
applicable to trailers with these types of brakes. The agency notes, 
however, that a trailer equipped with an air-over-hydraulic brake 
system will have to comply with the ABS requirement, since an air-over-
hydraulic system is a subsystem of an air-braked system, and is 
therefore subject to Standard No. 121.
2. Hydraulically Braked Vehicles
    NAFA stated that it is premature to mandate ABSs on medium vehicles 
with a GVWR between 10,000 and 26,000 pounds, claiming that there are 
no accident or safety data supporting an ABS requirement for these 
vehicles. In response to both the NPRM and the SNPRM, ATA commented 
that the agency should not require ABSs on hydraulically braked 
commercial vehicles until proven ABSs are available. It stated that it 
is not aware of [[Page 13251]] any proven ABS for hydraulic systems nor 
of any effort by the government to obtain such systems for fleet tests, 
which it believed is necessary before mandating such equipment. In 
response to the SNPRM, UPS stated that this requirement should not be 
adopted because NHTSA has performed no tests or demonstrations on 
hydraulically braked vehicles. Moreover, it stated that it is aware of 
no proven technology that could be applied to satisfy the new NHTSA 
rule.
    Allied Signal and Midland-Grau, two antilock brake system 
manufacturers, commented on the proposed requirements for ABSs on 
hydraulically braked heavy vehicles. Allied Signal stated that the 
technology for ABSs on heavy vehicles is the same as that used on 
passenger cars and light trucks and should not present significant 
technological problems. It indicated that some components such as the 
modulator and ECU are identical or nearly identical to those used in 
light vehicle applications. In addition, wheel speed sensors for 
hydraulically braked heavy vehicles incorporate the same technology 
used in wheel speed sensors for light vehicles and air braked heavy 
vehicles. Allied Signal commented that the agency's time frame can be 
achieved with proven technology. (i.e., ABS are increasing in use in 
this country on vehicles under 10,000 pounds GVWR). Midland-Grau 
commented that the industry is only about three years away from having 
ABSs for hydraulic braked single-unit trucks. In response to the SNPRM, 
AAMA stated that it is optimistic that validated ABSs will be available 
for all hydraulic vehicles within the proposed time frames. 
Nevertheless, because the availability of such systems is uncertain, it 
stated that there may be delays for certain types of hydraulic vehicles 
if development problems arise.
    Based on the available information, NHTSA believes that a March 
1999 effective date for requiring antilock brake systems on hydraulic 
braked single-unit trucks and buses provides sufficient time for 
vehicle manufacturers and ABS manufacturers to complete the development 
and testing of these systems. In addition, some Japanese and European 
manufacturers are currently marketing ABS for medium and large 
hydraulically braked vehicles. In their comments, brake manufacturers 
expressed confidence that such antilock systems will be available in 
this country.
    NHTSA notes that ATA and UPS are incorrect in their belief that the 
agency can only issue a requirement after conducting tests or 
demonstrations on that specific subcategory of vehicles. Nothing in the 
Safety Act mandates such specific vehicle testing. Based on comments by 
vehicle and ABS manufacturers and the positive experience in other 
countries with ABS-equipped hydraulic vehicles, NHTSA has determined 
that requiring hydraulic vehicles with ABS is practicable and 
appropriate. Moreover, the agency notes that manufacturers, which have 
fully developed antilock systems for hydraulic brakes on passenger cars 
and light vehicles, will be able to apply the underlying technology 
(i.e., wheel speed sensors, ECU, and modulators) to heavy vehicles. The 
agency has provided a lead time of four years to ensure that 
manufacturers will have sufficient time to develop and test antilock 
systems for hydraulic braked heavy vehicles.
    The agency is aware that Isuzu and Mitsubishi Fuso have marketed 
hydraulic braked heavy trucks with GVWRs of up to 16,000 pounds, with 
optional ABS since 1991. The ECU of the hydraulic ABS available on the 
Isuzu trucks is manufactured by Akebono and the remainder of the system 
is manufactured by Transtron. The hydraulic ABS on the Mitsubishi Fuso 
Trucks is manufactured by Japan ABS Co. Mercedes-Benz, offers 
hydraulic-braked heavy trucks with GVWRs of up to 26,000 pounds, with 
Bosch's ABS.
    Based on this information on the current availability of hydraulic 
ABS in Europe and Japan and comments by vehicle and ABS manufacturers, 
NHTSA is confident that there will be sufficient time for the 
development and testing of reliable antilock brake systems for 
hydraulically braked vehicles. Accordingly, NHTSA believes that it is 
appropriate and necessary for motor vehicle safety to require 
hydraulically-braked vehicles to be equipped with antilock brake 
systems. Nevertheless, the agency plans to monitor this development 
closely and could modify the implementation schedule if development of 
antilock systems for hydraulically braked vehicles faced unexpected 
development problems.

J. Implementation

    In the NPRM, NHTSA stated that its goal is to achieve significant 
improvements in braking performance at a reasonable cost to 
manufacturers and consumers. The agency proposed the following 
implementation schedule:

Truck Tractors.....................  2 years after final rule (1996).   
Trailers, including converter        3 years after final rule (1997).   
 dollies.                                                               
Single-unit trucks.................  4 years after final rule (1998).   
Buses..............................  5 years after final rule (1999).   
                                                                        

NHTSA stated that this implementation schedule was appropriate, given 
the current state of ABS technology. The agency believed that the 
schedule would provide the industry, ABS manufacturers, and maintenance 
personnel sufficient leadtime to prepare for the changes that would be 
required to accommodate the new technology.
    AAMA recommended that the effective dates for the proposed heavy 
vehicle stability and control requirements and the previously proposed 
stopping distance requirements be ``synchronized for the various 
vehicle types.''<SUP>60 AAMA recommended that the agency adopt the 
following effective dates for both the stability and control 
requirements and the stopping distance requirements, assuming that the 
two rules are issued before September 1994:

     <SUP>60On February 23, 1993, NHTSA proposed that the stopping 
distance requirements take effect two years after the final rule for 
all applicable vehicles. (58 FR 11009)

Truck tractors.....................  2 years after final rule (1996).   
Trailers, including converter        3 years after final rule (1997).   
 dollies.                                                               
Air-braked single-unit trucks and    3 years after final rule (1997).   
 buses.                                                                 
Hydraulic-braked single-unit trucks  4 years after final rule (1998).   
 and buses.                                                             
                                                                        

    Similarly, HDBMC requested that the implementation schedule for the 
directional stability and control requirements be accelerated and that 
the effective dates of this rulemaking and the stopping distance 
rulemaking be ``made coincident to allow the industry to maximize its 
efforts by effectively utilizing its limited resources.''
    ATA recommended effective dates of December 31, 1999 for tractors 
and December 31, 2001 for trailers, claiming that this schedule would 
permit each fleet, through its own tests, to determine which ABS is 
best suited to its operations and to phase in ABS accordingly. In 
contrast, Advocates favored the proposed implementation schedule and 
opposed any schedule that moved the compliance calendar to the next 
century.
    Based on its analysis of these comments, NHTSA issued a SNPRM that 
proposed the following implementation schedule for both sets of 
requirements:

                                                                        
[[Page 13252]]                                                          
Truck tractors.....................  2 years after final rule (1996).   
Trailers...........................  3 years after final rule (1997).   
Air-braked single-unit trucks and    3 years after final rule (1997).   
 buses.                                                                 
Hydraulic-braked single unit trucks  4 years after final rule (1998).   
 and buses.                                                             
                                                                        

    The agency stated that making the effective dates for the two 
rulemakings concurrent would facilitate a more orderly implementation 
process, avoid the need for manufacturers to redesign the brakes on 
individual vehicles twice, and reduce the development and compliance 
costs that manufacturers would face as a result of these regulations. 
NHTSA requested comments about the implementation schedule proposed in 
the supplemental notice.
    AAMA, HDBMC, Ford, GM, White GMC, Bosch, Eaton, Midland-Grau, 
Allied Signal, Advocates, and Gillig favored the implementation 
schedule proposed in the SNPRM. AAMA stated that the supplemental 
proposal would provide a more orderly and cost effective implementation 
of new requirements, thereby helping to avoid unnecessary redesign and 
redundant testing. Ford requested that the agency specify that the 
requirements have September 1 effective dates. Strait-Stop favored 
keeping the stopping distance requirements separate from the stability 
and control ones.
    ATA favored a phased in implementation schedule under which 
manufacturers would be required to sell (or consumers would be required 
to purchase) air braked powered vehicles with at least 25 percent ABS 
in 1996, 50 percent in 1997, 75 percent in 1998, and 100 percent in 
1999. Trailers would have a similar phase-in beginning in 1998. ATA 
stated that a phase-in is necessary to allow manufacturers the 
opportunity to offer a wider selection of ABS and to provide time to 
improve existing systems. Moreover, ATA claimed that a phase-in was 
essential to users because it would allow experimentation with 
different systems, thereby increasing public acceptance of the ABS 
mandate. Similarly, Tramec favored introducing the requirements over a 
period of time instead of all at once. Eaton cautioned that unforeseen 
manufacturing problems may impact product quality and availability. 
Therefore, it stated that a gradual increase in ABS usage would reduce 
concerns about manufacturer capacity and end-user support abilities.
    After reviewing the available information, NHTSA has decided to 
adopt an implementation schedule similar to the one proposed in the 
SNPRM. Specifically, truck tractors manufactured on or after March 1, 
1997 will have to be equipped with ABS and comply with the braking-in-
a-curve test and high coefficient of friction stopping distance 
requirements; trailers and single-unit air braked trucks and buses 
manufactured on or after March 1, 1998 will have to be equipped with 
ABS, and single-unit air braked trucks and buses will also have to 
comply with the high coefficient of friction stopping distance 
requirements; and hydraulic braked trucks and buses manufactured on or 
after March 1, 1999 will have to be equipped with ABS and comply with 
the high coefficient of friction stopping distance requirements. The 
agency has decided that these effective dates, which were widely 
supported by vehicle manufacturers, brake manufacturers, and safety 
advocacy groups, will provide for an efficient implementation of 
Congress's desire that NHTSA require heavy vehicles to be equipped with 
ABSs. This implementation schedule phases in ABS for heavy vehicles 
over a three-year period. Truck tractors, the vehicle type with the 
largest potential safety benefit from ABS, are required to comply with 
the rule first.
    This phase-in should facilitate consumer acceptance, since truck 
tractors, the most standardized type of heavy vehicle, will be subject 
to the regulation first. Only after this relatively uniform type of 
vehicle is equipped with ABS, will single unit vehicles which include 
more niche vehicles (e.g., dump trucks) be required to comply with the 
regulation?
    In deciding on the most appropriate implementation schedule, NHTSA 
gave serious consideration to ATA's suggestion that the requirements of 
this rule be phased in on a percentage basis over a four-year period. 
However, for the reasons set forth below, NHTSA has determined that the 
implementation schedule being adopted in today's final rule will 
provide the most benefits in the most cost effective manner. The agency 
emphasizes that adopting ATA's recommended phase-in would have resulted 
in needless and protracted delay, thereby resulting in a significantly 
less safe highway environment.
    Such a delay is unnecessary given the current state of development 
for ABS. At the time of publication of this final rule, six of the 
seven major U.S. manufacturers of heavy trucks, Freightliner 
Corporation, Peterbilt Motors Corporation, Kenworth Truck Company, Ford 
Motor Company, Mack Corporation, and Navistar International 
Corporation, have publicly announced that some or all of their product 
line of truck tractors, and in some cases single-unit trucks, will be 
equipped with ABS, as standard equipment, beginning with the 1995 model 
year. For heavy vehicle manufacturers, that model year began the summer 
of 1994. Thus, it appears that the marketplace has already addressed 
ATA's concern that manufacturers cannot meet increasing market demand 
for ABS. Also, manufacturers are typically warranting ABS for 300,000 
miles or three years, a fact that should allay ATA's concerns that 
manufacturers will not support their product offerings.
    NHTSA further notes that the final rule includes a phase-in 
requirement in which the vehicles for which braking stability is the 
greatest concern (truck tractors and trailers) are required to be 
equipped with ABS first. Single-unit trucks and buses follow at a later 
date. This will facilitate vehicle manufacturers' efforts to engineer 
these systems into their entire line of product offerings over a period 
of time spanning four years, instead of having to do it all in one 
year. This should substantially reduce burdens on manufacturers and 
give them sufficient time to engineer and accomplish high quality 
installations of ABS, which is a major concern of ATA.

