[Federal Register: July 27, 2009 (Volume 74, Number 142)]
[Rules and Regulations]
[Page 37121-37158]
[DOCID:fr27jy09-10]

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Part III

Department of Transportation

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National Highway Traffic Safety Administration

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49 CFR Part 571

Federal Motor Vehicle Safety Standards; Air Brake Systems; Final Rule

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DEPARTMENT OF TRANSPORTATION

National Highway Traffic Safety Administration

49 CFR Part 571

[Docket No. NHTSA-2009-0083]
RIN 2127-AJ37

Federal Motor Vehicle Safety Standards; Air Brake Systems

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

ACTION: Final rule.

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SUMMARY: This document amends the Federal motor vehicle safety standard
on air brake systems to improve the stopping distance performance of
truck tractors. The rule requires the vast majority of new heavy truck
tractors to achieve a 30 percent reduction in stopping distance
compared to currently required levels. For these heavy truck tractors
(approximately 99 percent of the fleet), the amended standard requires
those vehicles to stop in not more than 250 feet when loaded to their
gross vehicle weight rating (GVWR) and tested at a speed of 60 miles
per hour (mph). For a small number of very heavy severe service
tractors, the stopping distance requirement will be 310 feet under
these same conditions. In addition, this final rule requires that all
heavy truck tractors must stop within 235 feet when loaded to their
``lightly loaded vehicle weight'' (LLVW).
    The purpose of these amendments is to reduce the number of
fatalities and injuries associated with crashes involving tractor-
trailer combinations and other vehicles. In addition, we anticipate
that this rule will prevent a substantial amount of property damage
through averting or lessening the severity of crashes involving these
vehicles. Once all subject heavy truck tractors on the road are
equipped with enhanced braking systems, we estimate that annually,
approximately 227 lives will be saved and 300 serious injuries will be
prevented. In addition, this final rule is expected to prevent over
$169 million in property damage annually, an amount which alone is
expected to exceed the total cost of the rule.
    There are a number of simple and effective manufacturing solutions
that vehicle manufacturers can use to meet the requirements of this
final rule. These solutions include installation of enhanced drum
brakes, air disc brakes, or hybrid disc/drum systems. We note that
currently a number of vehicles in the commercial fleet already utilize
these improved braking systems and already realize performance that
would meet the requirements of the amended standard.

DATES: Effective Date: This final rule is effective November 24, 2009.
    Compliance Date: Three-axle tractors with a GVWR of 59,600 pounds
or less must meet the reduced stopping distance requirements specified
in this final rule by August 1, 2011. Two-axle tractors and tractors
with a GVWR above 59,600 pounds must meet the reduced stopping distance
requirements specified in this final rule by August 1, 2013. Voluntary
early compliance is permitted before those dates.
    Petitions for Reconsideration: If you wish to submit a petition for
reconsideration of this rule, your petition must be received by
September 10, 2009.

ADDRESSES: Petitions for reconsideration should refer to the docket
number above and be submitted to: Administrator, Room W42-300, National
Highway Traffic Safety Administration, 1200 New Jersey Avenue, SE.,
Washington, DC 20590.
    See the SUPPLEMENTARY INFORMATION portion of this document (Section
VI; Rulemaking Analyses and Notice) for DOT's Privacy Act Statement
regarding documents submitted to the agency's dockets.

FOR FURTHER INFORMATION CONTACT: For non-legal issues, you may call Mr.
Jeff Woods, Office of Crash Avoidance Standards (Telephone: 202-366-
6206) (Fax:<PHONE_NUMBER>).
    For legal issues, you may call Mr. Ari Scott, Office of the Chief
Counsel (Telephone:<PHONE_NUMBER>) (Fax:<PHONE_NUMBER>).
    You may send mail to both of these officials at National Highway
Traffic Safety Administration, 1200 New Jersey Avenue, SE., Washington,
DC 20590.

SUPPLEMENTARY INFORMATION:

Table of Contents

I. Executive Summary
    a. Background and Safety Problem Addressed by the Regulation
    b. Notice of Proposed Rulemaking
    c. Summary of Public Comments
    d. Requirements of the Final Rule
    e. Lead Time
    f. Specific Decisions and Differences Between the Final Rule and
the Notice of Proposed Rulemaking
    g. Costs and Benefits
II. Background
    a. Existing Brake Technologies for Heavy Air-Braked Trucks
    b. Current Requirements of FMVSS No. 121
    c. Summary of the NPRM
    d. Summary of Public Comments on the NPRM
III. The Final Rule and Response to the Public Comments
    a. The Final Rule
    i. Summary of Requirements
    ii. Compliance Dates
    iii. Margin of Compliance
    b. Summary of NHTSA Testing and Results Conducted After
Publication of the NPRM
    i. Testing Conducted on Three-Axle Truck Tractors
    ii. Testing Conducted on Two-Axle Truck Tractors
    iii. Testing Conducted on Severe Service Tractors
    c. Response to Public Comments
    i. Braking Performance of Heavy Truck Tractors With Improved
Brake Systems
    1. Braking Performance of Typical Three-Axle Tractors With
Improved Brake Systems in the Loaded-to-GVWR Condition
    2. Braking Performance of Two-Axle Tractors With Improved Brake
Systems in the Loaded-to-GVWR Condition
    3. Braking Performance of Severe Service Tractors With Improved
Brake Systems in the Loaded-to-GVWR Condition
    a. Definition of Severe Service Tractor and Specific Safety
Benefits
    b. Three-Axle Severe Service Tractors With a GVWR Under 70,000
Pounds
    c. Three-Axle Severe Service Tractors With GVWR Over 70,000
Pounds
    d. Severe Service Tractors With Four or More Axles
    e. Two-Axle Severe Service Tractors
    f. Summary of Severe Service Tractors
    4. Braking Performance of Tractors With Improved Brake Systems
in the Unloaded Weight Condition
    5. Emergency Braking Performance of Tractors With Improved Brake
Systems
    a. Background Information on the Emergency Braking Performance
Requirement
    b. Commenters' Responses to Proposed Emergency Braking
Performance Requirement
    ii. Ancillary Issues Arising From Improved Brake Systems
    1. Stability and Control of Tractors With Improved Brake Systems
    2. Brake Issues on Tractors With Improved Brake Systems
    3. Brake Balance and Trailer Compatibility Issues for Tractors
With Improved Brake Systems
    a. Brake Balance Between the Steer and Drive Axles
    b. Tractor-Trailer Compatibility
    c. Brake Balance and Trailer Compatibility Issues for Two-Axle
and Severe Service Tractors
    iii. Cargo Securement
    iv. Testing Procedures
    1. Brake Burnish Issues for Tractors With Improved Brake Systems
    2. Brake Dynamometer Test Requirements
    v. Stopping Distances at Reduced Initial Test Speeds
    vi. Comments Regarding Foreign Trade Agreements
    vii. Miscellaneous Comments
    viii. Costs and Benefits of Shorter Tractor Stopping Distances
    1. Estimated Benefits of a 30 Percent Reduction in Stopping
Distance

    2. Cost of Improved Brake Systems
    3. Additional Costs Incurred Resulting From Improved Brake
Systems
    4. Summary of Cost-Benefit Analysis
    ix. Lead Time
IV. Rulemaking Analyses and Notices
    a. Vehicle Safety Act
    b. Executive Order 12866 and DOT Regulatory Policies and
Procedures
    c. Regulatory Flexibility Act
    d. Executive Order 13132 (Federalism)
    e. Executive Order 12988 (Civil Justice Reform)
    f. Executive Order 13045 (Protection of Children From
Environmental Health and Safety Risks)
    g. Paperwork Reduction Act
    h. National Technology Transfer and Advancement Act
    i. Unfunded Mandates Reform Act
    j. National Environmental Policy Act
    k. Regulatory Identifier Number (RIN)
    l. Privacy Act
Regulatory Text