K. Intermediate and Final Stage Manufacturers/Trailer Manufacturers

    In the NPRM, NHTSA provided an extensive discussion about the 
potential effect of the proposed requirements on intermediate, final 
stage, and trailer manufacturers. The agency explained that it is aware 
of the concerns of final stage and intermediate stage manufacturers 
about road testing their vehicles. In particular, the agency explained 
how an incomplete vehicle manufacturer could pass through certification 
to the final stage manufacturer and how a final stage manufacturer 
could certify compliance with the proposed requirements.
    NTEA commented that many of its members, most of whom are final 
stage manufacturers of vehicles produced in two or more stages, would 
not be able to use the pass-through certification because it believed 
that the guidelines provided by the incomplete vehicle manufacturer 
would be very restrictive. NTEA stated that these final stage 
manufacturers would, therefore, have no practicable and objective means 
of demonstrating compliance with the braking-in-a-curve requirement 
because they have neither the financial nor engineering resources to 
conduct their own compliance testing. NTEA [[Page 13253]] therefore 
requested that the agency exclude from this requirement all ``multi-
staged produced vehicles that are equipped with a cargo-carrying body 
or work-related equipment.'' Likewise, Midland-Grau stated that final 
stage manufacturers do not have the resources to certify their 
vehicles, and believed that it would be difficult for chassis 
manufacturers to establish comprehensive guidelines for final stage 
manufacturers to follow. AM General commented that small vehicle 
manufacturers will face undue burdens, and suggested that the 
rulemaking be limited to only Class 7 and 8 vehicles (which are the 
largest heavy vehicles, typically truck tractors over 26,000 pounds).
    As explained above, NHTSA has decided to apply the braking-in-a-
curve test only to truck tractors at this time. These vehicles are 
manufactured almost exclusively by large, single stage manufacturers. 
This final rule does not require manufacturers of single-unit vehicles 
and trailers, such as NTEA's members, to establish compliance with 
today's amendments through road testing. While incomplete single unit 
vehicles and trailers will have to be equipped with ABSs, the final 
stage and trailer manufacturers can ensure the presence of the 
equipment on their vehicles and can reasonably rely on a brake 
manufacturer's assurances that its ABS complies with the standard. 
Specifically, certification of compliance with the equipment 
requirement for ABS does not necessitate road testing.
    Nothing in the preceding discussion should be understood as 
indicating that the agency agrees with NTEA's comment that it would be 
impracticable for a final stage manufacturer to certify compliance with 
the braking-in-a-curve test. As explained in the NPRM, while a 
manufacturer must certify that its vehicles meet all applicable safety 
standards, a manufacturer need not necessarily conduct the specific 
tests set forth in an applicable standard. Certifications may be based 
on, among other things, engineering analyses, actual testing, and 
computer simulations. Moreover, a manufacturer need not conduct these 
operations itself. A manufacturer can utilize the services of 
independent engineers and testing laboratories. It can also join 
together with other manufacturers through trade associations to sponsor 
testing or analysis. Finally, it can rely on testing and analysis 
performed by other parties, including the brake manufacturers.

L. Benefits

    As detailed in the FRE, NHTSA estimates that the use of ABS on all 
heavy vehicles will help prevent between 320 and 506 fatalities, 
between 15,900 and 27,413 injuries, and between $458 million and $553 
million of property damage each year. Based on its evaluation, NHTSA 
believes that the rulemaking is cost beneficial since a significant 
number of crashes resulting in fatalities and property damage will be 
prevented by this rulemaking.
    In its comments, ATA questioned NHTSA's benefit analysis, arguing 
that recent accident data analyses have indicated that ABS on passenger 
cars does not result in significant reductions in crashes. The agency 
believes that it is neither appropriate nor possible to project 
effectiveness estimates for ABS, or for that matter, other safety 
equipment/features from one type of vehicle to another. As ATA is 
aware, vehicle loading characteristics for heavy vehicles differ 
significantly from those of passenger cars. Although the study upon 
which NHTSA based its benefit estimates did not specifically analyze 
whether heavy vehicles equipped with ABS have statistically lower 
accident rates, the results of that study carefully analyzed and 
reconstructed heavy vehicle crashes to estimate the likely benefit of 
ABS. The agency believes that its benefit analysis accurately estimates 
the benefits of heavy vehicle ABS.
    ATA also argues that ``the presence of ABS did not lead to a 
reduction in the accident rate, since in NHTSA's tractor fleet study, 
the proportion of crashes involving ABS-equipped tractors is the same 
as their proportion of the total fleet. NHTSA disagrees with this 
contention. The agency's fleet studies of ABS were never intended to 
result in estimates of the safety benefit of ABS. The total number of 
crashes that occurred during the tractor fleet study, fourteen, is too 
small to draw any statistically significant conclusions about the 
relative safety of ABS-equipped versus non-ABS-equipped vehicles.

M. Costs

    In the ANPRM, NHTSA estimated that the unit cost to a manufacturer 
for a complete six-channel ABS installed on a 6 x 4 tractor would be 
approximately $1400 or approximately $1100 for a full Select Low ABS. 
It estimated that the unit cost to a manufacturer to install ABS on a 
trailer would be $900.
    In response to comments to the ANPRM, NHTSA reevaluated its initial 
cost estimates to include several additional components including the 
wiring harnesses, mounting hardware, and in-cab warnings. As the 
Preliminary Regulatory Impact Analysis (PRIA) explained in detail, the 
agency estimated that the unit cost for a vehicle purchaser to comply 
with the proposed requirements (including the connectors and cables 
that provide full-time power) would be approximately $2900 for the 
average truck tractor, $2350 for the average single-unit truck and bus, 
$1850 for a non-towing trailer, $1700 for a towing trailer, and $1475 
for a trailer converter dolly. Based on these estimates of consumer 
costs and estimated annual production of 137,000 truck tractors, 
160,000 single unit trucks and school buses, and 7000 transit and 
intercity buses, the agency estimated that the annual costs for these 
vehicles would be $790 million. For trailers, these consumer cost 
estimates together with an annual production of 115,000 non-towing 
trailers, 32,000 towing trailers, and 3,000 trailer converter dollies 
yields an estimated annual cost of $272 million.
    Since the preparation of the PRIA, NHTSA has completed a detailed 
engineering process-cost analysis study in which antilock braking 
systems from three ABS manufacturers were evaluated. The cost 
evaluation entailed a physical tear-down of the system, in which the 
cost of each part was determined based on the actual manufacturing 
process used in its production. The study estimated the weight and 
various costs related to the production and installation of three 4S/4M 
tractor ABS, each from a different ABS manufacturer, and three 
different trailer ABS configurations, a 6S/3M, a 4S/2M and a 2S/1M, 
each from a different manufacturer. Based on that cost information, the 
agency estimates that the cost for the minimum ABS needed to comply 
with the requirements in this amendment would be: $749.33 for a truck 
tractor, $682.51 for a single-unit truck, and $439.64 for a trailer. 
Separate analyses estimated the cost and weight of tractor-to-trailer 
connectors/cables and related wiring ($93.97 for a truck tractor, 
$39.52 for a non-towing trailer, and $133.49 for a towing trailer or 
trailer converter dolly), and of in-cab ABS malfunction indicator lamps 
(MIL) for tractors and trailer-mounted ABS MILs for trailers ($13.66 
for a truck tractor, $9.47 for a single-unit truck, and $9.43 for a 
trailer). The total estimated cost to the vehicle purchaser is 
estimated to be: $856.96 for a truck tractor, $691.98 for a single-unit 
truck or bus, $488.59 for a non-towing trailer, and $582.56 for a 
towing trailer or trailer converter dolly. Based on these estimates of 
increased cost to the vehicle purchaser and estimated annual production 
of 147,600 truck tractors, 248,300 single unit trucks and school 
[[Page 13254]] buses, and 7000 transit and intercity buses, the agency 
estimates that the annual costs for these vehicles would be $303 
million. For trailers and trailer converter dollies, these estimates of 
increased cost to the vehicle purchaser together with an annual 
production of 139,400 non-towing trailers, 46,700 towing trailers, and 
2,900 trailer converter dollies yields an estimated annual cost of $97 
million. Therefore, the agency estimates that the total annual 
increased cost for equipping heavy vehicles with ABS will be $400 
million.
    Along with estimating the cost increases to the new vehicle 
purchaser, NHTSA also estimated the increases in the cost of operating 
heavy vehicles equipped with ABS. Three categories of operating costs 
were examined: lifetime maintenance costs, lifetime fuel costs due to 
the additional weight of the ABS, and lifetime revenue loss due to 
payload displacement. The range of the increase in total lifetime 
operating costs related to equipping heavy vehicles with ABS is from 
$201.47 to $786.65. Since the estimates for these various operating 
costs are dependent upon the type of fuel used for powered vehicles and 
on the estimated lifetime vehicle miles travelled (VMT) for the various 
vehicle types, the heavy vehicles were divided into 18 different fuel 
type/VMT categories. The total estimated increase in vehicle operating 
costs associated with ABS for all heavy vehicles is $232 million. The 
reader is referred to the FEA for a detailed discussion of the costs 
for these different categories.
    In its comments, ATA questioned NHTSA's portrayal of the increases 
in vehicle maintenance costs as not being significant compared to 
overall cost of maintaining the air brake system on heavy vehicles. ATA 
did not, however, question the actual increased maintenance cost per 
mile estimates derived from the agency's fleet studies. It is these 
estimates of the increased maintenance cost per mile that were used in 
estimating the total cost impact of this rulemaking and determining 
that the amendment is cost effective. As such, the agency believes that 
the relative increase in vehicle maintenance that would result in 
different fleets is not the important factor in evaluating the impact 
of this Final Rule.

IX. Rulemaking Analyses and Notices

A. Executive Order 12866 and DOT Regulatory Policies and Procedures

    NHTSA has considered the impacts of this rulemaking action and 
determined that it is ``significant'' within the meaning of the 
Department of Transportation's regulatory policies and procedures. In 
addition, the Office of Management and Budget has determined that it is 
``significant'' within the meaning of Executive Order 12866. The agency 
has prepared a Final Economic Assessment describing the economic and 
other effects of this rulemaking action. Summary discussions of those 
effects are provided above. For persons wishing to examine the full 
analysis, a copy is being placed in the docket.

B. Regulatory Flexibility Act

    NHTSA has also considered the effects of this rulemaking action 
under the Regulatory Flexibility Act. I hereby certify that it will not 
have a significant economic impact on a substantial number of small 
entities. Accordingly, the agency has not prepared a final regulatory 
flexibility analysis.
    The primary cost effect of the requirements will be on 
manufacturers of heavy vehicles which are generally large businesses. 
However, final stage manufacturers are generally small businesses. A 
detailed discussion about the anticipated economic impact on these 
businesses is provided in the FRIA.

C. National Environmental Policy Act

    NHTSA has analyzed this rulemaking action for the purposes of the 
National Environmental Policy Act. The agency has determined that 
implementation of this action will not have any significant impact on 
the quality of the human environment.

D. Executive Order 12612 (Federalism)

    NHTSA has analyzed this action under the principles and criteria in 
Executive Order 12612. The agency has determined that this notice does 
not have sufficient Federalism implications to warrant the preparation 
of a Federalism Assessment. No State laws will be affected.

E. Civil Justice Reform

    This final rule does not have any retroactive effect. Under 49 
U.S.C. 30103, whenever a Federal motor vehicle safety standard is in 
effect, a State may not adopt or maintain a safety standard applicable 
to the same aspect of performance which is not identical to the Federal 
standard, except to the extent that the State requirement imposes a 
higher level of performance and applies only to vehicles procured for 
the State's use. 49 U.S.C. 30161 sets forth a procedure for judicial 
review of final rules establishing, amending or revoking Federal motor 
vehicle safety standards. That section does not require submission of a 
petition for reconsideration or other administrative proceedings before 
parties may file suit in court.

List of Subjects in 49 CFR Part 571

    Imports, Incorporation by reference, Motor vehicle safety, Motor 
vehicles, Rubber and rubber products, Tires.

    In consideration of the foregoing, the agency is amending Section 
571.3, Standard No. 101, Controls and Displays, Standard No. 105, 
Hydraulic Brake Systems and Standard No. 121, Air Brake Systems, in 
Title 49 of the Code of Federal Regulations at Part 571 as follows:

PART 571--[AMENDED]

    1. The authority citation for Part 571 continues to read as 
follows:

    Authority: 49 U.S.C. 322, 30111, 30115, 30117, and 30166, 
delegation of authority at 49 CFR 1.50.

    2. Part 571.3 is amended in paragraph (b) to add a definition of 
``Full Trailer'' as follows:


Sec. 571.3  Definitions.

* * * * *
    Full trailer means a trailer, except a pole trailer, that is 
equipped with two or more axles that support the entire weight of the 
trailer.
* * * * *
    3. In Sec. 571.101, Table 2 is revised to appear as follows:


Sec. 571.101  Standard No. 101; Controls and Displays.