I. Executive Summary

a. Background and Safety Problem Addressed by the Regulation

    On March 10, 1995, NHTSA published three final rules \1\ as part of
a comprehensive effort to improve the braking ability of medium and
heavy vehicles.\2\ While the major focus of that effort was to improve
directional stability and control through adoption of antilock brake
system (ABS) requirements, the 1995 rules also reinstated stopping
distance requirements for medium and heavy vehicles, replacing earlier
requirements that had been invalidated in 1978 by the United States
Court of Appeals for the 9th Circuit due to reliability issues (see
PACCAR v. NHTSA, 573 F.2d 632 (9th Cir. 1978)).
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    \1\ 60 FR 13216 (Dockets 92-29 and 93-69), 60 FR 13287
(Docket 93-06), March 10, 1995.
    \2\ Medium and heavy weight vehicles are hydraulic-braked
vehicles over 10,000 pounds GVWR, and all vehicles equipped with air
brakes; hereinafter referred to collectively as heavy vehicles.
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    Currently, stopping distance requirements under FMVSS No. 121, Air
Brake Systems, vary according to vehicle type. Vehicles are tested
under three different test conditions: (1) Loaded-to-GVWR; (2)
unloaded; and (3) emergency braking conditions. Under the loaded-to-
GVWR condition, when stopping from 60 mph, air-braked buses must stop
within a distance of 280 feet, air-braked single unit trucks must stop
within 310 feet, and air-braked truck tractors must comply within 355
feet.\3\ Under the unloaded \4\ condition at 60 mph, air-braked buses
are required to stop within 280 feet, while single-unit trucks and
truck tractors must stop within 335 feet. Under the emergency brake \5\
60 mph requirements, air-braked buses and single-unit trucks must stop
within 613 feet, while tractors must stop within 720 feet.
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    \3\ For heavy truck tractors (tractors), the current stopping
distance test in the loaded-to-GVWR condition is conducted with the
tractor coupled to an unbraked control trailer, with weight placed
over the fifth wheel of the tractor, and a 4,500 pound load on the
single axle of the trailer. This test method isolates the braking
performance of the tractor so that only that system's performance is
evaluated. The performance of a tractor in an FMVSS No. 121 stopping
distance test does not directly reflect the on-road performance of a
tractor/semi-trailer combination vehicle that has braking at all
wheel positions.
    \4\ In the unloaded condition, vehicles are tested at lightly
loaded vehicle weight (LLVW).
    \5\ Emergency brake system performance is tested with a single
failure in the service brake system of a part designed to contain
compressed air or brake fluid.
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    Data from the agency's 2000-2002 GES database and the agency's
2004-2006 FARS database indicate that the involvement of large trucks
in fatal and injury-producing crashes has slightly declined, while
vehicle-miles-traveled (VMT) has increased. However, because the number
of registered heavy vehicles has increased, the net effect is that the
total number of crashes remains high. According to the 2006 data: \6\
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    \6\ See Traffic Safety Facts 2006--Large Trucks, National Center
for Statistics and Analysis (NCSA), report number DOT HS 810 805,
http://www.nrd.nhtsa.dot.gov/Pubs/810805.pdf. The NCSA report uses
the term ``large trucks,'' which in practical terms describes the
same segment of the vehicle population as ``heavy vehicles.''
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     385,000 large trucks were involved in traffic crashes in
the U.S.
     4,732 large trucks were involved in fatal crashes,
resulting in 4,995 fatalities (12 percent of all highway fatalities
reported in 2006). Seventy-five percent of the fatally injured people
were occupants of another vehicle; 16 percent were truck occupants, and
8 percent were nonoccupants.
     106,000 people were injured in crashes involving large
trucks. Seventy-six percent of the injured people were occupants of
another vehicle; 22 percent were truck occupants, and 2 percent were
nonoccupants.
    According to a report \7\ published by the Analysis Division of the
Federal Motor Carrier Safety Administration (FMCSA), the fatality rate
for large truck crashes was 66 percent higher than the fatality rate
for crashes involving only passenger vehicles (defined as a car or
light truck) in 2005. When the FMCSA report considered combination
trucks (e.g., tractor and trailer combinations) separately, the crash
fatality rate was nearly double that of passenger vehicles. Conversely,
the crash fatality rate for single-unit trucks was approximately 23
percent higher than for passenger vehicles. The FMCSA data indicate
that for all types of crashes involving large trucks, those involving
trucks with a GVWR over 26,000 pounds have the highest rate of crash
involvement.
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    \7\ Large Truck Crash Facts 2005 (report number FMCSA-RI-07-046,
http://www.fmcsa.dot.gov/facts-research/research-technology/report/
Large-Truck-Crash-Facts-2005/Large-Truck-Crash-Facts-2005.pdf.
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    It is expected that in most cases reductions in stopping distances
for large trucks will result in a reduction of the impact velocity, and
hence the severity of a crash. In some cases, reduced stopping
distances will prevent a crash from occurring entirely (i.e., a vehicle
with a reduced stopping distance will stop short of impacting another
vehicle). Based on the crash data in the June 2005 NHTSA report titled
``An Analysis of Fatal Large Truck Crashes,'' \8\ improvements in
stopping distance will provide benefits in the following types of
crashes: Rear-end, truck striking passenger vehicle; passenger vehicle
turned across path of truck; and straight path, truck into passenger
vehicle. It is estimated that these types of crashes account for 26
percent of fatalities involving large trucks, or 655 fatalities
annually. In addition, it is possible that some head-on collisions
could be reduced in severity, since improvement in braking performance
could reduce impact speeds.
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    \8\ DOT HS 809 569, http://www.nrd.nhtsa.dot.gov/Pubs/809-
569.pdf; Docket  NHTSA-2005-21462-5 via Web site
references.
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    NHTSA has been exploring the feasibility of reducing the stopping
distance under FMVSS No. 121 for heavy air-braked vehicles by 20-30
percent based on testing of current vehicles. We have initially focused
on air-braked truck tractors, since the available crash data indicate
that these vehicles are the ones most frequently involved in fatal
truck crashes. By promulgating a more stringent requirement for air-
braked heavy tractor stopping distances, it is our intent to reduce
fatalities and injuries relating to this class of vehicles. It is our
belief that development of advanced air disc brakes, enhanced larger
capacity drum brakes, and advanced ABS, offer cost-effective means to
reduce heavy truck stopping distances and to reduce injuries and damage
from large tractor crashes effectively.

b. Notice of Proposed Rulemaking

    On December 15, 2005, NHTSA published a Notice of Proposed
Rulemaking (NPRM) in the Federal Register (70 FR 74270) \9\ proposing
to amend FMVSS No. 121 so as to reduce

the required stopping distances for the loaded and unloaded service
brake distances and emergency brake distances for truck tractors by 20
to 30 percent. These amendments would apply to nearly all of the
130,000 tractors manufactured annually. NHTSA also proposed a lead time
of two years to implement these amendments, given that vehicles tested
by the agency and industry were able to meet the proposed requirements
without modifications other than the use of improved foundation brakes.
Finally, NHTSA indicated that it was considering revising the
dynamometer testing procedures to ensure adequate braking capability
for trailer foundation brakes.
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    \9\ Docket No. NHTSA-2005-21462.
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    The NPRM included figures from the accompanying Preliminary
Regulatory Impact Analysis (PRIA) indicating that enhanced brake system
specifications would result in a range of costs and benefits based on
the specific requirements and the choices made to reach those
requirements. We note that in some instances, the cost estimates in the
PRIA do not correspond to the numbers in the FRIA or those cited in the
Final Rule. This is because NHTSA has updated its cost estimates during
the interim period, and the FRIA uses 2007 dollars.
    The NPRM also discussed the results of testing conducted at NHTSA's
Vehicle Research and Test Center (VRTC), as well as data from Radlinski
and Associates provided to NHTSA. These data strongly suggested that
with improved foundation brakes, typical three-axle tractors \10\ would
be able to meet the proposed requirements for reduced stopping
distance, although the Radlinski data did not include data on two-axle
or severe service \11\ tractors. The data also indicated that some
vehicles in service today would meet the enhanced requirements with no
additional modifications.
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    \10\ As explained below, ``typical'' three-axle tractors have a
GVWR less than or equal to 59,600 pounds.
    \11\ As explained below, ``severe service'' tractors refer to
tractors with a GVWR over 59,600 pounds.
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    NHTSA requested comments on a number of subjects in the NPRM.
Comments were requested generally on the proposal to reduce stopping
distances 20-30 percent and on the costs of the proposal. Comments were
also requested on a variety of specific subjects, such as the possible
changes in dynamometer testing procedures, the application of Advanced
ABS and Electronically Controlled Braking Systems (ECBS), and the lead
time that would be required to implement the proposed changes. Finally,
NHTSA requested comments on the VRTC and Radlinski testing, as well as
information from vehicle manufacturers regarding vehicle modifications
(other than to foundation brakes) that might be required to meet the
proposal's enhanced braking specifications.

c. Summary of Public Comments

    Commenters brought up a variety of issues in response to the NPRM.
Most commenters supported NHTSA's proposal to reduce the stopping
distance requirements for heavy truck tractors. In general, safety
organizations recommended adopting the 30 percent reduction in stopping
distances for all heavy truck tractors. On the other hand, truck
manufacturing groups recommended that the agency reduce the stopping
distance requirements by 20-25 percent, and limit the scope of the
reductions to standard three-axle tractors. In their comments,
manufacturers cited the increased costs and complexity of upgrading to
the stricter stopping distance requirements, as well as potential
problems that could be encountered with upgrading the requirements for
two-axle and severe service tractors. Many commenters also discussed
the vehicle testing NHTSA cited in the NPRM, along with providing
independent test and cost-benefit data.
    Other aspects of Standard No. 121 mentioned in the NPRM received
comments as well. Several commenters recommended against making any
changes to the emergency braking requirements in the Standard.
Regarding brake dynamometer specifications, some commenters also
recommended that no changes be made. Several commenters suggested that
the brake burnish procedure could be returned to an older procedure,
known as a ``hot burnish,'' that existed before 1993. Finally,
attention was called to the possible ramifications of the stopping
distance changes for issues like cargo securement and brake power at
lower speeds.

d. Requirements of the Final Rule

    After careful consideration of the public comments on the NPRM, we
are promulgating this final rule, which amends the requirements of
FMVSS No. 121 by reducing the specified stopping distance for the vast
majority of heavy truck tractors by 30 percent. For a small number of
very heavy, severe service tractors, the stopping distance requirement
is reduced by a smaller amount. The reduction applies to service brake
stopping distance but does not, however, apply to emergency braking
distances.
    For heavy trucks in the loaded-to-GVWR condition, the stopping
distance requirements from an initial speed of 60 mph are as follows:
     A tractor with two or three axles and a GVWR of 70,000
pounds or less must stop within 250 feet.
     A tractor with three axles and a GVWR greater than 70,000
pounds must stop within 310 feet.
     A tractor with four or more axles and a GVWR of 85,000
pounds or less must stop within 250 feet.
     A tractor with four or more axles and a GVWR greater than
85,000 pounds must stop within 310 feet.\12\
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    \12\ We note that tractors with any axle with a GAWR of 29,000
pounds or greater will continue to be excluded from FMVSS No. 121
requirements in accordance with paragraph S3.
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    For heavy trucks in the unloaded condition, the agency is reducing
the specified stopping distance from 60 mph by 30 percent, to a 235-
foot requirement. This requirement applies to all tractors, including
those severe service tractors for which the loaded-to-GVWR stopping
distance requirement has been set at 310 feet.
    Stopping distance requirements for heavy air-braked tractors are
provided in Tables I through III (See Section III). The tables list the
following information:
     Table I lists the requirements and details the explanation
for stopping distance requirements in the loaded-to-GVWR condition for
two- and three-axle tractors with a GVWR of 70,000 pounds or less, and
tractors with four or more axles with a GVWR of 85,000 pounds or less.
     Table II lists the requirements and details the
explanation for stopping distance requirements in the loaded-to-GVWR
condition for three-axle tractors with a GVWR greater than 70,000
pounds, and tractors with four or more axles and a GVWR greater than
85,000 pounds.
     Table III lists the stopping distance requirements and
details the explanation for all tractors in the unloaded condition.
    In addition, to reduce a possible source of test variability, the
agency is adding a specification to the unloaded condition testing
requirement in FMVSS No. 121 that the fuel tank is filled to 100
percent of capacity at the beginning of testing and may not be less
than 75 percent of capacity during any part of the testing.
    Finally, it should be noted that there were several changes
suggested in the NPRM that we are not incorporating into this final
rule amending FMVSS No. 121. These include:

     There is no change in the emergency brake stopping
distance requirement.
     There are no changes to the dynamometer test requirements.

e. Lead Time

    After carefully considering the public comments on the NPRM, the
agency has decided to tie the lead time to the specific type of heavy
truck in light of the anticipated challenges in making the necessary
modifications. For the reasons discussed below, we have decided to
provide the majority of three-axle tractors with two years lead time
from the date of today's final rule, and we are providing two-axle and
severe service tractors with four years lead time.
    NHTSA's test data indicate that for typical three-axle tractors
with improved brake systems (i.e., enhanced drum brakes or air disc
brakes), compliance with the new stopping distance requirements can be
readily achieved. Therefore, the agency is specifying a compliance date
that is two years from the date of publication of the final rule for
typical three-axle tractors. ``Typical three-axle'' tractors are
defined as having three axles and a GVWR less than or equal to 59,600
pounds.
    Available test data also indicate that two-axle tractors with
improved brake systems can meet a 250-foot loaded-to-GVWR stopping
distance requirement. However, we believe additional lead time is
needed for manufacturers to evaluate new brake systems more fully to
ensure compatibility with existing trailers and converter dollies when
used in multi-trailer combinations, and to minimize the risk of vehicle
stability and control issues. With regard to severe service tractors,
available test data and analysis indicate that the 250-foot and 310-
foot loaded-to-GVWR stopping distance requirements, depending on the
vehicle's GVWR, are achievable. However, only limited development work
has been performed on these vehicles, and additional lead time is
needed for manufacturers to complete testing and validation of new
brake systems for these vehicles. In light of these facts, NHTSA has
decided that additional lead time is necessary for all two-axle
tractors, and severe service tractors with a GVWR greater than 59,600
pounds. Accordingly, for those vehicles the compliance date for today's
final rule is four years from the date of publication.

f. Specific Decisions and Differences Between the Final Rule and the
Notice of Proposed Rulemaking

    In the NPRM, NHTSA discussed a number of potential actions intended
to improve vehicle safety by reducing heavy air-braked tractor stopping
distance through amendments to FMVSS No. 121. The available data showed
that it was both technically feasible and cost-effective to require
improved foundation brakes on air-braked tractors that could achieve a
20-30 percent reduction in stopping distance. The main differences
between the NPRM and the final rule include decisions to: (1) Specify a
30 percent reduction in stopping distance for the vast majority of
tractors, with a smaller reduction for a small number of very heavy
severe service tractors; (2) continue the standard's emergency braking
requirements without change; (3) alter the stopping distance
requirements for reduced speed tests to account for brake system
reaction time and the available tire-road friction; and (4) extend the
effective date for compliance by two-axle and severe service tractors.
The rationales for these decisions are discussed briefly below,
followed by a more complete explanation later in this document.
    In the NPRM, NHTSA proposed reducing the required stopping distance
for heavy air-braked tractors by 20-30 percent. This range was based on
available test results and cost analyses (described below). In the
final rule, NHTSA is requiring a 30 percent reduction in the required
stopping distance for the vast majority of tractors. We note that the
agency's final regulatory impact analysis (FRIA) estimated that greater
safety benefits would be attained with a 30-percent reduction in
stopping distance requirements compared to the benefits estimated for a
20-percent reduction. It estimated that more than twice as many
benefits in fatalities and serious injuries prevented are projected for
the 30-percent case versus the 20-percent case. The differential in
estimated property damage reductions is even greater, with
approximately five times the property damage prevented for the 30-
percent case versus the 20-percent case. NHTSA testing and analysis
demonstrated that nearly all two-axle and three-axle tractors will be
able to meet the 30 percent reduction by using improved foundation
brakes that are readily available. For a small percentage of severe
service tractors (estimated to be approximately one percent), namely
three-axle tractors with a GVWR over 70,000 pounds and tractors with
four or more axles and a GVWR over 85,000 pounds, we concluded that a
30 percent reduction is not currently practicable. For those vehicles,
the stopping distance is reduced by 13 percent, from the currently
mandated level to the level of similar single-unit trucks.
    While the NPRM proposed reducing emergency brake stopping distances
by 20-30 percent, we decided not to adopt this part of the proposal.
Comments received from the Truck Manufacturers Association (TMA)
indicated that in order to meet the agency's proposed emergency brake
stopping distance requirements, manufacturers would need to modify the
ABS algorithms to allow more drive wheel lockup. This modification
could be detrimental to vehicle stability and control. NHTSA considered
this, as well as the relative rarity of a crash-imminent situation
during a brake failure, and decided to maintain the status quo.
    In the final rule, NHTSA is also altering the stopping distance
requirement for speeds less than 60 mph from the original figures cited
in the NPRM. Several commenters argued that the reduced stopping
distance values in the proposed Table V of FMVSS No. 121 did not take
into account the brake system reaction time and average deceleration.
In the final rule, the stopping distances for speeds less than 60 mph
have been adjusted to take these factors into consideration.
    Finally, the final rule provides additional lead time for several
types of tractors to comply with the reduced stopping distance
requirements. The NPRM had proposed a two-year lead time for all
tractors to meet the reduced stopping requirements. With regards to
typical three-axle tractors (three-axle tractors with a GVWR of 59,600
pounds or less), the available test data showed that compliance to the
new stopping distance requirements can be readily achieved without the
need to make significant modifications to other vehicle systems. As
stated above, however, the agency believes that additional lead time is
needed for manufacturers to develop and evaluate improved braking
systems more fully for two-axle and severe service tractors. Therefore,
the lead time has been extended for those types of vehicles by an
additional two years.

g. Costs and Benefits

    A 30 percent reduction in required stopping distance will realize
significant benefits, both in terms of injuries and fatalities
prevented, as well as in property damage prevented. The agency's
analysis in the FRIA estimates that, with a 30 percent reduction in
stopping distance requirements, 227 fatalities and 300 serious injuries
will be prevented. In addition, it is estimated that a 30 percent
reduction in stopping distance will realize significant

reductions in property damage. According to the FRIA, using a 3 percent
discount rate, $205M of property damage will be prevented annually.
Using a 7 percent discount rate, the figure is $169M.
    The range of figures in terms of net costs are based on what types
of foundation brakes, disc brakes or enhanced drum brakes, are used to
meet the new stopping distance requirements. The figures are derived
based on an average annual production of about 130,000 truck tractors
(82 percent of which are typical three-axle tractors, ten percent two-
axle tractors, and eight percent severe service tractors). Each typical
three-axle tractor contains one steer axle and two drive axles, as do
most severe service tractors. Each two-axle tractor contains one steer
axle and one drive axle. Therefore, the agency estimates that in total,
the final rule will require the upgrading of 130,000 steer axle brakes
and 247,000 drive axle brakes. In order to compute the total cost of
complying with the reduced stopping distance rule, the agency
calculated the number of axles that will need to be upgraded with
improved foundation brakes, and multiplied that number by the cost of
the brake. The agency estimated the cost of enhanced drum brakes for
the steer axle at $85, and for drive axles at $65. The agency estimated
the cost of disc brakes to be $500 per axle at all wheel positions.
    Because the agency is not certain how truck manufacturers will
choose to comply with the final rule, using the above figures, the
agency created a range of costs of compliance. The most expensive means
of compliance would be to use a $500 disc brake at all wheel positions,
while the least expensive means of compliance would be to use enhanced
drum brakes at all wheel positions. The FRIA estimates that the
incremental cost to add disc brakes to all wheel positions would be
$1,475 per tractor ($192M total cost), while the incremental cost to
add enhanced drum brakes would be $211 ($27M total cost). One commenter
(Freightliner) provided cost information, stating that the cost of disc
brakes would be $1,627 for a three-axle tractor and $963 for a two-axle
tractor, while the cost of drum brakes for a three-axle tractor would
be $222. In addition, the commenter stated that development and
manufacturing costs would need to be added, although it did not
elaborate on what these costs would be. The agency notes that these
figures are very similar to its own estimates.
    NHTSA testing indicated that for standard three-axle tractors, it
is likely enhanced drum brakes at the steer axle and drive axle
positions will enable the tractors to meet a 250-foot stopping distance
requirement in FMVSS No. 121. For two-axle tractors and severe service
tractors, it is likely that disc brakes would be required at all wheel
positions. Considering that standard three-axle tractors comprise
roughly 82 percent of all tractors, it seems likely that the total
costs will be skewed toward the lower end of the range. In the FRIA,
the agency estimates that the incremental average cost per tractor,
given these assumptions, will be $413 per vehicle ($54M total). NHTSA
notes that this figure is substantially lower than the lowest figure in
the range of estimated savings in property damage ($169M).
    The FRIA estimates that the net cost per equivalent life saved
(NCELS) will range from $108,000 to net benefits based on property
damage savings alone (that is, the costs of implementing this final
rule will be less than the costs saved in damaged property,
irrespective of the injuries and fatalities prevented). The high figure
($108,000 NCELS) is derived by taking the highest estimated cost figure
and the lowest estimated property damage prevented. Conversely, the low
figure (net benefits) is derived from using the low cost estimate and
the high benefits estimate.