* * * * *

                                                 BILLING CODE 4910-59-P
[[Page 13255]]

[GRAPHIC][TIFF OMITTED]TR10MR95.000



BILLING CODE 4910-59-C
[[Page 13256]]

* * * * *
    4. Section 571.105 is amended in S4 by removing the definition of 
``Antilock system'' and by adding the definitions of ``Antilock brake 
system,'' ``Directly controlled wheel,'' ``Indirectly controlled 
wheel,'' and ``Peak friction coefficient;'' by revising S5.1, S5.3, 
S5.3.1(c), S5.3.3; and S5.5; and adding S5.3.3(a), S5.3.3(b), S5.5.1 
and S5.5.2 to read as follows:


Sec. 571.105  Standard No. 105, hydraulic brake systems.

* * * * *
    Antilock brake system or ABS means a portion of a service brake 
system that automatically controls the degree of rotational wheel slip 
during braking by:
    (1) Sensing the rate of angular rotation of the wheels;
    (2) Transmitting signals regarding the rate of wheel angular 
rotation to one or more controlling devices which interpret those 
signals and generate responsive controlling output signals; and
    (3) Transmitting those controlling signals to one or more 
modulators which adjust brake actuating forces in response to those 
signals.
* * * * *
    Directly Controlled Wheel means a wheel at which the degree of 
rotational wheel slip is sensed and corresponding signals are 
transmitted to one or more modulators that adjust the brake actuating 
forces at that wheel. Each modulator may also adjust the brake 
actuating forces at other wheels in response to the same signal[s].
* * * * *
    Indirectly Controlled Wheel means a wheel at which the degree of 
rotational wheel slip is not sensed, but at which the modulator of an 
antilock braking system adjusts its brake actuating forces in response 
to signals from one or more sensed wheels.
* * * * *
    Peak friction coefficient or PFC means the ratio of the maximum 
value of braking test wheel longitudinal force to the simultaneous 
vertical force occurring prior to wheel lockup, as the braking torque 
is progressively increased.
* * * * *
    S5.1  Service brake systems. Each vehicle shall be equipped with a 
service brake system acting on all wheels. Wear of the service brake 
shall be compensated for by means of a system of automatic adjustment. 
Each passenger car and each multipurpose passenger vehicle, truck, and 
bus with a GVWR of 10,000 pounds or less shall be capable of meeting 
the requirements of S5.1.1 through S5.1.6 under the conditions 
prescribed in S6, when tested according to the procedures and in the 
sequence set forth in S7. Each school bus with a GVWR greater than 
10,000 pounds shall be capable of meeting the requirements of S5.1.1 
through S5.1.5 under the conditions prescribed in S6, when tested 
according to the procedures and in the sequence set forth in S7. Each 
multipurpose passenger vehicle, truck, and bus (other than a school 
bus) with a GVWR greater than 10,000 pounds shall be capable of meeting 
the requirements of S5.1.1, S5.1.2, and S5.1.3 under the conditions 
prescribed in S6, when tested according to the procedures and in the 
sequence set forth in S7. Except as noted in S5.1.1.2 and S5.1.1.4, if 
a vehicle is incapable of attaining a speed specified in S5.1.1, 
S5.1.2, S5.1.3, or S5.1.6, its service brakes shall be capable of 
stopping the vehicle from the multiple of 5 mph that is 4 to 8 mph less 
than the speed attainable in 2 miles, within distances that do not 
exceed the corresponding distances specified in Table II. If a vehicle 
is incapable of attaining a speed specified in S5.1.4 in the time or 
distance interval set forth, it shall be tested at the highest speed 
attainable in the time or distance interval specified.
* * * * *
    S5.3  Brake system indicator lamp. Each vehicle shall have a brake 
system indicator lamp or lamps, mounted in front of and in clear view 
of the driver, which meet the requirements of S5.3.1 through S5.3.5. A 
vehicle with a GVWR of 10,000 pounds or less may have a single common 
indicator lamp. A vehicle with a GVWR of greater than 10,000 pounds may 
have an indicator lamp which is common for gross loss of pressure, drop 
in the level of brake fluid, or application of the parking brake, but 
shall have a separate indicator lamp for antilock brake system 
malfunction. However, the options provided in S5.3.1(a) shall not apply 
to a vehicle manufactured without a split service brake system; such a 
vehicle shall, to meet the requirements of S5.3.1(a), be equipped with 
a malfunction indicator that activates under the conditions specified 
in S5.3.1(a)(4). This warning indicator shall, instead of meeting the 
requirements of S5.3.2 through S5.3.5, activate (while the vehicle 
remains capable of meeting the requirements of S5.1.2.2 and the 
ignition switch is in the ``on'' position) a continuous or intermittent 
audible signal and a flashing warning light, displaying the words 
``STOP-BRAKE FAILURE'' in block capital letters not less than one-
quarter of an inch in height.
* * * * *
    S5.3.1 * * *
    (c) A malfunction that affects the generation or transmission of 
response or control signals in an antilock brake system, or a total 
functional electrical failure in a variable proportioning brake system.
* * * * *
    S5.3.3  (a)Each indicator lamp activated due to a condition 
specified in S5.3.1 shall remain activated as long as the malfunction 
exists, whenever the ignition (start) switch is in the ``on'' (run) 
position, whether or not the engine is running.
    (b) For vehicles with a GVWR greater than 10,000 pounds, each 
message about the existence of a malfunction in an antilock brake 
system shall be stored after the ignition switch is turned to the 
``off'' position and automatically reactivated when the ignition switch 
is turned to the ``on'' position. The indicator lamp shall also be 
activated as a check of lamp function whenever the ignition is turned 
to the ``on'' (run) position. The indicator lamp shall be deactivated 
at the end of the check of the lamp function unless there is a 
malfunction or a message about a pre-existing malfunction.
* * * * *
    S5.5.  Antilock and Variable Proportioning Brake Systems.
    S5.5.1  Each vehicle with a GVWR greater than 10,000 pounds, except 
for any vehicle that has a speed attainable in 2 miles of not more than 
33 mph, shall be equipped with an antilock brake system that directly 
controls the wheels of at least one front axle and the wheels of at 
least one rear axle of the vehicle. Wheels on other axles of the 
vehicle may be indirectly controlled by the antilock brake system.
    S5.5.2  In the event of any failure (structural or functional) in 
an antilock or variable proportioning brake system, the vehicle shall 
be capable of meeting the stopping distance requirements specified in 
S5.1.2 for service brake system partial failure.
* * * * *


Sec. 571.121  Standard No. 121, air brake systems.

    5. Section 571.121 is amended in S4 by removing the definitions of 
``Antilock system'' and ``skid number'' and by adding the definitions 
of ``Antilock brake system,'' ``Directly Controlled Wheel,'' ``Full-
treadle brake application,'' ``Independently Controlled Wheel,'' 
``Indirectly Controlled Wheel,'' ``Maximum drive- 
[[Page 13257]] through speed,'' ``Peak friction coefficient;'' by 
revising S5.1.6 and adding S5.1.6.1, S5.1.6.2, and S5.1.6.3; by adding 
S5.2.3, S5.2.3.1, S5.2.3.2, and S5.2.3.3; by revising S5.3; by adding 
S5.3.6, S5.3.6.1, and S5.3.6.2; by revising S5.5.1, S5.5.2, S6.1.7, 
S6.1.10, S6.1.10.2, S6.1.10.3, and S6.1.10.4; by removing and reserving 
S6.1.10.1; by removing S6.1.10.5, S6.1.10.6, and S6.1.10.7, and by 
adding S6.1.15 to read as follows:


Sec. 571.121  Standard No. 121; air brake systems.

* * * * *
    Antilock Brake System or ABS means a portion of a service brake 
system that automatically controls the degree of rotational wheel slip 
during braking by:
    (1) Sensing the rate of angular rotation of the wheels;
    (2) Transmitting signals regarding the rate of wheel angular 
rotation to one or more controlling devices which interpret those 
signals and generate responsive controlling output signals; and
    (3) Transmitting those controlling signals to one or more 
modulators which adjust brake actuating forces in response to those 
signals.
* * * * *
    Directly Controlled Wheel means a wheel at which the degree of 
rotational wheel slip is sensed and corresponding signals are 
transmitted to one or more modulators that adjust the brake actuating 
forces at that wheel. Each modulator may also adjust the brake 
actuating forces at other wheels in response to the same signal[s].
* * * * *
    ``Full-treadle brake application'' means a brake application in 
which the treadle valve pressure in any of the valve's output circuits 
reaches 100 psi within 0.2 seconds after the application is initiated.
* * * * *
    Independently Controlled Wheel means a directly controlled wheel 
for which the modulator does not adjust the brake actuating forces at 
any other wheel on the same axle.
* * * * *
    Indirectly Controlled Wheel means a wheel at which the degree of 
rotational wheel slip is not sensed, but at which the modulator of an 
antilock braking system adjusts its brake actuating forces in response 
to signals from one or more sensed wheel(s).
* * * * *
    ``Maximum drive-through speed'' means the highest possible constant 
speed at which the vehicle can be driven through 200 feet of a 500-foot 
radius curve arc without leaving the 12-foot lane.
* * * * *
    Peak friction coefficient or PFC means the ratio of the maximum 
value of braking test wheel longitudinal force to the simultaneous 
vertical force occurring prior to wheel lockup, as the braking torque 
is progressively increased.
* * * * *
    S5.1.6  Antilock Brake System.
    S5.1.6.1(a)  Each single-unit vehicle manufactured on or after 
March 1, 1998 shall be equipped with an antilock brake system that 
directly controls the wheels of at least one front axle and the wheels 
of at least one rear axle of the vehicle. Wheels on other axles of the 
vehicle may be indirectly controlled by the antilock brake system.
    S5.1.6.1(b)  Each truck tractor manufactured on or after March 1, 
1997 shall be equipped with an antilock brake system that directly 
controls the wheels of at least one front axle and the wheels of at 
least one rear axle of the vehicle, with the wheels of at least one 
axle being independently controlled. Wheels on other axles of the 
vehicle may be indirectly controlled by the antilock brake system. A 
truck tractor shall have no more than three wheels controlled by one 
modulator.
    S5.1.6.2  Antilock Malfunction Circuit and Signal.
    (a) Each truck tractor manufactured on or after March 1, 1997 and 
each single unit vehicle manufactured on or after March 1, 1998 shall 
be equipped with an electrical circuit that is capable of signalling a 
malfunction that affects the generation or transmission of response or 
control signals in the vehicle's antilock brake system.
    (b) Each truck tractor manufactured on or after March 1, 1997 and 
each single unit vehicle manufactured on or after March 1, 1998 shall 
have an indicator lamp, mounted in front of and in clear view of the 
driver, which is activated whenever there is a malfunction that affects 
the generation or transmission of the response or control signals in an 
antilock brake system. The indicator lamp shall remain activated as 
long as the malfunction exists, whenever the ignition (start) switch is 
in the ``on'' (run) position, whether or not the engine is running. 
Each message about the existence of a malfunction in an antilock brake 
system shall be stored after the ignition switch is turned to the 
``off'' position and automatically reactivated when the ignition switch 
is turned to the ``on'' position. The indicator lamp shall also be 
activated as a check of lamp function whenever the ignition is turned 
to the ``on'' or ``run'' position. The indicator lamp shall be 
deactivated at the end of the check of lamp function unless there is a 
malfunction or a message about a pre-existing malfunction.
    (c) Each truck tractor manufactured on or after March 1, 1997 and 
each single unit vehicle manufactured on or after March 1, 1998 that is 
equipped to tow another air-braked vehicle, shall be equipped with an 
electrical circuit that is capable of transmitting information about a 
malfunction in the antilock brake system on one or more towed 
vehicle(s) (e.g., trailer(s) and dolly(ies)). Each such vehicle shall 
also be equipped with an indicator lamp, mounted in front of and in 
clear view of the driver, capable of receiving, from one or more 
antilock equipped towed vehicle(s), information transmitted about a 
malfunction of a towed vehicle's antilock system and then activating 
the indicator lamp when there is a malfunction in the towed vehicle's 
antilock brake system. The indicator lamp shall remain activated as 
long as the malfunction exists, whenever the ignition (start) switch is 
in the ``on'' (run) position, whether or not the engine is running. The 
indicator lamp shall also be activated as a check of lamp function 
whenever the ignition is turned to the ``on'' or ``run'' position. The 
indicator lamp shall be deactivated at the end of the check of lamp 
function unless there is a malfunction or a message about a pre-
existing malfunction.
    S5.1.6.3  Antilock Power Circuit for Towed Vehicles. Each truck 
tractor manufactured on or after March 1, 1997 and each single unit 
vehicle manufactured on or after March 1, 1998 that is equipped to tow 
another air-braked vehicle shall be equipped with one or more separate 
electrical circuits, specifically provided to power the antilock system 
on the towed vehicle(s). Such a circuit shall be adequate to enable the 
antilock system on each towed vehicle to be fully operable.
* * * * *
    S5.2.3  Antilock Brake System.
    S5.2.3.1(a)  Each semitrailer (including a trailer converter dolly) 
manufactured on or after March 1, 1998 shall be equipped with an 
antilock brake system that directly controls the wheels of at least one 
axle of the vehicle. Wheels on other axles of the vehicle may be 
indirectly controlled by the antilock brake system.
    (b) Each full trailer manufactured on or after March 1, 1998 shall 
be equipped with an antilock brake system that directly controls the 
wheels of at least [[Page 13258]] one front axle of the vehicle and at 
least one rear axle of the vehicle. Wheels on other axles of the 
vehicle may be indirectly controlled by the antilock brake system.
    S5.2.3.2  Antilock Malfunction Circuit and Signal. Each trailer 
(including a trailer converter dolly) manufactured on or after March 1, 
1998 that is equipped with an antilock brake system shall be equipped 
with an electrical circuit that is capable of signalling a malfunction 
in the trailer antilock brake system, and shall comply with the 
requirements of S5.2.3.3. A trailer manufactured on or after March 1, 
1998 that is not designed to tow another air brake equipped trailer 
shall have the means for connection of the antilock malfunction signal 
circuit and ground, at the front of the trailer. A trailer manufactured 
on or after March 1, 1998 that is designed to tow another air brake 
equipped trailer shall be capable of transmitting a malfunction signal 
from the antilock systems of additional trailers in a combination and 
shall have means for the connection of the antilock malfunction signal 
circuit and ground, at both the front and rear of the trailer. Each 
message about the existence of a malfunction in an antilock brake 
system shall be stored whenever power is no longer supplied to the 
system. The indicator lamp shall also be activated as a check of lamp 
function whenever power is supplied to the antilock brake system. The 
indicator lamp shall be deactivated at the end of the check of lamp 
function unless there is a malfunction or a message about a pre-
existing malfunction.
    S5.2.3.3  Antilock Malfunction Indicator. Each trailer (including a 
trailer converter dolly) manufactured on or after March 1, 1998 and 
before March 1, 2006 shall be equipped with a lamp indicating a 
malfunction of a trailer's antilock brake system. Such a lamp shall 
remain activated as long as the malfunction exists whenever the power 
is supplied to the antilock brake system. The display shall be visible 
within the driver's forward field of view through the rearview 
mirror(s), and shall be visible once the malfunction is present and 
power is provided to the system.
* * * * *
    S5.3  Service Brakes--road tests. The service brake system on each 
truck tractor manufactured before March 1, 1997 shall, under the 
conditions of S6, meet the requirements of S5.3.3 and S5.3.4, when 
tested without adjustments other than those specified in this standard. 
The service brake system on each truck tractor manufactured on or after 
March 1, 1997 shall, under the conditions of S6, meet the requirements 
of S5.3.1, S5.3.3, S5.3.4, and S5.3.6, when tested without adjustments 
other than those specified in this standard. The service brake system 
on each bus and truck (other than a truck tractor) manufactured before 
March 1, 1998 shall, under the conditions of S6, meet the requirements 
of S5.3.3, and S5.3.4, when tested without adjustments other than those 
specified in this standard. The service brake system on each bus and 
truck (other than a truck tractor) manufactured on or after March 1, 
1998 shall, under the conditions of S6, meet the requirements of 
S5.3.1, S5.3.3, and S5.3.4 when tested without adjustments other than 
those specified in this standard. The service brake system on each 
trailer shall, under the conditions of S6, meet the requirements of 
S5.3.3, S5.3.4, and S5.3.5 when tested without adjustments other than 
those specified in this standard. However, a heavy hauler trailer and 
the truck and trailer portions of an auto transporter need not meet the 
requirements of S5.3.
* * * * *
    S5.3.6  Stability and Control During Braking--Truck Tractors. When 
stopped three consecutive times for each combination of weight, speed, 
and road condition specified in S5.3.6.1 and S5.3.6.2, each truck 
tractor manufactured on or after March 1, 1997 shall stop each time 
within the 12-foot lane, without any part of the vehicle leaving the 
roadway.
    S5.3.6.1  Using a full-treadle brake application, stop the vehicle 
from 30 mph or 75% of the maximum drive-through speed, whichever is 
less, on a 500-foot radius curved roadway with a wet level surface 
having a peak friction coefficient of 0.5 when measured using an 
American Society for Testing and Materials (ASTM) E1136 standard 
reference test tire, in accordance with ASTM Method E1337-90, at a 
speed of 40 mph, with water delivery.
    S5.3.6.2  Stop the vehicle with the vehicle
    (a) loaded to its GVWR, and
    (b) at its unloaded weight plus up to 500 pounds (including driver 
and instrumentation), or at the manufacturer's option, at its unloaded 
weight plus up to 500 pounds (including driver and instrumentation) and 
plus not more than an additional 1000 pounds for a roll bar structure 
on the vehicle.
* * * * *
    S5.5.1  Antilock System Malfunction. On a truck tractor 
manufactured on or after March 1, 1997 and a single unit vehicle 
manufactured on or after March 1, 1998 that is equipped with an 
antilock brake system, a malfunction that affects the generation or 
transmission of response or control signals of any part of the antilock 
system shall not increase the actuation and release times of the 
service brakes.
* * * * *
    S5.5.2  Antilock System Power--Trailers. On a trailer (including a 
trailer converter dolly) manufactured on or after March 1, 1998 that is 
equipped with an antilock system that requires electrical power for 
operation, the power shall be obtained from one or more separate 
electrical circuits specifically provided to power the trailer antilock 
system. The antilock system shall automatically receive power from the 
stop lamp circuit, if the separate power circuit or circuits are not in 
use. Each trailer (including a trailer converter dolly) manufactured on 
or after March 1, 1998 that is equipped to tow another air-braked 
vehicle shall be equipped with one or more separate electrical circuits 
specifically provided to power the antilock system on the towed 
vehicle(s). Such circuits shall be adequate to enable the antilock 
system on each towed vehicle to be fully operable.
* * * * *
    S6.1.7  Unless otherwise specified, stopping tests are conducted on 
a 12-foot wide level, straight roadway having a peak friction 
coefficient of 0.9. For road tests in S5.3, the vehicle is aligned in 
the center of the roadway at the beginning of a stop. Peak friction 
coefficient is measured using an ASTM E1136 standard reference test 
tire in accordance with ASTM method E1337-90, at a speed of 40 mph, 
without water delivery for the surface with PFC of 0.9, and with water 
delivery for the surface with PFC of 0.5.
* * * * *
    S6.1.10  In a test other than a static parking test, a truck 
tractor is tested at its GVWR by coupling it to an unbraked flatbed 
semi-trailer (hereafter, control trailer) as specified in S6.1.10.2 to 
S6.1.10.4.
* * * * *
    S6.1.10.1  [RESERVED]
    S6.1.10.2  The center of gravity height of the ballast on the 
loaded control trailer shall be less than 24 inches above the top of 
the tractor's fifth wheel.
* * * * *
    S6.1.10.3  The control trailer has a single axle with a gross axle 
weight rating of 18,000 pounds and a length, measured from the 
transverse centerline of the axle to the centerline of the kingpin, of 
258 <plus-minus> 6 inches. [[Page 13259]] 
    S6.1.10.4  The control trailer is loaded so that its axle is loaded 
at 4,500 pounds and the tractor is loaded to its GVWR, loaded above the 
kingpin only, with the tractor's fifth wheel adjusted so that the load 
on each axle measured at the tire-ground interface is most nearly 
proportional to the axles' respective GAWRs, without exceeding the GAWR 
of the tractor's axle or axles or control trailer's axle.
* * * * *
    S6.1.15  Initial Brake Temperature. Unless otherwise specified, the 
initial brake temperature is not less than 150  deg.F and not more than 
200  deg.F.
* * * * *
    Issued on: March 1, 1995.
Ricardo Martinez,
Administrator.

    Note.--The following appendix will not appear in the Code of 
Federal Regulations:

Appendix--Braking Systems, Tires, Wheel Lockup, and Loss-of-Control 
Crashes

1. Introduction

    NHTSA is providing a brief discussion\1\ of braking systems, 
tires, wheel lockup, and loss-of-control crashes in this Appendix; 
interested persons are referred to several agency reports<SUP>2 for 
a more complete discussion.

    \1\Much of the discussion which follows is adapted from U.S. v. 
General Motors Corp., 656 F.Supp 1555, 1562-1566 (D.D.C. 1987,), 
``The Anatomy of a Tractor Trailer Jackknife'' by Richard Radlinski, 
Vehicle Research and Test Center, National Highway Traffic Safety 
Administration, and ``Antilock Systems for Air-Braked Vehicles'' by 
William A. Leasure, Jr. and Sidney F. Williams, Jr., National 
Highway Traffic Safety Administration, SP-789, Society of Automotive 
Engineers, Inc., February 1989.

---------------------------------------------------------------------------

    An ABS is a closed-loop feedback control system that, above a 
preset minimum speed, automatically modulates brake pressure in 
response to measured wheel speed performance to control the degree 
of wheel slip during braking and provide improved utilization of the 
friction available between the tires and the road. These systems, 
therefore, could justifiably be called antilock brake/tire systems 
since their function is to balance brake torque with tire/road 
friction to obtain that wheel slip which optimizes braking 
performance. Antilock system designers must take into consideration 
the characteristics of brake systems and tires--both must be 
understood in order to optimize the performance of antilock systems.

2. Heavy Vehicle Brake Systems

    The function of a motor vehicle's brake system is to slow or 
stop the vehicle or to hold it stationary. Service brake systems\3\ 
consist of foundation brake assemblies (the portion of the system 
that actually creates brake torque and the resulting retarding 
forces at the tire/road interface) and a service brake control 
system.

    \3\A vehicle's brake system includes both the service brake 
system which the driver uses to slow or stop the vehicle, and the 
parking brake system which the driver uses to hold the vehicle 
stationary while unattended. The notice only addresses the service 
brake system and does not discuss parking brake system performance.
---------------------------------------------------------------------------

    There are two principal types of foundation brakes in use: drum 
and disc brakes. Drum brakes create retarding friction by pressing 
contoured brake linings against the inside walls of brake drums that 
are attached the vehicle's wheels. Disc brakes perform the same 
function by squeezing or clamping both sides of a brake rotor 
between two or more brake pads.
    There are two principal types of service brake control systems, 
hydraulic and pneumatic. These service brake control systems consist 
of the components necessary to distribute and control the fluid 
pressure to the foundation brake assemblies. In the case of an air 
brake system, this is pneumatic pressure; i.e., compressed air, and 
in the case of an hydraulic brake system, this is hydrostatic 
pressure; i.e., pressurized brake fluid.
    In the case of an air brake system, the service brake control 
system modulates the air pressure in the service brake system. 
Pressurized (compressed) air stored in reservoirs is supplied 
through a foot-actuated service brake control valve (treadle valve). 
This air pressure, which varies in proportion to how far the treadle 
valve is depressed, is then applied through a series of pneumatic 
valves (relay valves, and in the case of vehicles equipped with 
antilock brake systems, modulator valves) to the service brake 
chambers located near each wheel on the vehicle. This air pressure 
in the service brake chambers in turn applies forces to the brake 
linings or pads within the foundation brakes to create brake torque. 
Pneumatic systems are open, in that air, once utilized at a brake 
chamber, is exhausted to atmosphere. Air pressure levels in 
reservoirs are maintained by an engine-driven compressor.
    Hydraulic brake systems utilize an incompressible fluid (a 
petroleum-based brake fluid), metered through a combined valve and 
reservoir (brake master cylinder), to create variable amounts of 
hydrostatic pressure within a closed system of brake lines. The 
brake lines transmit this pressure to wheel cylinders or brake 
caliper pistons which, in turn, apply force to the brake linings or 
pads in proportion to the amount of manual force being applied to 
the brake pedal.
    It should be noted that hydraulic foundation brake assemblies 
(either drum or disc brakes) are sometimes used in air brake systems 
(commonly called air-over-hydraulic brake systems) with the 
hydraulic pressure produced by a hydraulic master cylinder which is 
powered by an air brake chamber.
    One important characteristic of brake systems that effects the 
control modes used by ABSs to control wheel slip is the 
hysteresis<SUP>4 of both the service brake control systems and 
foundation brakes. In the case of service brake control systems, the 
hysteresis of concern is the time lag between the ECU signalling the 
modulator valve to release (reduce) or apply (increase) brake 
application pressure and the time at which that increased or 
decreased pressure is actually applied at the foundation brakes. 
This pneumatic hysteresis time lag can be up to several tenths of a 
second for an air braked system, but for a hydraulic brake system, 
this time lag is very short, usually less than one-tenth of a 
second.