II. Background

a. Existing Brake Technologies for Heavy Air-Braked Trucks

    The relevant brake technologies at issue in this rulemaking can be
divided into two categories, S-cam drum brakes (drum brakes) and air
disc brakes (disc brakes).
    The most common type of foundation brake used in air brake systems
for heavy vehicles is the S-cam brake. This is a leading/trailing type
of brake with fixed pivot type shoes. Upon brake application, air
pressure enters the brake chamber causing the diaphragm to push the
pressure plate, which in turn applies a force to the end of the brake
slack adjuster. This force creates a torque on the camshaft, and
rotates the camshaft to which the S-cam is attached. The camshaft head,
which is S-shaped, forces the brake shoes against the surface of the
brake drum to create the retardation force for braking. Enhanced S-cam
drum brakes are essentially larger and wider versions of standard S-cam
drum brakes. On the steer axle, for example, the diameter of the brake
drum is 16.5 inches versus 15 inches for the standard steer axle drum,
and this produces more braking torque. Typically the enhanced steer
axle drum brake lining is 5 inches wide instead of the standard steer
axle brake lining width of 4 inches. On the drive axles, both standard
and enhanced S-cam drum brakes use a 16.5 inch diameter drum, while the
standard lining width is 7 inches versus 8 or 8.625 inches for the
enhanced drum brake. The increased width of the lining and brake drum
provides greater thermal capacity, so that enhanced S-cam drum brakes
operate cooler, contributing to longer life, and they are also less
prone to fade during high-speed stops.
    Air disc brakes are also used on commercial vehicles, but are still
used in relatively small numbers in the U.S. A disc brake is basically
a C-clamp with the retardation force applied by friction pads that
squeeze the brake rotor mounted between them. All air disc brake
systems are composed of a rotor, brake linings, a caliper, an adjusting
mechanism, and an air brake chamber, among other parts, and there are
many different designs to accomplish their function. Disc brakes offer
a number of favorable performance characteristics including linear
torque output and high resistance to fade, although they are
substantially more expensive than drum brakes.

b. Current Requirements of FMVSS No. 121

    Under the current FMVSS No. 121 requirements, most truck tractors
are required to stop within 355 feet, when tested at 60 mph in the
loaded-to-GVWR condition while pulling an unbraked control trailer.
Standard No. 121 also requires that truck tractors stop within 335
feet, when tested at 60 mph in the unloaded condition. Finally, the
standard requires an emergency brake stopping distance of 720 feet,
when tested at 60 mph in the unloaded condition. Currently, the
standard does not specify different requirements for different vehicles
based on their number of axles or on their GVWR, except that vehicles
with a GAWR (gross axle weight rating) of 29,000 pounds or more are
exempt from the standard, as are certain vehicles with a GVWR greater
than 120,000 pounds.
    Before testing, brakes are burnished according to the procedure
specified in paragraph S6.1.8 of the standard. The tractor is coupled
to an unbraked control trailer and loaded so that the combined weight
of the tractor and trailer equals the GVWR of the tractor.
Thermocouples are installed in the brake linings to measure the brake
temperatures. The burnish consists of 500 snubs (reductions in speed)
from 40 mph to 20 mph using the service brakes at a deceleration rate
of 10 ft/sec\2\;. Each subsequent snub is conducted at a distance
interval of 1 mile from the

point of the beginning of the previous snub.

c. Summary of the NPRM

    On December 15, 2005, NHTSA published an NPRM in the Federal
Register (70 FR 74270) \13\ proposing to amend FMVSS No. 121 to reduce
the required stopping distance for the loaded and unloaded service
brake conditions and emergency brake conditions for heavy truck
tractors by 20 to 30 percent. NHTSA proposed a lead time of two years
to implement this requirement, given that vehicles tested by the agency
and private industry were able to meet the proposed requirements
without modifications other than improved foundation brakes. In
addition, NHTSA suggested that it was considering revising dynamometer
testing procedures to ensure adequate braking capability for trailer
foundation brakes.
---------------------------------------------------------------------------

    \13\ Docket No. NHTSA-2005-21462.
---------------------------------------------------------------------------

    In the NPRM, NHTSA stated that it believed the reason that many
truck operators had not progressed to readily-available, more advanced
brake systems was because truck operators did not have this cost
savings information available. Further, the proposal stated that truck
operators are cost-sensitive in terms of the initial purchase price of
the vehicle and are reluctant to add different types and sizes of brake
components to their specifications. The agency noted that the proposed
requirements would result in net cost savings for truck operators if
the savings resulting from decreased property damage are taken into
consideration.
    NHTSA also provided data from its Vehicle Research and Test Center
(VRTC) to compare the performance of air-braked tractors and trailers
equipped with a variety of brake system configurations. These data
indicated that the tested vehicles would be able to comply with a 20-30
percent reduction in the stopping distance requirements with
modifications only to the foundation brake systems. Testing was also
conducted on heavy trucks with a failed primary reservoir in order to
generate data on emergency stopping distances; the results indicated
that the same modifications that improved service brake stopping
distances also improved emergency braking stopping distances.
    Industry data provided by Radlinski and Associates (Radlinski),
commissioned by two brake lining manufacturers, were also cited in the
NPRM. These data related to standard three-axle tractors equipped with
enhanced, larger-capacity S-cam drum brakes at all axle positions.
These data indicated that the tractors were able to meet the 30 percent
reduced stopping distance requirement without disc brakes, and the
braking performance in these tests exceeded that of NHTSA's own tests
at the VRTC, in some cases even when disc brakes were applied at all
positions.
    In the NPRM, NHTSA requested comments on a variety of topics to
further the agency's understanding of the ramifications of various
measures for improving braking systems. As a preliminary matter,
comments were solicited on the safety need for improved braking
distances. Comments were also requested on the implications of
improving stopping distances by 20 percent and 30 percent, including
necessary lead time, needed vehicle modifications, and issues regarding
brake balance. The agency also sought comments on the Radlinski data,
as well as information on developments in electronically-controlled
braking systems (ECBS) and advanced ABS, and how these systems could
benefit heavy vehicle safety.

d. Summary of Public Comments on the NPRM

    NHTSA received 27 comments on the December 2005 NPRM, from heavy
vehicle manufacturers (International Truck and Engine Corporation
(International); Freightliner LLC (Freightliner)), brake suppliers
(Arvin Meritor; Meritor WABCO (Meritor); WABCO Vehicle Control Systems
(WABCO); Honeywell Bremsbelag GmbH (Honeywell); Bendix Commercial
Systems/Spicer Foundation Brake (Bendix); Haldex Brake Products
Corporation (Haldex); Brake Pro), industry organizations and
associations (Truck Manufacturers Association (TMA); Heavy Duty Brake
Manufacturers Council (HDBMC); American Trucking Associations (ATA);
Owner Operators Independent Drivers Association (OOIDA); National
Automobile Dealers Association (NADA)), automobile safety advocates
(Insurance Institute for Highway Safety (IIHS); Advocates for Highway
and Auto Safety (Advocates)), a foreign government (People's Republic
of China), and concerned organizations and individuals (John W. Klegey;
Automotive Safety Office (ASO); Roger L. Adkins; Graham Lower; Timothy
Larrimore; Anonymous; University of Washington; Roger Sauder). All of
the comments on the NPRM can be reviewed in Docket No. NHTSA-2005-
21462. Commenters expressed a range of views, with vehicle
manufacturers, brake suppliers, and trade associations generally
supporting the NPRM. Advocacy groups generally recommended that the
agency adopt a standard at the stricter end of the range (toward 30
percent) for all tractors, while most of the trucking industry comments
recommended that NHTSA reduce the stopping distances by 20-25 percent
(instead of 20-30 percent), and only for typical three-axle tractors.
As part of its comments, TMA provided a crash data analysis indicating
that typical three-axle tractors comprise 82 percent of tractor
production and are involved in 91 percent of fatal crashes involving
tractors.
    The following overview of the public comments reflects the key
issues raised by the commenters, including the safety and cost benefits
of reducing stopping distances, recommended percentages for reducing
stopping distances, as well as issues of technical feasibility and
stability that arise from increasing brake torque. Other issues were
raised as well, including reduced stopping distances in the unloaded
vehicle condition, emergency brake stopping distances, maintenance
issues, recommended dynamometer testing changes, and brake burnish
procedures. Comments were also received in response to NHTSA's
questions about the validity and applicability of the Radlinski testing
data, the impact of ECBS and advanced ABS, and on the margin of
compliance for testing in accordance with FMVSS No. 121. A few
commenters recommended that the government undertake additional,
cooperative studies with industry in order to gather data for two-axle
and severe service tractors. Finally, comments were provided on the
implications of reduced stopping distance for reduced test speed
stopping distance testing and for issues of cargo securement under
high-deceleration conditions.
    Although the agency also requested comments on trailer stopping
distance test data and efforts to improve the braking performance of
single-unit trucks, few comments were received regarding those issues.
Likewise, only a small number of comments addressed the agency's
requests for information about the costs of improved braking systems,
as well as any increase in weight. The issues raised in the public
comments are discussed in further detail and addressed below in Section
III, The Final Rule and Response to Public Comments.
General Need To Reduce Stopping Distance Performance for Tractors
    Support for NHTSA's proposal to reduce the stopping distance
performance of heavy truck tractors was