     <SUP>4Hysteresis is:
    1. the time lag exhibited by a body in reacting to changes in 
the forces affecting it, and
    2. the phenomenon exhibited by a system in which the reaction of 
the system to changes is dependent upon its past reactions to 
change.
---------------------------------------------------------------------------

    The foundation brakes' hysteresis significantly affects ABS 
design. This hysteresis is characterized by the foundation brake's 
torque output not immediately falling in response to and in 
proportion to a reduction in brake application pressure. This is 
shown in Figure 1 for an air-actuated foundation brake. As is the 
case for service brake control systems, the hysteresis in hydraulic 
foundation brakes is much less than that of air-actuated foundation 
brakes.
    The amount of deceleration that a braking vehicle can attain is 
dependent on three factors: the amount of brake torque that can be 
generated; tire-friction properties; and road surface friction 
characteristics.
    The ability to generate braking torque is primarily dependent 
upon the size of the foundation brake components used (i.e., brake 
drums, linings, and actuating chambers or pistons) and the amount of 
hydraulic or pneumatic pressure delivered to these components. Brake 
system designers size systems to provide sufficient brake torque 
generating capability to lock (or come relatively close to locking) 
the brakes (wheels) on the vehicle (except those on the steering 
axle) when it is loaded with the maximum weight it is designed to 
carry and when operating on all types of road surfaces. It is 
necessary to provide such brake torque generating capability if a 
vehicle is to have adequate stopping distance performance when it is 
fully loaded.
    Most heavy trucks built today can thus generate sufficient brake 
torque to lock (or come relatively close to locking) all their 
wheels (except those on the steering axle) on all road surfaces at 
all loading conditions. If a brake is ``big'' enough to lock a 
wheel, the issue of stopping capability of that wheel then focuses 
on tire properties and not the brake since, in effect, any further 
increase in braking torque cannot be utilized. The limit of tire 
traction in such a case determines the maximum capability of each 
wheel (brake) to contribute to the vehicle's stopping ability.
    For passenger cars, maximum loaded weight includes the empty 
weight of the vehicle, up to as many as six adult passengers, 
assorted luggage or cargo, and a full tank of fuel. For a heavy 
truck, maximum loaded weight includes the empty weight of the 
vehicle, typically one or two passengers, a full load of fuel, and 
the maximum weight of cargo that can be carried in the truck. The 
ratio of loaded to empty weight for passenger cars is generally in 
the range of 1.5 to 1 or less. For heavy vehicles, especially 
combination-unit trucks, this ratio can exceed 3 to 1.
    Standard design practice in the U.S. is to use fixed brake force 
distributions on heavy vehicles (i.e., a brake force distribution 
that does not change with axle load changes). The 
[[Page 13260]] force distribution is established by selecting 
particular ``size'' or torque capacity brakes for each axle. Because 
load distribution is so variable on heavy vehicles, a fixed brake 
balance is a compromise and cannot be expected to provide high 
braking efficiency (i.e., high braking rates without locking wheels) 
under all conditions. Generally speaking, heavy vehicle brakes are 
balanced for the fully loaded, low deceleration stop. This results 
in too much braking at the rear axle(s) when the vehicles are empty.
    Heavy vehicles have a comparatively much greater propensity for 
brake-induced wheel lockup than passenger cars for two reasons. The 
first is the much less than optimum brake force distribution in the 
lightly loaded and empty load conditions, which leads to rear wheel 
lockup under such conditions. The second is the difference in loaded 
to empty weight ratio and the resulting difference in brake sizing. 
Since a heavy vehicle's brakes must be sized for the fully loaded 
condition, such a vehicle tends to be very overbraked when it 
operates lightly loaded or empty or when it operates on a slippery, 
low friction road surface. Under either of these operating 
conditions, and especially when both conditions exist, it is very 
easy for the driver to inadvertently lock some or all of the 
vehicle's wheels, even when making only a moderate or light brake 
application.

3. Tire/Road Friction

    Ultimately, the retarding (braking) forces at the tire/road 
interface, that result from the braking torque that is applied to 
the vehicle's wheels, are transmitted to the road surface at that 
interface. Tire and road surface friction properties that affect 
these forces are significant factors in determining the amount of 
deceleration that the vehicle can achieve. In fact, the forces and 
moments<SUP>5 that the vehicle's tires are capable of generating at 
the tire/road interface are not only the only means by which a 
driver is able to control the velocity of the vehicle (not only 
slowing and stopping the vehicle by applying the brakes, but also 
accelerating the vehicle by actuating the accelerator), but they are 
also the only means by which the driver is able to control the 
direction and path of a vehicle by turning the steering wheel.

    \5\A moment, or the moment of a force, is a torque, and is a 
measure of the tendency of that force acting on an object to produce 
torsion and rotation of that object about an axis.
---------------------------------------------------------------------------

    These forces and moments result when the driver turns the 
steering wheel, applies the brakes and/or actuates the accelerator 
and are reactions to the inertial forces<SUP>6 and moments<SUP>7 
that act on the vehicle. Therefore, in order to understand those 
factors that influence the control and stability (and the loss 
thereof) of a vehicle, it is necessary to understand how tires 
generate those forces and moments.

    \6\Inertial forces are those forces occurring within an object 
that resist the tendency of external forces on the object to 
accelerate the object. They are defined by Newton's Second Law, 
which basically states that an object at rest tends to remain at 
rest and an object in motion tends to remain in motion, and are 
equal to the mass of the object times its rate of acceleration.
    \7\Inertial moments are those moments occurring within an object 
that resist the tendency of external moments on the object to 
accelerate the rotation of the object. They are also defined by 
Newton's Second Law and are equal to the moment of inertia of the 
object times its rate of rotational acceleration.
---------------------------------------------------------------------------

    Tire-road friction is an interaction between the tire and the 
road resulting in reaction forces and moments acting in the plane of 
the road at the tire-road interface. Reaction forces and moments 
result from control inputs (e.g., braking, accelerating, steering) 
and/or external disturbances (e.g., wind, road geometry and 
condition, etc.). The direction and magnitude of the resultant 
reaction forces and moments are determined by these inputs.
    Before discussing these tire-road friction properties, several 
terms need to be defined. In order to understand the conditions 
under which a tire generates forces at the tire-road interface, the 
axis system used to define a tire's operating condition needs to be 
defined.<SUP>8 First, the position of the tire is defined by the 
wheel plane, the road plane, and the center of tire contact. The 
wheel plane is the central plane of the tire, normal (perpendicular) 
to the spin axis, which is the axis of rotation of the wheel (tire). 
The road plane is the plane of the road surface. The center of tire 
contact is the intersection of the wheel plane and the vertical 
projection of the spin axis of the wheel onto the road plane. The 
axis system is then defined as follows:

    \8\The following definitions are based on those which appear in 
``SAE J670e--Vehicle Dynamics Terminology, Society of Automotive 
Engineers, Inc. July 1976. The reader is referred to that document 
for a more complete description of these terms.
---------------------------------------------------------------------------

    1. The origin of the tire axis system is the center of the tire 
contact.
    2. The X' axis is the intersection of the wheel plane and the 
road plane with a positive direction forward. The X' axis defines 
the longitudinal<SUP>9 axis of the tire and is positive in the 
direction in which the tire is pointed.

    \9\Similarly for the vehicle, the vehicle's longitudinal axis, 
direction, is the direction in which the vehicle is pointed.
---------------------------------------------------------------------------

    3. The Z' axis is perpendicular to the road plane with a 
positive direction downward. If the road surface is flat and level, 
the Z' axis is vertical.
    4. The Y' axis is in the road plane, its direction being chosen 
to make the axis system orthogonal and right-handed. The Y' axis 
defines the lateral<SUP>10 axis of the tire and is perpendicular to 
the direction in which the tire is pointed and positive to the right 
when looking in the direction in which the tire is pointed.

    \10\Similarly for the vehicle, the vehicle's lateral axis, 
direction, is perpendicular to the direction in which the vehicle is 
pointed.
---------------------------------------------------------------------------

    With this axis system as a basis, the tire angles which affect 
the forces and moments generated by a tire are defined as follows:
    1. Slip angle is the angle between the X' axis and the direction 
of travel of the center of tire contact. In simple terms, the slip 
angle is the angle between the direction in which the tire is 
pointed and the direction in which the tire is moving.
    2. Inclination (camber) angle is the angle between the Z' axis 
and the wheel plane. In simple terms, the inclination angle is a 
measure of how far the top of the tire is tilted to one side or the 
other when looking in the direction in which the tire is pointed.
    The other important operating condition of a tire is that which 
produces braking and driving forces. This condition, which is 
referred to as longitudinal slip in the SAE terminology, is also 
called percent slip, wheel slip, or simply, slip. Throughout this 
notice, the term wheel slip is used and is defined as: the ratio of 
the longitudinal slip velocity to the spin velocity of the straight 
free-rolling tire, expressed as a percentage, where:
    1. the longitudinal slip velocity is the difference between the 
spin velocity of the driven or braked tire and the spin velocity of 
the straight free-rolling tire, with both spin velocities measured 
at the same linear velocity at the wheel center in the X' direction,
    2. the spin velocity is the angular velocity of the wheel on 
which the tire is mounted, about its spin axis, and
    3. the straight free-rolling tire is a loaded rolling tire 
operated without application of driving or braking torque moving in 
a straight line at zero inclination angle and zero slip angle.
    It should be noted that wheel slip is sometimes expressed as the 
ratio of the difference between the velocity of the wheel center and 
the velocity of a point on the tread of the tire that is not in 
contact with the road to the velocity of the wheel center. Using 
this definition, a free-rolling tire operates at a small amount of 
wheel slip, usually less than 1 or 2 percent, due to the rolling 
resistance of the tire. Throughout the preamble, the definition of 
longitudinal slip given above is used.
    The final terms that need to be defined are those that describe 
the forces and moments generated by the tire. Tire force is the 
external force acting on the tire by the road. Longitudinal force is 
the component of tire force in the X' direction, i.e., in the 
direction which the tire is pointed. Braking force is the negative 
longitudinal force resulting from braking force application. Lateral 
force is the component of tire force in the Y' direction, i.e., 
perpendicular to the direction the tire is pointed. Normal force is 
the component of tire force in the Z' direction. Vertical force is 
the normal reaction of the tire on the road which is equal to the 
negative of the normal force. Braking force coefficient, mu<INF>X, 
is the ratio of the braking force to the vertical load. Lateral 
force coefficient, mu<INF>Y, is the ratio of the lateral force to 
the vertical load.
    With these definitions as a basis, the following discusses the 
forces and moments generated by a tire, how those forces are 
affected by wheel slip, and how those forces influence a vehicle's 
control and stability.
    Tire-road traction properties determine the maximum limits of 
forces and moments which can be developed at the tire-road interface 
at given operating and environmental conditions. They also affect 
tire force and moment slip characteristics, i.e., relationships 
between lateral tire force and slip angle (and camber angle);<SUP>11 
and [[Page 13261]] braking or driving torque and wheel slip. These 
properties have a substantial effect on a vehicle's dynamics and its 
control and stability characteristics.

    \11\Throughout the remainder of this discussion, the effects of 
camber angle are not addressed, and when discussing the operating 
condition of a tire, the camber angle is assumed to be zero.
---------------------------------------------------------------------------

a. Braking (Longitudinal) Friction

    Application of braking torque inputs to a wheel, rolling at zero 
slip and camber angles, results in a longitudinal force acting 
parallel to the wheel plane in a direction opposite to the direction 
of wheel motion. Longitudinal reaction force is modified by the 
rolling resistance of the tire which increases braking force.
    As the braking force at the wheel is increased, slippage will 
occur between the tire and the road surface. To generate slippage, 
the rotational speed of the tire must be less than the speed of the 
wheel center and, therefore, the vehicle. This slippage between the 
tire and road surface is the longitudinal slip defined earlier.
    Longitudinal friction properties of tires have been measured and 
tabulated for numerous combinations of tire/load/road/environmental 
conditions in the form of mu<INF>X-slip curves. (This type of data 
is quite prevalent in the public domain for passenger car tires 
while similar data for truck tires are sparse.)
    The braking force that a tire is capable of developing varies 
with wheel slip in accordance with the typical curve shown in Figure 
2. The shape of the mu<INF>X-slip curve illustrates the classic 
features of longitudinal force generation. The braking or 
longitudinal force is zero when the tire is free rolling, reaches a 
peak at about 10-20 percent slip and then falls off to a somewhat 
lower level when the tire is operating at 100 percent slip, i.e., 
fully locked (sliding).
    The initially steep increase of longitudinal force with 
increasing slip reflects the circumferential elasticity of the 
tire's carcass and tread structure. As the brakes are applied with 
increasing amounts of torque, the elastic capability of the tire in 
the footprint area is exceeded and sliding begins to take place at 
the rear of the footprint. Beyond the elastic region, the force 
output reaches a peak as all of the tread elements traversing the 
contact patch begin to slide relative to the roadway. Beyond peak 
friction, any increase in brake torque causes sliding across the 
entire footprint and the tire rapidly goes into full lockup. In this 
regime, frictional coupling between the tire and road degrades due 
to rubbing speed and heating effects, hence, the characteristically 
negative slope at high slip level.
    The shape of this curve (see Figure 3) is dependent upon the 
tire characteristics and the road surface properties. Typically, the 
peak is relatively high on dry roads but tire force fall-off is 
small. On wet roads, the peak is lower and the fall-off as the wheel 
locks is much greater.
    Another form of hysteresis that affects ABS design is related to 
the braking force versus wheel slip characteristics. As both the 
peak and slide coefficients of friction become lower on more 
slippery road surfaces, the time necessary for a locked (or nearly 
locked) wheel to spin up to near the vehicle's velocity increases. 
This results from the reduced force generating capability of tires 
on low friction road surfaces together with mass of the rotating 
components that include the wheel. On the drive axles of heavy 
vehicles, this mass, which includes the tire, wheel, axle assembly 
and axle differential components, can be great enough to require 
more than one-half second for a locked wheel to spin up to the 
vehicle's speed on very slippery road surfaces such as ice.
    For pneumatic tires, the magnitude of the braking force is 
dependent upon tire construction properties, tread depth, amount of 
loading, wheel speed (velocity), the type and condition of the road 
surface and the amount of slippage between the tire and the roadway. 
With regard to maximum braking capability, the pertinent features of 
the mu<INF>X-slip curve are the peak value of braking force 
coefficient, the peak coefficient of friction, and the slide value 
under the locked-wheel condition at 100 percent slip, the sliding 
coefficient of friction.
    In the preamble of this notice, the terms skid number and peak 
friction coefficient (PFC) are used. These terms represent the 
results of a test to determine the longitudinal friction 
characteristics of a road surface using a specific test procedure, 
the American Society for Testing and Materials (ASTM) Method E1337-
90 procedure, a specific tire, the ASTM E1136 SRTT tire, and a 
specific test device, an ASTM traction trailer. Skid number is the 
result of the ASTM test which characterizes the slide value of the 
friction coefficient between the ASTM tire and the road surface 
being measured. The peak friction coefficient, PFC, is the result of 
the ASTM test which characterizes the peak value of the friction 
coefficient between the ASTM tire and the road surface being 
measured.
    The friction force potential of truck tires is significantly 
less than that for car tires. The difference is due primarily to the 
rubber compounding used to achieve the high tread life typically 
achieved with truck tires and the higher pressures in the tires that 
result in higher footprint loading. The braking performance of any 
vehicle is ultimately limited by its tire properties. Thus, given 
current truck tire properties, heavy vehicles cannot perform as well 
as passenger cars in braking situations even if they have braking 
systems that are 100 percent efficient (i.e., a braking system that 
would utilize all of the available tire/road friction).

b. Cornering (Lateral) Friction

    In addition to braking forces, tires must also generate 
lateral--or cornering--forces to direct the vehicle in accordance 
with steering inputs from the driver or in response to other lateral 
forces such as crosswinds.
    Tire friction characteristics in cornering are described by the 
relationship between lateral force coefficient and slip angle.
    The lateral force that an unbraked tire is capable of developing 
varies with slip angle in accordance with the typical curve shown 
for the free rolling tire in Figure 4. The single most important 
feature of the force generating capability of a tire, as it relates 
to vehicle control and stability, is the ability of a rolling tire 
to generate forces perpendicular to the tire's direction of travel.

c. Combined Braking/Cornering Friction

    When braking a vehicle, it is necessary to generate both braking 
and cornering forces at the wheels if the vehicle is to be stopped 
without deviating from its intended path. The situation is identical 
when a driver must brake severely while negotiating a curve or lane 
change where cornering forces are required to keep the vehicle from 
sliding towards the outside of the turn while the braking forces 
decelerate the vehicle.
    In braking-in-a-curve maneuvers, tire friction properties are 
determined primarily by the peak and slide values of the resultant 
braking-cornering coefficients. Figure 4 shows the lateral force 
coefficient versus slip angle relationships for a free rolling tire 
and for a braked tire at different amounts of wheel slip, including 
100 percent (locked wheel condition). All of the curves converge at 
a slip angle of 90 deg. as expected, since the tire is perpendicular 
to the direction of travel.
    At small slip angles, the lateral force capability under locked 
wheel conditions is much lower than that of a free-rolling wheel. It 
should be noted that although this figure shows that the tire is 
capable of generating lateral force in the locked wheel, 100 percent 
wheel slip condition, this force is ``lateral'' in relation to the 
tire itself. In this situation, the only force generated by the tire 
is opposite to its direction of travel, and its ``lateral'' 
component results from the tire's being steered away from its 
direction of travel. This locked wheel, ``lateral'' force is 
basically equal to the sliding coefficient of friction of the tire 
times the vertical load on the tire times the sine of the slip 
angle.
    Lateral is a relative term whose meaning depends upon the object 
or direction to which it relates, i.e., lateral in relation to the 
vehicle is not the same as lateral in relation to a tire steered 
relative to the vehicle, and is also not the same as lateral with 
respect to the vehicle's direction of travel.<SUP>12 Earlier in this 
notice and in the previous notices related to this Final Rule, the 
phrase lateral stability has been used to describe whether or not a 
vehicle can resist yawing or spinning in response to some external 
lateral force acting on the vehicle. As long as the vehicle's 
direction of travel is the same as or very close to the direction in 
which the vehicle is pointed no significant confusion results. 
However, once a vehicle has begun to yaw or spin and its direction 
of travel is significantly different than the direction in which the 
vehicle is pointed, confusion can result regarding the meaning of 
lateral stability and lateral tire forces. To eliminate any 
confusion, the term directional stability (or directional stability 
and control) will be used throughout the remainder of this notice in 
place of lateral stability (or lateral stability and steering 
control).

    \12\To eliminate confusion regarding the meaning of lateral, 
several technical terms are defined that will be used throughout the 
remainder of this notice.
---------------------------------------------------------------------------

    With respect to the tire forces related to a vehicle's 
directional stability and control, the phrase, ``stabilizing tire 
forces'' will be used to describe tire forces that act perpendicular 
to the vehicle's direction of travel, instead of [[Page 13262]] the 
phrase ``lateral tire forces'' the meaning of which can be unclear 
relative to the vehicle's direction of travel. As indicated earlier, 
a tire's ability to generate such ``stabilizing tire forces'' is the 
single most important feature of the force generating capability of 
a tire, as it relates to vehicle directional control and stability.
    The graph in Figure 4 can be used to illustrate how tire 
traction characteristics influence vehicle directional stability. 
For example, if a single-unit vehicle negotiates a cornering 
maneuver with the front wheels at 4 deg. slip angle and the rear 
wheel at 3 deg. slip angle, and the application of braking pressure 
results in 20 percent slip at the front tires while the rear tires 
become locked, the data indicate that the lateral force coefficient 
at the front would decrease from 0.55 to 0.25 while the 
corresponding decrease at the rear would be from 0.45 to 0.03. In 
this case, the lateral force capability at the front would be eight 
times greater than at the rear. Because of the greatly reduced 
stabilizing forces on the rear tires, they would no longer be 
capable of resisting the vehicle yaw induced by the forces on the 
front tires, and the vehicle would spin out.
    Tire loading also affects the amount of slip which occurs at the 
various wheels on a vehicle. For example, weight is transferred from 
the inside to the outside wheels of a vehicle when it is driven 
around a corner. Therefore, the wheels on the inside of the vehicle 
will operate at a lighter tire load and hence, when generating the 
same braking force, will operate at a higher percentage of wheel 
slip than their counterparts on the outside of the vehicle. In 
tractor-trailer combinations, improper load distribution can produce 
unequal axle loadings between the tractor and trailer. If both the 
tractor and trailer brakes are applied equally, increased wheel slip 
will occur at the wheels which are carrying the lightest load. If 
the improper load distribution is severe enough, wheel lockup and 
skidding can occur at otherwise normally acceptable deceleration 
rates.

4. Vehicle Loss of Control

    Heavy vehicles are likely to experience wheel lockups in maximum 
braking situations because of the friction properties of their tires 
and the less than optimal force distributions of their brake 
systems. Lockup of all of the wheels on one or more of a vehicle's 
axles, if not responded to by the driver, will usually result in 
either a loss of steering control or loss of the vehicle's 
directional stability.

a. Single-Unit Trucks

    A single-unit truck behaves much like a passenger car when wheel 
lockup occurs. Figure 5 shows a simple single unit vehicle (car or 
truck) with only its front wheels locked. Such a vehicle, with only 
the front wheels locked and the rear wheels rolling, will experience 
a loss of steering control. The vehicle cannot be steered, but it is 
stable due to the stabilizing forces provided by the rolling rear 
wheels and does not tend to yaw or spin out.
    Figure 6 shows a simple single unit vehicle (car or truck) with 
only its rear wheels locked. In this case, the vehicle will 
experience a loss of directional stability. It is very unstable and 
the slightest side force disturbance (i.e., lateral force due to 
steering, side slope or road crown, crosswind, unequal front axle 
braking, etc.) results in the vehicle yawing significantly or 
spinning out.
    If all wheels are locked, the vehicle cannot be steered but is 
not as likely to spin.

b. Combination-Unit Vehicles

    With combination-unit vehicles, the effect of wheel lockup is 
more complex but can easily be inferred from the simple single-unit 
vehicle case by treating each vehicle in the combination as a 
single-unit vehicle.
    If the wheels on the steering axle lock, the vehicle, 
experiencing a loss of steering control, will travel essentially in 
a straight path, stable but unsteerable, as illustrated in Figure 7. 
Usually a driver immediately senses this condition and, if 
conditions permit, can modulate the brakes to allow the steering 
axle wheels to spin up and regain steerability.
    If the trailer wheels lock, the trailer will experience a loss 
of directional stability and (if side force disturbances are 
present) will swing to the outside of the vehicle path, as shown in 
Figure 8. However, because trailer wheelbases are long in comparison 
to the tractor, this unstable yawing response is slower. Thus, a 
driver again, if conditions permit and if the driver is aware of the 
condition soon enough, may have time to modulate the brakes to spin 
up the trailer wheels and bring the trailer back in line. As a 
trailer becomes shorter, this possibility of correction becomes less 
likely.
    If the tractor's drive axle wheels lock, the truck tractor will 
experience a loss of directional stability and the combination 
vehicle will begin to jackknife if a side force disturbance exists, 
as shown in Figure 9. When this occurs, the process usually becomes 
irreversible as the driver is unable to react fast enough to prevent 
total loss of vehicle control, particularly when the tractor has a 
short wheelbase. This instability condition is the one which a 
driver is least likely to be able to control.
    As more units (and more articulation points) are added to the 
combination, the situation becomes more complex and the modes of 
instability increase in number.

5. The Need for Antilock

    As mentioned earlier, the only means by which a driver is able 
to control the direction, velocity, and path of a vehicle is to 
apply steering, braking, and/or accelerator inputs to the vehicle 
which in turn result in forces and moments being generated by the 
vehicle's tires. A tire can only generate a limited amount of 
frictional force. As the tire is required to generate more force for 
braking, its capability to generate stabilizing force is reduced. 
Since the capability of a tire to generate both braking 
(longitudinal) and stabilizing (lateral) forces is determined by the 
amount of wheel slip at which the tire is operating, controlling 
wheel slip is the only means by which it is possible to have a tire 
generate a significant amount of longitudinal force to decelerate a 
vehicle while still maintaining the capability to also produce 
sufficient amounts of stabilizing force to steer the vehicle and to 
retain directional stability.
    As illustrated earlier, when the wheel slip goes beyond the 
point at which maximum (peak) braking force occurs, the tire's 
stabilizing force capability drops dramatically, leading to a 
situation that can result in loss of vehicle control. By sensing and 
controlling wheel slip, an antilock system automatically reduces the 
amount of brake application pressure to prevent prolonged, excessive 
wheel slip which would compromise the vehicle's directional 
stability by reducing the stabilizing force capabilities of the 
vehicle's tires. An antilock system which operates in such a manner 
is referred to as a closed-loop system. The basic closed-loop 
control algorithm for an ABS is as follows:
    1. The driver actuates the brake pedal (or treadle valve) 
resulting in an application of brake pressure to the vehicle's 
foundation brakes,
    2. this generates brake torque at the vehicle's wheels that 
creates braking forces at the tire/road interface,
    3. this results in wheel slip (as discussed above), the level of 
which is determined by the ABS by sensing the rotational speed of 
the vehicle's wheels,
    4. if the amount of wheel slip is not within an ``acceptable'' 
range (which is determined by the ECU, based on a predetermined set 
of logic) the brake application pressure is adjusted to return the 
level of wheel slip to the acceptable range; i.e., if the level of 
wheel slip is excessive, the brake application pressure is reduced 
and if the level of wheel slip is too low, the brake application 
pressure is increased, but never to a level higher than that which 
results from the driver's actuation of the brake pedal (or treadle 
valve).
    Vehicles equipped with ABS, operating in such a manner, usually 
have shorter stopping distances compared to the same vehicle without 
ABS, particularly on low mu surfaces.\13\ An antilock system which 
controls the wheel slip at the level that results in the maximum 
amount of braking force at the tire/road interface maximizes a 
vehicle's stopping capability and also provides some directional 
stability enhancement. On the other hand, antilock systems which 
control wheel slip at levels below that which results in peak 
braking force generation will result in a greater degree of 
directional stability but provide lower levels of braking force 
resulting in longer stopping distances.