nearly universal. Highway safety advocacy organizations, such as
Advocates and IIHS, supported the largest reduction of stopping
distances within the range proposed by NHTSA (i.e., a 30 percent
reduction from the current requirements of FMVSS No. 121 for all
tractors). Most of the trucking industry comments favored a 25 percent
reduction in stopping distances, but those commenters recommended
limiting the new requirements to standard three-axle tractors, which
account for over 80 percent of tractor production. It should be noted
that some industry commenters suggested reducing stopping distances by
only 20 percent, the lowest reduction proposed by NHTSA.
Comments on the Proposal To Reduce Service Brake Stopping Distance
Performance by 20-30 Percent in the Loaded-to-GVWR Condition
    The majority of commenters fell into two groups, those who
supported 30 percent reductions in stopping distances for all tractors,
and those who supported less stringent requirements. Most trucking
industry comments (from truck manufacturers and brake suppliers) urged
25 percent reductions for standard three-axle tractors only. In making
these recommendations, the trucking industry commenters argued that
data had not been provided for two-axle and severe service tractors,
and that operational problems (e.g., brake balance, stability, and
steering pull) could occur if brake output is increased for those
tractors. Specifically, TMA suggested that amending FMVSS No. 121 to
require heavy trucks to stop within shorter distances may force
manufacturers to implement designs that could cause poorer real-world
stopping performance and instability. On this point, TMA stated that
one of the reasons current production tractors are equipped with low-
power steer axle brakes is for low-level brake applications, and that
tractors designed only to achieve maximum straight-line decelerations
when fully loaded may not perform well during normal brake
applications.
    In contrast, other commenters, including some brake suppliers
(Bendix and Wabco) as well as Advocates and IIHS, supported a 30
percent reduction in stopping distance for all tractors. These
commenters cited the agency's safety benefit analysis as justifying the
cost of the improvement. Advocates also argued that there are other
benefits associated with the use of disc brakes, including greater
resistance to fading.\14\ Bendix stated that more powerful brakes, both
disc and enhanced drum, are currently available and being used on the
road with no significant operational problems.
---------------------------------------------------------------------------

    \14\ ``Brake Fade'' is a term used to describe a temporary
decrease in torque output of a brake when exposed to certain
conditions, such as high heat.
---------------------------------------------------------------------------

Comments on the Proposal To Reduce Service Brake Stopping Distance
Performance by 20-30 Percent in the Lightly Loaded Condition
    Few comments were received on this topic. However, TMA stated that
currently, standard three-axle unloaded tractors start to experience
rear wheel slip during brake applications of approximately 30 psi or
more.
Comments on the Proposal To Reduce Emergency Braking Stopping Distance
by 20-30 Percent
    Comments from the trucking industry opposed the proposed reduction
in emergency braking stopping distance. Many commenters stated that
NHTSA had not provided any crash data or any other rationale to justify
why any such reduction is necessary. These commenters also stated that
the occurrence of a crash-imminent situation at the same time as a
primary or secondary brake system failure is likely to be extremely
rare.
Comments on the Proposed Two-Year Lead Time
    Trucking industry commenters and NADA argued that, for standard
three-axle tractors, a two-year lead time is adequate to meet a 25
percent reduction in stopping distance. No specific recommendations
were offered for two-axle or severe service tractors, although ATA
suggested a two-stage implementation strategy for standard three-axle
tractors and all other tractors. These commenters also stated that if
the agency decides on a 30 percent reduction in stopping distance,
longer lead times would be required for brake system development and
evaluation.
    Haldex and other commenters also recommended that the stopping
distance reduction be timed as to not coincide with the 2010 effective
date for new engine emission standards, set to become effective by the
Environmental Protection Agency.
Vehicle Modifications Necessary To Meet Proposed Reductions in Stopping
Distance
    Commenters from the trucking and brake industry stated that the
largest percentage of improvements in stopping distance would be
achieved by using more powerful steer axle brakes; either enhanced drum
brakes (larger in width and/or diameter than standard drum brakes) or
disc brakes. Most commenters added that more powerful brakes on the
drive axles would further contribute to braking performance.
Freightliner indicated that 97 percent of its fleet would require brake
improvements to meet a 25 percent stopping distance reduction.
    Commenters from the trucking industry suggested, but provided
little specific information on, other modifications to the vehicle that
may be necessary to achieve the improved braking performance. These
modifications include chassis structural analysis, redesign, and
validation. TMA stated that packaging larger steer axle brakes could
result in steering problems. On the other hand, brake suppliers
suggested that these issues could be resolved.
    For two-axle tractors, several commenters stated that instability
could prove to be a problem. Accordingly, TMA stated that for two-axle
tractors with a short wheelbase, the following modifications would be
necessary to allow the tractor to comply with a 30 percent reduction in
the FMVSS No. 121 test: (1) Steer axle brakes would need to be
enhanced; and (2) drive axle brake torque would need to be reduced to
prevent wheel lockup (a condition which would prove hazardous during
normal road braking situations). TMA indicated that these problems
could be mitigated by added electronic stability systems, but that such
systems could increase stopping distance and dramatically increase
cost.
Margin of Compliance Issues
    Commenters on this issue stated that tractor manufacturers target a
10 percent margin of compliance to account for test conditions and
vehicle variability. Haldex stated that with a 10 percent margin of
compliance on a 25 percent reduction in stopping distance,
manufacturers would strive to achieve a total reduction in stopping
distance of 35 percent.
Cost and Weight of Improved Braking Systems
    Few commenters provided information on the issues of cost and
weight of improved braking systems in response to NHTSA's request.
Freightliner provided cost information on improved foundation brakes,
but without supporting data. According to Freightliner's figures,
installing enhanced drum brakes on a three-axle tractor would add $222
to the cost,

while adding disc brakes would cost an additional $1,627; the cost of
adding disc brakes to a two-axle tractor would be $963. TMA commented
that for two-axle and severe service tractors, NHTSA did not provide a
cost analysis, and it argued that increasing stopping performance would
result in cessation of production of certain vehicles manufactured in
low volumes because manufacturers would not be able to amortize the
manufacturing/engineering costs, which would in turn limit market
choice.
    With regard to weight, Bendix stated that, currently, the heaviest
drum brake weighs 32 lbs. more than the lightest disc brake, while the
heaviest disc brake weighs 134 lbs. more than the lightest drum brake.
WABCO stated that its disc brakes are equivalent in weight to high
performance drum brakes.
Brake Balance Issues With Existing Trailers
    Commenters provided relatively little information on the issue of
brake balance with existing trailers. Truck manufacturers stated that
brake balance information will need to be further evaluated. Some brake
manufacturers provided comments as well. For example, Bendix stated
that its tests of disc-braked tractors had shown no objectionable brake
balance issues. ArvinMeritor, however, stated that if stopping distance
were reduced by more than 25 percent, drive axle torque would need to
be increased, which would cause disruptive issues with the existing
trailer fleet.
Braking Performance of Single-Unit Trucks
    Commenters provided relatively little information regarding single-
unit trucks. Haldex and Bendix suggested that further testing needs to
be done, and that the government should work with industry to develop
test data on the subject. Bendix stated that currently, single-unit
trucks have a higher center of gravity than tractors, and that their
stopping distances are about 15 percent shorter than tractors.
Developments in Advanced ABS and ECBS Systems and Their Effects on
Stopping Distance Performance
    Several brake suppliers provided comments on the state of advanced
ABS and ECBS on stopping distance performance. Specifically, WABCO
stated that currently, ABS systems installed on tractors uses modified
individual regulation (MIR), which reduces yaw movement \15\ on split-
coefficient road surfaces. According to the commenter, with larger
foundation brakes, this system should not require significant
modification, and it could help alleviate potential problems with
larger brakes. Bendix also stated that electronic stability programs
for rollover prevention and yaw stability are available on a variety of
truck tractors.
---------------------------------------------------------------------------

    \15\ Yaw movement refers to vehicle rotation producing lateral
sliding, due to tires on one side of the road producing more
friction than tires on the other side.
---------------------------------------------------------------------------