    \13\A low mu surface is one that is relatively slippery and thus 
provides lower levels of braking force and poorer directional 
stability and control during braking. These surfaces, which are 
typical on wet roads, are also referred to as low coefficient of 
friction surfaces.
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6. General Antilock System Operation

    The following discussion addresses three different aspects of 
ABS operation. The first aspect discussed is the control strategies 
used by an ABS to monitor wheel rotational speed and adjust brake 
application pressure to control wheel slip at an individual wheel. 
The second relates to the various component configurations that are 
used to control the wheels on an axle or a tandem axle set. The 
third is the control strategies used to control the wheels on an 
axle or tandem axle set. [[Page 13263]] 

a. ABS Wheel Slip Control Strategies

    The goal of an antilock system is to prevent wheel slip on the 
controlled wheels from exceeding that which provides a good 
compromise between providing near maximum levels of braking force 
and providing sufficient levels of stabilizing forces to assure that 
the vehicle will remain directionally stable without reducing the 
wheel slip below that which produces braking force which utilizes 
most of the friction (adhesion) that is available at the tire/road 
interface. Once wheel slip goes beyond the point which provides peak 
braking traction, both braking and cornering traction are reduced, 
as shown in Figure 10 for a truck tire cornering at an 8 deg. slip 
angle.
    Early mechanical antilock systems controlled slip by the use of 
an assembly at the wheel which contained an inertia disc that 
rotated freely with the wheel when brakes were not applied. Braking 
the wheel caused it to decelerate while the inertia disc tried to 
continue to rotate at the original speed, but was restrained by a 
triggering mechanism. This triggering mechanism controlled an air 
valve (modulator valve) which when activated, shut off air pressure 
to the foundation brake air chambers and exhausted pressure already 
in the chambers. When deceleration of the wheel exceeded about 
``1g,'' the inertia of the disc generated enough force to trip the 
mechanism activating the modulator valve. As braking force decreased 
and the wheel speeded up, the force exerted by the inertia disc 
decreased, allowing the trip mechanism to deactivate the modulator 
valve, thus, reapplying the brakes.
    Electronic antilock systems act in a manner similar to the early 
mechanical systems except they are more sophisticated as a result of 
their computational capability. With electronic systems, the 
mechanical wheel assembly is replaced by a wheel speed sensor and an 
electronic control module (ECU). Wheel speed sensors, which are 
located at the wheels or within the axle housings, constantly 
monitor wheel speed (or a component whose speed is proportional to 
the wheel speed) sending electrical signals to the ECU which are 
proportional to the wheel speed. The ECU determines wheel speed and 
changes in wheel speed (acceleration and deceleration) based on 
these signals.
    The following discusses two basic control modes\14\ used by 
electronic ABSs to control brake applications at a wheel in response 
to wheel speed sensor signals.

    \14\The following discussion, which is largely based on the 
previously referenced Leasure and Williams SAE, paper specifically 
addresses ABS control modes for air brake systems. Similar control 
strategies are used in hydraulic ABSs with the specific parameters 
of the control modes differing due to differences in the brake 
torque versus brake pressure application characteristics of air and 
hydraulic brake systems.
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    In the acceleration/deceleration threshold mode of operation, 
the ECU recognizes the rapid wheel deceleration that occurs as wheel 
slip exceeds the peak friction wheel slip (Figure 11), and 
electrically commands the modulator valve to reduce brake 
application pressure and, thus, brake torque. When brake torque 
decreases enough to cause braking force to be less than the friction 
force at the tire/road interface, the wheel stops decelerating and 
begins to accelerate. The rate of acceleration increases with the 
increasing friction associated with a reduction in wheel slip. When 
wheel slip falls to the level corresponding to peak braking force, 
the acceleration rate peaks and starts to decrease with wheel slip. 
The ECU senses this change in acceleration rate and commands the 
modulator valve to start increasing brake application pressure and 
the cycle repeats.
    In the reference speed mode of operation, the ECU tracks wheel 
speed information which it uses to estimate vehicle speed. The 
antilock system uses this estimated speed to compute a ``reference 
speed'' which is less than the estimated speed by a preprogrammed 
factor. The reference speed is updated throughout a stop as 
illustrated in Figure 12. This figure also illustrates how the ECU 
in one manufacturer's 1970's system uses this reference speed as a 
cue to modulate the brake application pressure. When the brakes are 
applied as shown in the figure, the wheel starts decelerating. As 
wheel speed falls below the reference speed (point ``G-1''), the 
antilock system acts to reduce brake pressure. After brake pressure 
has been reduced long enough to allow the wheel speed to roll up to 
that of the reference speed (point ``G-2''), the antilock system 
acts to increase brake pressure. This cycle continues until the 
vehicle is stopped.
    Today's antilock systems usually combine acceleration/
deceleration threshold logic and speed reference logic in some 
fashion. Both are believed necessary to improve the efficiency of 
antilock systems to account for the variance of tractor performance 
with surface (Figure 13), slip angle (Figure 14, vehicle speed 
(Figure 15), etc.
    If an antilock system waits for the threshold deceleration 
associated with peak braking friction under some conditions, the 
ability of the wheel to provide cornering friction will have been 
compromised severely. Therefore, a threshold reference speed needs 
to be established around 30 percent to prevent excessive wheel slip.
    Figure 16 shows a typical control cycle for one manufacturer's 
antilock system which uses a ``hold'' pressure phase, as well as a 
release pressure phase. This ECU uses two wheel slip thresholds (K1 
and K2) and two deceleration thresholds (-b and b) in making 
decisions regarding control of the modulator. The ECU tracks the 
information from all of the vehicle's wheel speed sensors (even when 
the brakes are not applied) and uses this information to compute a 
reference speed which it continually updates. In the panic stop in 
Figure 16, the wheel decelerates until the wheel speed sensors 
indicate a deceleration which the vehicle cannot physically attain 
(point 1). At this point, the reference speed, which until this 
instant has corresponded to the wheel speed, now separates from the 
wheel speed and decreases according to an empirically determined 
rate of deceleration.
    At point 2, the deceleration threshold -b is reached and the 
wheel runs into the unstable range of the traction curve. The wheel 
has exceeded the maximum braking force and any further increase in 
braking torque only increases wheel deceleration. Brake pressure is, 
therefore, quickly reduced and wheel deceleration falls after a 
short time. This deceleration time is determined by the hysteresis 
time lag between the time the modulator valve actuates to release 
the air pressure to the time that the air pressure in the air brake 
chamber, the hysteresis of the foundation brakes and the hysteresis 
related to the time needed for the wheel (and its associated 
rotating components) to spin up after it has been locked. Only after 
this delay does a further pressure fall also lead to reduction of 
wheel deceleration.
    The deceleration signal -b is traversed at point 3 and brake 
pressure is held constant for a fixed time T1. Normally wheel 
acceleration will rise above the threshold +b at a point 4 within 
this holding time T1.
    Provided this happens, brake pressure will continue to be held 
constant. (Were the +b signal not produced within the time T1, as 
with very low friction surfaces, then brake pressure would be again 
reduced in response to the slip signal. The time constant, T1, is 
determined for each vehicle/brake system based on the influences of 
the various kinds of hysteresis previously discussed).
    During the constant pressure phase, the wheel accelerates in the 
stable slip range, the +b signal being traversed again at point 5 at 
which time utilized adhesion is just below the maximum on the 
traction curve. The +b threshold is used this time to signal a rapid 
pressure increase over time T2 to overcome brake hysteresis.
    The time T2 is preprogrammed for the first control cycle and 
then recalculated for each subsequent control operation depending 
upon the response of the wheels. After this rapid pressure increase 
stage, brake pressure is raised again but at a lesser gradient by 
alternate pressure increase and hold pulses.
    As a rule, the deceleration threshold -b is again reached during 
the pulsing phase at point 9, and brake pressure falls. The 
procedure repeats itself as long as the brake pedal is depressed too 
forcefully for the existing road conditions or until the vehicle 
speed drops below a specified value.
    The logic presented here in principle is not fixed, but, matched 
by microcomputers to the dynamic response of the wheel under 
differing adhesion conditions. Not only are ABSs capable of 
``adapting'' to various conditions by employing complex algorithms 
to control wheel slip, but they are also able to ``adapt'' the 
parameters of those algorithms, as with the T2 parameter discussed 
above, to improve the system's ability to control wheel slip over 
the broad range of road surface and vehicle load conditions under 
which heavy vehicles operate. One obvious result of this 
adaptability is the range of ABS cycle times, or controlling 
frequencies, that result when controlling wheel slip under various 
road surface and vehicle load conditions.
    Figures 17 and 18 illustrate the effects of two very different 
situations of load and road surface conditions on the ABS cycle 
times and how an air brake ABS adapts its control of wheel slip. 
Figure 17 shows treadle valve pressure, and brake chamber pressure, 
wheel speed and ABS modulator solenoid activity [[Page 13264]] for 
the left wheel of the intermediate drive axle for a full treadle 
application stop of a Freightliner 6 x 4 conventional truck tractor 
with a WABCO 6S/6M ABS in a lightly loaded condition on a very low 
friction surface, ice. The figure shows the first five ABS cycles 
for that stop. To characterize ABS cycle time, the ABS cycle is 
assumed to begin when brake pressure begins to rise in the brake 
chamber and that rising brake chamber pressure leads to excessive 
wheel slip or wheel lockup. This excessive wheel slip is sensed by 
the ECU which actuates the modulator valve to decrease brake chamber 
pressure to reduce wheel slip to an acceptable level. This 
``initial'' rise in brake chamber pressure can result from an 
increase in the driver's level of brake application, i.e., rising 
treadle valve pressure, or from an increase resulting from the ECU 
signaling the modulator valve to increase brake chamber pressure. 
The ABS cycle ends when, after the reduction in brake chamber 
pressure resulting from actuation of the modulator valve, the brake 
chamber pressure begins to again rise in response to the ECU 
signaling the modulator valve to increase brake chamber pressure. 
For the five ``ABS cycles'' shown in Figure 17-a, the ABS cycle 
times range from 0.72 seconds to 0.80 seconds, i.e., an ABS 
controlling frequency of from about 1.2 to 1.4 cycles per second.
    Two things shown in Figure 17-b are of note. The first is the 
time required for the wheel to lock after the initial brake 
application which is very short, about 0.04 seconds. The second is 
the time required for the wheel's speed to increase to that of the 
vehicle after the wheel has locked, i.e., the wheel's spin up time. 
The spin up times shown in Figure 17-b range from 0.20 seconds for 
the fourth ABS cycle (wheel spin up begins at about 3 seconds on the 
time scale) to 0.34 seconds for the first ABS cycle (wheel spin up 
begins at about 0.5 seconds on the time scale). The rate of wheel 
spin up can be characterized by the acceleration of the outer 
surface of the tire, i.e., the tread of the tire, relative to the 
wheel center. In the case of the wheel spin up during the first ABS 
cycle, the spin up time is 0.34 seconds and the change in wheel 
speed over that time is 11.3 mph; the wheel's acceleration is 
therefore 33.2 mph per second or 48.8 feet per second per second.
    In contrast to the ABS cycle time and wheel spin up rates shown 
in Figure 17, Figure 18 illustrates a situation where the ABS cycle 
times are much shorter and the wheel spin up rates are much faster. 
Figure 18 shows treadle valve pressure, and brake chamber pressure, 
wheel speed and ABS modulator solenoid activity for the left wheels 
of the tandem drive axles for a full treadle application stop of a 
Volvo-GM 6 x 4 conventional truck tractor with a Bosch 6S/4M ABS in 
a lightly loaded or bobtail condition on what is believed to be a 
high friction surface. The reason for the uncertainty of the 
conditions under which this stop took place is that this data 
resulted from the monitoring and recording of ABS event occurrences 
during the agency's truck tractor fleet study and no details are 
available regarding the exact circumstances of this stop. However, 
given the high average deceleration rate of this stop, more than 16 
feet per second per second which if sustained during a stop from 60 
mph would result in a stopping distance of less than 240 feet, it is 
reasonable to assume that the surface had a rather high coefficient 
of friction. Given this and the low level of brake chamber pressure 
at which excessive wheel slip occurs, between 15 and 30 psi, it is 
reasonable to assume that the vehicle was lightly loaded and may 
even have been a bobtail situation.
    It should be noted that the various data traces shown in Figure 
18 are rough ``stairsteps'' during the first second of data. The 
reason for this is that the data monitoring/recording equipment used 
in the truck tractor fleet study used a data sampling rate of 10 
samples per second while monitoring ABS activity. When an ``ABS 
braking event'' was detected the equipment began to use a data 
sampling rate of 50 samples per second. The equipment then stored 
the data for the one second prior to the ``ABS braking event'' at a 
10 sample per second rate and for the entire ``event'' at a 50 
sample per second rate.
    With regard to the ABS controlling frequency shown in Figure 18, 
unlike the situation shown in Figure 17, the ABS cycles are not 
discrete cycles where the wheel goes to complete lockup and then the 
brake application pressure is reduced to zero. To estimate the ABS 
controlling frequency in this situation, an ABS cycle is 
characterized by a decrease in brake chamber pressure followed by an 
increase in brake chamber pressure where these pressures are less 
than the treadle valve pressure so as to be sure that the brake 
chamber pressure is being controlled by the ABS. Using this 
criteria, Figure 18-a shows that between two and three seconds on 
the time scale the brake chamber pressure goes through about 9 such 
``cycles'', i.e., an ABS controlling frequency of about 9 cycles per 
second. This is more than 6 times faster than the fastest ABS 
controlling frequency shown in Figure 17-a for the stop on an ice 
surface.
    With regard to the wheel spin up time for the stop shown in 
Figure 18-b, just after time equals 3 seconds, there is a large 
decrease in wheel speed for left rear drive wheel followed by a 
steep increase in speed of that wheel. This wheel speed increase is 
7.3 mph and occurs over 0.06 seconds, i.e., a wheel acceleration of 
121.7 mph per second or 178.4 feet per second per second. This is 
more than 3.5 times higher than the wheel acceleration rate for the 
``ice'' stop shown in Figure 17-b. Since, as indicated earlier, 
hydraulic brake systems generally have much lower levels of 
hysteresis than air brake systems, everything tends to happen faster 
in hydraulic brake systems and, as such, the controlling frequency 
for hydraulic brake ABS can be significantly higher. The logic used 
in different systems also varies with the control strategy utilized 
and the number of wheel speed sensors.
    A difficult task for air brake antilock systems, with regard to 
controlling slip, is the prevention of wheels going into ``deep 
cycles'' (wheel slips in the high wheel slip part of the friction 
curve where both braking and cornering friction are reduced). Deep 
cycles are particularly undesirable in the first cycle of an 
antilock system operation where the demand for cornering friction 
can be the highest because of the speed of the vehicle. The extent 
to which an antilock system goes into a deep cycle depends on how 
effectively the modulator controls air into and out of the air 
chambers. Figure 19 shows how a 1970's antilock system was not able 
to reduce the air pressure fast enough in a panic application to 
prevent some wheel lockup. The electronic antilock systems of today, 
because of the versatility of digital technology (and compatible 
pneumatic valving) have an expanded control range that provides for 
better air pressure control to respond to conditions and to prevent 
overpressurizing air chambers. This makes possible the reductions in 
deep cycling shown in Figure 20.
    The hysteresis of foundation brakes can have significant effect 
on the ability of an antilock system to prevent ``deep cycles.'' 
Although an antilock system may quickly detect impending wheel lock 
and rapidly actuate the modulator valve to reduce the air pressure 
in the air chambers, the three types of brake system hysteresis 
discussed earlier may prevent an immediate reduction in brake torque 
and rapid spin up of the wheel causing deeper wheel cycles than 
desired. Figure 19 shows an example of how the inherent hystereses 
of the pneumatic components and foundation brakes of air brake 
systems, and the hysteresis related to wheel spin up times affect 
how quickly an ABS can respond to and control wheel slip. The effect 
of the pneumatic hysteresis can easily be seen in the release of 
chamber pressure portions of the ABS cycles. It takes from 0.08 to 
0.22 seconds for the chamber pressure to decrease to 3 pounds per 
square inch, the chamber pressure at which wheel spin up begins for 
several of the ABS cycles. The effect of foundation brake hysteresis 
can not be estimated without data on the brake torque acting on the 
wheel. However, it may not be significant since this type of 
hysteresis is most significant at high brake chamber pressures. As 
shown in Figure 17, the hysteresis time lags related to wheel spin 
up range from 0.20 to 0.34 seconds. The ABS cycle times of up to 
0.80 seconds shown in Figure 17, are the result of these properties 
of the foundation brakes and tires used on heavy vehicles today.
    The inherent hystereses of the pneumatic components and 
foundation brakes of air brake systems and in the tire spin up rates 
of heavy vehicle wheel/tire assemblies have to be considered in the 
design of antilock systems. It also has to be recognized that 
different brake types/configurations can have different amounts of 
hysteresis. An antilock system which works efficiently with one type 
of brake may not work as efficiently with another type of brake.

b. ABS Single and Tandem Axle Component Configurations

    Several types of ABS configurations are currently available for 
heavy vehicles. In order of decreasing complexity and cost, the 
systems for tractors include those with: (1) individual control of 
the wheels on an axle; (2) side-to-side control of the wheels on a 
tandem axle set; (3) axle-by-axle control of [[Page 13265]] the 
wheels on a tandem axle set; and (4) tandem control of all of the 
wheels on a tandem set. With individual wheel control, the most 
complicated and costly type of ABS, each of the wheels on an axle is 
individually monitored and controlled using wheel-speed sensors and 
modulator control valves for each wheel. This prevents lockup at 
each wheel and thus provides optimum stability and control, 
especially on a split mu surface.\15\ With side-to-side control,\16\ 
all of the wheels on one side of a tandem axle set are controlled 
together by one modulator in response to wheel speed sensor signals 
from one or more of those wheels. With axle-by-axle (or simply, 
axle) control, the wheels on an axle (either on a single axle or on 
each axle of a tandem axle set) are controlled together by one 
modulator in response to wheel speed signals from the wheels on that 
axle. With tandem control,\17\ all four (or in some cases, six) 
wheels on a tandem (or tridem) axle set are controlled together by 
one modulator in response to wheel speed signals from the wheels on 
one or more of the axles in the tandem (or tridem) axle set.

    \15\With a split mu surface, the road is divided along its 
length so that the wheels on one side of the vehicle are on a high 
friction surface and the wheels on the other side are on a low 
friction surface. One example of a split mu surface is when one 
portion of a lane is dry and another part is covered with ice.
    \16\Side-by-side control ABS can have two different wheel speed 
sensor configurations. Either all of the wheels on the tandem axle 
set have their own wheel speed sensors, or only the wheels on one 
axle of the tandem axle set have wheel speed sensors.
    \17\Tandem control ABS can have two different wheel speed sensor 
configurations. Either all of the wheels on the tandem axle set have 
their own wheel speed sensors, or only the wheels on one axle of the 
tandem axle set have wheel speed sensors.
---------------------------------------------------------------------------

    ABS technology has improved dramatically in recent years given 
the use of computerized components. Unlike the antilock brake 
systems in the 1970s that primarily relied on an analog control 
technology, current generation antilock systems use advanced digital 
control technology that enhances the systems' efficiency. Digital 
logic permits the use of more complex and sophisticated control 
strategies and reduces the time lags in the antilock computer. More 
generally, digital technology applied to motor vehicles has been 
significantly refined in the last twenty years to control motor 
vehicle fuel systems so that vehicles can comply with fuel 
efficiency and pollution prevention regulations.

c. ABS Single and Tandem Axle Control Strategies

    As discussed above, there are several different component 
configurations used to equip an axle or axles with ABS.
    For each of the configurations for which more than one wheel is 
controlled by one modulator, different wheel slip control strategies 
can be used by the ABS to control wheel slip of those wheels. These 
are select low regulation (SLR), select high regulation (SHR), and 
modified select high regulation (MSHR), also called ``Select Smart'' 
(Bendix) or select low high regulation (SLHR).
    The select low regulation strategy modulates the brake pressure 
application at both wheels of an axle at the same level based on the 
wheel speed signals from the wheel that experiences the higher level 
of wheel slip. On split mu surfaces, this control strategy results 
in near peak braking force on the wheel that experiences the higher 
level of wheel slip (the wheel that is on the lower friction side of 
the road) and less than peak braking force on the wheel with the 
lower level of wheel slip (the wheel on the higher friction side of 
the road). One wheel operating at a lower level of wheel slip and on 
a surface with a higher friction level means that wheel has a 
greater capability to provide additional stabilizing force; 
therefore, providing a higher level of directional stability and 
control for the vehicle. However, on split mu surfaces with the 
coefficient of friction on one side of the road surface being very 
different than on the other side, this can result in extended 
stopping distances since the wheel on the high coefficient of 
friction side is providing much less than the maximum level of 
braking force than can be provided by that surface.
    The select high regulation strategy modulates the brake pressure 
application at both wheels of an axle at the same level based on the 
wheel speed signals from the wheel that experiences the lower level 
of wheel slip. On split mu surfaces, this control strategy results 
in lockup of the wheel that experiences the higher level of wheel 
slip, which results in that wheel providing less than peak braking 
force and near peak braking force on the wheel with the lower level 
of wheel slip. One wheel operating at a locked wheel condition means 
that wheel has essentially no capability to provide any stabilizing 
force, and the other wheel operating at a higher level of braking 
force (near the maximum available on the high friction side of the 
road) means that wheel would have a reduced capability to provide 
stabilizing force. This results in a reduced level of directional 
stability and control for the vehicle compared to the SLR strategy. 
However, on split mu surfaces with the coefficient of friction on 
one side of the road surface being very different than on the other 
side, SHR results in shorter stopping distances compared to the SLR 
strategy since the wheel on the high coefficient of friction side is 
providing near peak braking force.
    The modified select high regulation strategy combines the SLR 
and SHR control strategies. At the beginning of a stop which results 
in excessive wheel slip at one wheel, the ABS controls wheel slip 
using the SLR strategy. While doing so, the ECU monitors the level 
of wheel slip on the wheel which has the lower level of wheel slip 
(the wheel on the high friction side of the road), and from that 
information, the ECU estimates the ratio of the coefficient of 
friction on the high friction side of the road to that on the low 
friction side. If this ratio exceeds a preset threshold and the 
vehicle speed is above a preset threshold, the ECU increases the 
brake application pressure to the wheels which increases the braking 
force provided by the wheel with the lower level of wheel slip (the 
wheel on the high friction side of the road) and which locks the 
wheel with the higher level of wheel slip (the wheel on the low 
friction side of the road). The ECU then begins to control wheel 
slip using the SHR strategy which results in a higher level of 
vehicle deceleration (shorter stopping distance) than would result 
from the use of the SLR strategy. However, as noted above, this 
results in a reduced capability of the both wheels to provide 
stabilizing forces, therefore reducing the vehicle's overall level 
of directional control and stability. Another feature of the MSHR 
strategy is that even when the vehicle velocity and ratio of 
coefficients of friction of the split mu surface thresholds are 
exceeded, the ECU does not immediately switch to the SHR control 
strategy to reduce the risk that the driver will be surprised by an 
unexpected steering wheel ``pull'' that can result in that control 
mode. Instead, the time period over which the system transitions 
from SLR to SHR control is adjusted based on the vehicle's velocity.
    For the individual wheel control configuration in which each 
wheel is controlled by its own modulator, there are two wheel slip 
control strategies: independent regulation (IR) and modified 
independent regulation (MIR). As its name implies, the independent 
regulation control strategy controls the wheel slip of each wheel on 
the axle independently, allowing each wheel's ABS to modulate the 
brake application pressure to each wheel in response to the signals 
from the wheel speed sensor at that wheel to maximize braking forces 
while maintaining sufficient capability to produce stabilizing 
forces to ensure vehicle directional stability. Although this 
control strategy is the most effective at both minimizing stopping 
distance as well as ensuring vehicle stability, when used on the 
steering axle of trucks, truck tractors and buses, this can lead to 
significant steering wheel ``pull'' on split mu surfaces which can 
be difficult for the driver to control. Therefore, ABS manufacturers 
have developed the MIR control strategy in which the wheel slip is 
controlled using the SLR strategy at the beginning of the stop. This 
results in equal braking forces at each wheel which alleviates 
steering wheel ``pull'' that would occur on a split mu surface with 
IR control of the steering axle brakes. After a short period of 
time, the ECU smoothly transitions to true IR control so that the 
buildup of any steering wheel pull is gradual so that it can easily 
be controlled by the driver. NHTSA understands that MIR control 
strategy is used exclusively by all vehicle manufacturers on 
vehicles which have independent sensor/modulator ABS on the steering 
axle. It should be noted that the SLR strategy also eliminates the 
problem of steering wheel ``pull'' on split mu surfaces, but as 
indicated above does not provide as effective use of the friction 
available on the high friction side of such surfaces, resulting in 
longer stopping distances.

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[FR Doc. 95-5410 Filed 3-7-95; 8:45 am]
BILLING CODE 4910-59-C