    Haldex stated that ECBS may improve stopping distance by reducing
the interval it takes between the time when the vehicle operator
depresses the brake pedal to the time when brake forces are actually
generated. However, Haldex also stated that because FMVSS No. 121
requires redundant brake control systems, ECBS is not a viable option
for heavy vehicles at this time. Haldex, like a number of other
commenters, stated that advanced ABS does not reduce stopping distance.
Dynamometer Testing Requirements
    Truck manufacturers and brake suppliers both recommended that there
be no changes to the FMVSS No. 121 dynamometer requirements. Some brake
manufacturers, such as Haldex and HDBMC, stated that current
dynamometer testing procedures in FMVSS No. 121 impose no appreciable
limitations on the useable brake torque, and expressed concern that
changes in dynamometer requirements could have the effect of limiting
their options.
    Arvin Meritor and Bendix stated that they were planning on
conducting further dynamometer testing, and would present the results
to NHTSA. However, NHTSA has not received any additional information on
this issue.
Brake Burnish Issues
    A comment by HDBMC stated that in order to achieve a reduction in
stopping distance, higher torque front brakes would be required on
truck tractors. According to the commenter, the higher torque front
brakes would do more of the work during burnish, thus lowering the rear
brake temperatures and reducing the conditioning of the rear brakes.
HDBMC stated that coupled with the trend toward wider rear brake
configurations, this will result in lower temperatures for rear brakes,
and the critical temperature needed to properly condition the rear
brakes would not be achieved. In order to address this issue, HDBMC
recommended the agency reinstate the FMVSS No. 121 burnish procedure
that existed prior to 1993. HDBMC also stated that because the
specification for rear-axle burnishing was reduced when the standard
was amended in 1993,\16\ parking brake performance has been negatively
affected, and this problem would be expected to worsen under the
agency's reduced stopping distance proposal.
---------------------------------------------------------------------------

    \16\ Docket  2005-21462-20.
---------------------------------------------------------------------------

    Arvin Meritor also commented on the burnish issue, requesting that
an optional burnish procedure be added to the FMVSS No. 121 dynamometer
test. The commenter's recommended procedure calls for six optional
stops, using 100 PSI pressure from a starting speed of 60 mph, at the
conclusion of the 350 [deg]F brake burnish.
Comments on Tractor Stopping Distance Data
    Comments from manufacturers raised two objections to the stopping
distance data provided by NHTSA. To begin with, several commenters
stated that the agency's proposal was non-specific, because it
specified a range of potential stopping distance reductions, rather
than a pinpoint proposal. Further, commenters stated that NHTSA
performed testing only on typical three-axle tractors. For example, TMA
stated that the absence of data on two-axle and severe service tractors
should preclude the agency from issuing a rulemaking on those types of
tractors at this time. TMA and Bendix provided their own testing data
from tractors with enhanced foundation brakes, which in general showed
significant improvements in performance.
    With regards to the Radlinski testing data referred to in the NPRM,
few commenters provided specific comments. Instead, most commenters
simply noted that the data were limited to standard three-axle
tractors. Bendix added that it believes the Radlinski test data is
representative of improvements that can be achieved.
    A cooperative testing system for tractor stopping distance was
recommended by a variety of commenters, including International,
Freightliner, HDBMC, and Arvin Meritor. In addition, the TMA
recommended the agency initiate a test program for two-axle and severe
service tractors.
In-Use Truck Brake System Maintenance
    Several commenters (truck manufacturers and brake suppliers)
commented on the need for better servicing and maintenance of truck
brakes, noting that in-service brakes frequently fall short of the
standards set for brakes sold with new vehicles. Brake

Pro stated that the vast majority (85 percent) of trucks, tractor-
trailers, and trailers in North America have had some form of brake
system component maintenance work or replacement work done on them, and
would no longer necessarily meet the new vehicle stopping distance
standards. TMA stated that 45 percent of trucks involved in crashes
where brakes were the primary avoidance system had non-compliant
brakes.
Reduced Test Speed Stopping Distance Requirements
    HDBMC and Bendix argued that brake system reaction time is not
taken into account in the NPRM's proposed tables in the reduced speed
test requirements. They argued that this resulted in unrealistic
stopping distances. Both commenters provided recommendations for
adjusting the lower test speed stopping distances to account for brake
system reaction time.
Cargo Securement
    OOIDA commented that if tractors with improved brake systems are
able to achieve higher deceleration rates, this could affect the safety
of cargo securement systems, and they provided information on the
Federal Motor Carrier Safety Administration's (FMCSA's) recent
regulatory changes in this area.\17\
---------------------------------------------------------------------------

    \17\ This regulation assigns certain g-forces within which cargo
securement devices and systems must contain the vehicle's cargo
load. See 49 CFR 393.102.
---------------------------------------------------------------------------

III. The Final Rule and Response to Public Comments

a. The Final Rule

i. Summary of Requirements
    In light of the estimated benefits, in terms of lives saved and
property damage avoided, we are upgrading the brake performance
requirements of FMVSS No. 121 for air-braked tractors. The requirements
of this regulation have been drafted so as to advance the safety and
braking performance of truck tractors without imposing overly high
costs on the trucking industry or requiring technical advances beyond
what are available in the commercial market today. In overview, the
final rule specifies 30 percent decreases in required stopping distance
for the vast majority of air-braked tractors. The rule also sets
somewhat less stringent requirements for a small percentage of truck
tractors in light of practicability concerns.
    Specifically, the upgrade to FMVSS No. 121 set forth in this final
rule specifies a 30 percent reduction in stopping distance that is
expected to apply to approximately 99 percent of air-braked tractors.
The reduction lowers the maximum stopping distance from the current
distance of 355 feet to 250 feet when tractors are tested in the
loaded-to-GVWR condition from 60 mph. For three-axle tractors with a
GVWR of over 70,000 pounds, and four (or more) axle tractors with a
GVWR of over 85,000 pounds, the stopping distance requirement in the
loaded-to-GVWR condition is being set at 310 feet.
    The decision to adopt a 250-foot stopping distance is based on the
agency's analysis of the potential safety benefits that may be achieved
by using enhanced braking technology and the costs and feasibility of
upgrading the requirements to the new level. NHTSA research
demonstrated that for most tractors--including standard three-axle
tractors which comprise over 80 percent of the commercial fleet--the
upgrade could be achieved at relatively low cost and with minimal
impact to tractor design specifications. Specifically, research
demonstrated that relatively low-cost enhanced drum brakes would be
adequate to achieve stopping distances within 250 feet, with a margin
of compliance of 10 percent.\18\ For most of the remaining tractors,
including two-axle and most severe service tractors, NHTSA concluded
that the upgraded requirements were also attainable, although more
powerful disc brakes and other design changes may need to be
implemented in order to stop within the required limits without
detrimental effects on stability or brake balance.
---------------------------------------------------------------------------

    \18\ The issue of margin of compliance is discussed later in
this document.
---------------------------------------------------------------------------

    For a small number of severe service tractors with three axles and
a GVWR of 70,000 pounds or more, or equipped with four or more axles
and a GVWR of 85,000 pounds or more, the agency is setting a 310-foot
requirement (similar to the current loaded-to-GVWR requirement for air-
braked single-unit trucks). This is due to the fact that even when
fitted with current disc brakes at all wheel positions, it has been
demonstrated that these vehicles cannot achieve 30 percent reductions
in stopping distance.
    For all tractors, the stopping distance requirement in the lightly-
loaded test condition is set at 235 feet, as it was determined that
with improved foundation brakes, this requirement is well within the
capabilities of all heavy truck manufacturers to achieve.
    The required improvement in stopping distance performance is
limited to service brakes, and does not include emergency braking.
Several commenters argued persuasively that improvements to emergency
braking performance could have deleterious effects on lateral stability
and control, due to modifications to the ABS algorithms that would be
required to meet the emergency braking requirements. Further, there are
no data to show that tractors operating in the bobtail condition (i.e.,
with no trailer attached) and experiencing an emergency braking
situation are contributing to the heavy truck crash problem.
ii. Compliance Dates
    There are two compliance dates on which the new stopping distance
requirements become mandatory. For standard three-axle tractors, the
new stopping distance requirements become mandatory on August 1, 2011.
``Standard three-axle tractor'' refers to typical three-axle tractors
that have a steer axle GAWR less than or equal to 14,600 pounds and a
combined drive axle GAWR less than or equal to 45,000 pounds, for a
total GVWR equal to or less than 59,600 pounds. The agency's test data
show that, for these tractors, compliance with the new stopping
distance requirements can be readily achieved.
    The compliance date for all two-axle tractors, as well as severe
service tractors with a GVWR greater than 59,600 pounds, is August 1,
2013. NHTSA's test data indicate that two-axle tractors can meet a 250-
foot loaded-to-GVWR stopping distance requirement with improved brake
systems. However, additional lead time is needed for manufacturers to
more fully evaluate new brake systems to ensure compatibility with
existing trailers and converter dollies when used in multi-trailer
combinations. Further, more time is needed to minimize the risk of
vehicle stability and control issues. With regard to severe service
tractors, the available test data and analysis indicate that the
respective 250-foot and 310-foot stopping distance requirements can be
met by improved brake systems. However, as only limited development
work has been performed, these vehicles require additional lead time to
ensure complete testing and validation of new brake systems.
iii. Margin of Compliance
    Manufacturers need to ensure that all of their vehicles meet a test
requirement established by a Federal safety standard. To account for
variability, including vehicle-to-vehicle variability, they typically
design vehicles with a margin of compliance.

    With regard to stopping distance, the comments stated that the
traditional industry compliance margin is 10 percent.\19\ We note that
this does not necessarily mean that manufacturers do not sometimes
certify vehicles with a smaller margin of compliance. However, they do
need to take whatever steps are necessary to ensure that each vehicle
they certify complies with applicable requirements.
---------------------------------------------------------------------------

    \19\ Bendix stated, for example, that the traditional industry
compliance margin is 10 percent. Docket  NHTSA-2005-21462-
24, p. 5. TMA referred to ``a requisite 10 percent compliance
margin.'' Docket  NHTSA-2005-21462-34.
---------------------------------------------------------------------------

    We believe that calculations of 10 percent compliance margins are
useful for analytical and discussion purposes in considering what
stopping distance requirements are appropriate and practicable.
    We note that in this document, in many cases we have cited a ten
percent margin of compliance from the average stopping distance that a
vehicle test has demonstrated in testing despite the fact that a
vehicle is required to meet the requirement in only one of six stops.
However, since there is generally little variability in the distance
achieved among multiple stops due largely to the incorporation of anti-
lock braking systems, it generally doesn't make much difference whether
we look at the average or best stop distance.

b. Summary of NHTSA Testing and Results Conducted After Publication of
the NPRM

i. Testing Conducted on Three-Axle Truck Tractors
    Available test data demonstrate that typical three-axle tractors
can meet a requirement with a 30 percent reduction in stopping distance
using only enhanced drum brakes, the least expensive type of improved
foundation brake available. NHTSA used the same definition for a
``typical three-axle tractor'' as TMA and HDBMC, which is a 6x4
configuration (three axles with six wheel positions; a non-driven steer
axle and two rear drive axles) with a GVWR below 59,600 pounds, a steer
axle with a GAWR equal or less than 14,600 pounds, and tandem drive
axles rated equal or less than 45,000 pounds total capacity. According
to the test data from the Radlinski \20\ reports (7 tests), typical
three-axle tractors with enhanced S-cam drum brakes at all wheel
positions achieved the target 30 percent reduction in stopping
distance, with margins of compliance (based on a 250-foot stopping
distance requirement) ranging from 12 to 18 percent. This is superior
to the ten percent threshold used by most manufacturers.
---------------------------------------------------------------------------

    \20\ Docket  NHTSA-2005-21462-5, 6, 7.
---------------------------------------------------------------------------

    NHTSA also conducted testing at its Vehicle Research Test Center
(VRTC), using a variety of foundation brake systems.\21\ The VRTC tests
of two tractors showed that with disc brakes at all wheel positions,
both tractors could meet the 30 percent target with compliance margins
between six and 13 percent, while one of these tractors could meet the
30 percent target using a hybrid (disc/drum) configuration with disc
brakes on the steer axle and standard drive axle drum brakes (16.5''
diameter drum x 7'' wide brake linings) with a six percent margin of
compliance.
---------------------------------------------------------------------------

    \21\ See Class 8 Truck Tractor Braking Performance Improvement
Study, available at: http://www.nhtsa.dot.gov/staticfiles/DOT/NHTSA/
NRD/Multimedia/PDFs/VRTC/ca/capubs/DOTHS809700.pdf
---------------------------------------------------------------------------

    The above tests show that disc brakes provide an alternative means
to achieve compliance with a 30 percent reduction in the stopping
distance requirement. All of the all-disc braked examples could meet or
exceed the ten percent margin of compliance with one exception (one
VRTC test). Moreover, the agency is confident that the performance of
that one example could readily be improved by increasing the torque
output of that disc brake (or switching to newer, readily-available,
and more powerful disc brakes).
    Results for the hybrid combination of disc brakes on the steer axle
and standard drum brakes on the drive axle were mixed, with one tractor
meeting the 30 percent reduction in stopping distance with a six
percent margin, even though the performance would be expected to match
or exceed the performance of a tractor with enhanced drum brakes at all
wheel positions (which, as the Radlinski testing showed, was able to
meet the 30 percent reduction with margins over ten percent). Also, the
agency did not test any hybrid configurations using enhanced drum
brakes (standard 16.5'' x 7'' drive axle brakes were used in the
agency's hybrid tests). Based on these results, one conclusion that can
be drawn regarding cost is enhanced drive axle S-cam drum brakes will
be necessary, at a minimum, whether used on the steer or drive axles of
a standard three-axle tractor, because the available data show that
standard drum brakes (15'' x 4'' steer, 16.5'' x 7'' drive) have not
been able to achieve the necessary performance to meet the requirements
in this final rule.
ii. Testing Conducted on Two-Axle Truck Tractors
    NHTSA's testing after publication of the NPRM indicated that a
Sterling 4x2 tractor is capable of complying with a 250-foot stopping
distance with enhanced foundation brakes.\22\ In the VRTC testing, the
test tractor was purchased new and was originally equipped with larger
steer axle S-cam drum brakes of 16.5'' diameter by 5'' lining width,
and standard S-cam drum brakes (16.5'' x 7'') on the drive axle. In the
as-received state (approximately 1,000 miles of normal road use, half
of the time in the bobtail condition and half of the time towing a 48-
foot flatbed trailer), the average stopping distance (based on six
stops) was 241 feet from 60 mph at GVWR plus 4,500 pounds of weight on
the single axle, unbraked control trailer as specified in FMVSS No.
121. However, when the foundation brakes were replaced with all new
components and subjected to a complete FMVSS No. 121 burnish, the
average stopping distance increased to 332 feet. Further investigation
of this problem indicated that the replacement brake linings generated
less torque than the original linings. This is discussed in further
detail in the brake burnish section below.
---------------------------------------------------------------------------

    \22\ Docket  NHTSA-2005-21462-39, p. 25.
---------------------------------------------------------------------------

    The same VRTC test tractor was also tested with disc brakes. The
first configuration of the VRTC testing was a hybrid brake system test.
In this test, the tractor was equipped with disc brakes on the steer
axle and the standard S-cam drum brakes on the drive axle (hybrid brake
configuration), and again subjected to an FMVSS No. 121 burnish. The
average loaded-to-GVWR stopping distance was 223 feet, meeting the
proposed 250-foot stopping distance requirement with a margin of
compliance of 11 percent. In the final configuration, the tractor was
equipped with disc brakes on both the steer axle and drive axle. Here,
the average loaded-to-GVWR stopping distance was 200 feet, a 20 percent
margin of compliance.
iii. Testing Conducted on Severe Service Tractors
    After publication of the NPRM, the agency conducted additional
testing on a severe service truck judged to have similar service
braking characteristics as a tractor of similar size and weight
dimensions.\23\ The test truck was a three-axle Peterbilt Model 357
with a steer axle GAWR of 18,000 pounds and tandem drive axle GAWR of
44,000 pounds. The total GVWR was 62,000

pounds, and the wheelbase was 275 inches. The vehicle was purchased as
a chassis-cab and manufactured as a single-unit truck, and a load frame
was attached to the frame rails for test loading purposes. Although a
single-unit truck differs in many ways from a truck tractor, based on
our testing we found that the single-unit truck was likely to
experience similar, if more severe, dynamic load transfer onto its
steer axle than if it had been tested as a tractor, thereby rendering
it a reasonable surrogate for a severe service tractor in this context.
---------------------------------------------------------------------------

    \23\ Docket  NHTSA-2005-21462-39, p. 10.
---------------------------------------------------------------------------

    The substantive difference in braking performance for this vehicle
in the truck versus tractor configuration would be apparent in
emergency braking performance, for which the truck configuration would
likely need to utilize spring brake modulation to meet the stopping
distance requirement at GVWR (this is because there is no equivalent
test requirement for tractors, since emergency braking requirements
only apply in the unloaded condition), and there are also differences
in parking brake performance requirements for single-unit trucks and
tractors. However, neither of these brake system differences were
factors during the normal service brake tests for the Peterbilt truck.
    The truck used in the VRTC testing was tested with a variety of
brake configurations in order to determine its stopping distance
performance. The truck was originally manufactured with enhanced 16.5''
x 6'' S-cam drum brakes on the steer axle, and standard 16.5'' x 7'' S-
cam drum brakes on the drive axles. It was also equipped with a 6S/6M
ABS system that should provide the highest braking efficiency because
the braking forces are modulated individually at each wheel position.
With the OEM S-cam drum brakes, the average loaded-to-GVWR, 60 mph
stopping distance was 280 feet, which would not meet the enhanced 250
feet stopping distance requirement. In a hybrid configuration with disc
brakes on the steer axle and standard S-cam drum brakes on the drive
axles, the average stopping distance was 251 feet. With disc brakes at
all wheel positions, the average stopping distance was 224 feet,
meeting the target reduced stopping distance with a better than 10
percent margin of compliance.
    Another test condition that was evaluated for the severe service
Peterbilt truck was to up-load the vehicle to a GVWR of 76,000 pounds
and conduct 60 mph stops using all disc brakes. The average stopping
distance for six stops was 254 feet and the minimum stopping distance
out of the six stops was 251 feet. The standard deviation of all six
stops was 3.2 feet, indicating that there was very little stop-to-stop
variability, and thus this vehicle achieved very repeatable performance
with disc brakes.\24\
---------------------------------------------------------------------------

    \24\ Docket  NHTSA-2005-21462-39, p. 23.
---------------------------------------------------------------------------

    In July 2006, the VRTC also ran simulation testing based on the
results of the Peterbilt truck testing to determine braking performance
at 80,000 pounds GVWR.\25\ This study used the Truck Sim vehicle
dynamics modeling software with which the VRTC staff has extensive
experience, including validation of many modules (such as foundation
brakes and ABS control systems) used in the program. This simulation
study determined that with the same all-disc brake configuration, but
with the GVWR increased to 80,000 pounds, a heavy truck's estimated
stopping distance would be 280 feet. By increasing the brake torque on
the steer axle (using type 30 brake chambers in place of type 24
chambers), the estimated stopping distance decreased to 262 feet at
80,000 pounds GVWR. Additional parametric studies (by modeling further
increases in brake torque at all wheel positions) showed that if brake
torque could be increased sufficiently to utilize all available tire-
road friction, stopping distances as low as 227 feet could be achieved
(meeting the 30 percent target with a nine percent margin of
compliance). However, the agency is not aware that there are any
available disc brakes currently capable of generating the requisite
torque and that would also be able to be packaged within the available
wheel envelope. Based upon this analysis, the agency has concluded that
the 30 percent reduction in stopping distance may not be feasible for
heavy truck tractors above 80,000 pounds GVWR.
---------------------------------------------------------------------------

    \25\ VRTC/R&D--Vehicle Modeling Research to Estimate Stopping
Distances for 80,000-lb GVWR Trucks and Tractors Using Current Brake
Technologies. Docket  NHTSA-2005-21462-39, p. 15.
---------------------------------------------------------------------------

c. Response to Public Comments

i. Straight-Line Braking Performance of Tractors With Improved Brake
Systems
    In this section, we discuss data and arguments relating to the
performance of tractors with improved braking systems. The purpose of
this section is to address whether various tractor configurations are
capable of meeting the proposed performance requirements of FMVSS No.
121 with improved braking systems. In addition, we provide additional
insight on what kind of improved brakes will be necessary for various
tractor configurations to meet the requirements of the standard, and
provide further refinement of our cost estimates. This portion of the
final rule deals only with straight-line braking performance. Issues of
stability, control, brake balance, burnish, and other issues are dealt
with later in the rule.
1. Braking Performance of Typical Three-Axle Tractors With Improved
Brake Systems in the Loaded-to-GVWR Condition
    In the NPRM, the agency proposed to amend the standard's fully-
loaded service brake stopping distance, at 60 mph, from the currently-
required 355 feet to a new, reduced distance in the range of 284 feet
(20 percent reduction) to 249 feet (30 percent reduction). The agency
requested comments on the proposed reductions in the required stopping
distance.
    A number of commenters supported the agency's decision to reduce
the stopping distance for typical three-axle tractors by 30 percent.
Advocates and IIHS supported the 30 percent reduction proposal over the
20 percent reduction proposal, citing the significantly higher
estimated benefits in terms of the number of injuries, fatalities, and
property damage prevented. Advocates also suggested that the agency
should mandate the use of disc brakes in addition to the reduced
stopping distances, arguing that under actual service conditions, disc
brakes will out-perform hybrid systems and drum brakes because disc
brakes are relatively immune to fade from either water or heat. IIHS
also stated that an additional benefit of the reduced stopping distance
would be encouraging the use of disc brake systems, citing similar
fade-resistant attributes of disc brakes.
    One brake manufacturer, Bendix, commented that it supported a 30
percent reduction in stopping distance for three-axle tractors, and
submitted test data to support the feasibility of this requirement.
Eight tests with disc brakes at all wheel positions showed that all of
the tractors tested could meet the 30 percent target with compliance
margins between 21 percent and 18 percent. Data on one hybrid three-
axle tractor showed that the 30 percent target was met with an eight
percent margin of compliance. Finally, one all drum brake equipped
tractor (drum brake sizes were not specified) met the 30 percent target
with a 14 percent margin of compliance.
    The TMA recommended that the stopping distance for three-axle
tractors be reduced by a maximum of 25 percent, a position shared by
International, Haldex, and NADA. TMA supplied test results for three-
axle

tractors as well. For three-axle tractors equipped with all disc brakes
(8 tests), the 30 percent target in stopping distance reduction was met
with margins of compliance ranging from 10-20 percent. In hybrid
configurations with disc brakes on the steer axle and enhanced drum
brakes on the drive axles (eight tests) and in all enhanced S-cam drum
configurations (eight tests), the margins of compliance ranged from two
to 20 percent.
    In its comments, ArvinMeritor stated that for typical three-axle
tractors to achieve tractor stopping distance reductions greater than
25 percent, an increase in drive axle torque would be needed. Based on
the vehicle testing conducted by NHTSA (see above, section III, B), the
agency agrees with this comment, and recognizes that improved drive
axle foundation brakes will be part of meeting a requirement that
reduces stopping distance by 30 percent.
    For the final rule, the agency has decided to reduce the stopping
distance for typical three-axle tractors in the loaded-to-GVWR
condition, at 60 mph, from the currently-required 355 feet to 250
feet.\26\ In arriving at this requirement, the agency reviewed the
available test data of typical three-axle tractors with improved brake
systems. That data showed that a 30 percent reduction is possible using
a variety of enhanced brake systems. In addition, to ensure that the
amended standard is practicable, the agency considered the margin of
compliance that truck manufacturers typically would use during
compliance to ensure that all similar production tractors would comply
with the requirement, which specifies a target stopping distance of 225
feet.
---------------------------------------------------------------------------

    \26\ A 30 percent reduction from 355 feet is, in fact, 249 feet,
which the agency has rounded to an even 250 feet.
---------------------------------------------------------------------------

    Given the totality of the data provided by TMA and Bendix, NHTSA
believes the test data demonstrate that for typical three-axle tractors
a 30 percent reduction in stopping distance is readily achievable. In
most cases a 10 percent margin of compliance was met or exceeded. Both
NHTSA and commenters'data are consistent with the agency's position
that a 30 percent reduction is feasible. For example, some tests
demonstrate that typical three-axle tractors with enhanced drum brakes
at all wheel positions are readily capable of attaining 30 percent
reductions with more than a 10 percent margin of compliance, although
the upper range (lowest performing) of the data from TMA on at least
one tractor with enhanced drum brakes showed that the margin of
compliance was approximately five percent.
    NHTSA does not agree with the recommendation from Advocates that it
mandate disc brakes for use in all heavy truck tractors. NHTSA has not
mandated the use of disc brakes because these presumed safety benefits
have not been quantified, and no data to this extent was provided by
Advocates. Further, we have no information as to what the net benefit
of any safety benefit unique to disc brakes would be, and how it would
compare to the increased costs of disc brakes.
    The agency believes that the available data demonstrate that 30
percent reductions in stopping distance are readily achievable on
typical three-axle tractors. A ten percent margin of compliance has
been demonstrated for the majority of tractors using disc brakes and
enhanced drum brakes (the exact percentage for margin of compliance
cannot be determined for some of the data for which only ranges in
performance for several tests were indicated). Therefore, the agency
concludes that it is practicable to achieve 30 percent reductions in
stopping distance when currently-available improved foundation brakes
are applied to typical three-axle tractors. We also note that many
tests demonstrate that enhanced drum brakes on the steer and drive
axles were sufficient for many standard three-axle tractors to meet the
30 percent reduction, allowing the lowest-cost option to be used for
the vast majority of heavy truck tractors.
2. Braking Performance of Two-Axle Tractors With Improved Brake Systems
in the Loaded-to-GVWR Condition
    NHTSA proposed in the NPRM to reduce the stopping distance for all
truck tractors, which includes two-axle tractors. As discussed below,
based on agency testing and comments received, the agency concludes
that all two-axle tractors can meet the 30 percent reduction in
stopping distance requirements with improved braking systems. Although
the agency did not include test data on two-axle tractors when the NPRM
was published, since that time, the agency has completed a foundation
brake study at the VRTC on a typical two-axle tractor. In addition,
testing data from the TMA and Bendix also indicate that two-axle
tractors are capable of meeting a 30 percent reduction in stopping
distance with a ten percent margin of compliance if equipped with disc
brakes.
    While industry commenters generally did not support reducing
stopping distance for two-axle tractors, TMA data submitted in response
to the NPRM indicated that for regular service two-axle tractors (i.e.,
with a drive axle GAWR below 23,000 pounds), the 250-foot stopping
distance requirement could be met using disc brakes.\27\ TMA tested
two-axle tractors in hybrid brake configurations and an all-disc
configuration. The first hybrid configuration (one test; disc brakes on
the steer axle and standard 16.5'' x 7'' S-cam drum brakes on the drive
axle) was able to meet the 250-foot requirement with a margin of
compliance of approximately 12 percent. A second hybrid configuration
(two tests; with disc brakes on the steer axle and enhanced 16.5'' x
8.625'' S-cam drum brakes on the drive axle) indicated that both test
vehicles met the 250 foot requirement, one with a margin of
approximately 15 percent, and the other with a margin of only two
percent. Finally, an all-disc configuration (one test) met the proposed
30 reduction with a 22 percent margin of compliance.
---------------------------------------------------------------------------

    \27\ Docket  NHTSA-2005-21462-26, p. 5.
---------------------------------------------------------------------------

    TMA also provided supplemental comments in October 2006,\28\ with
additional data on the performance of two-axle tractors with improved
foundation brakes. Two tractors with disc brakes at all wheel positions
indicated that the best of six stops ranged from 206 to 213 feet in the
loaded-to-GVWR condition from 60 mph, indicating margins of compliance
well over ten percent. A third tractor with a hybrid disc/drum
configuration was able to stop in 221 feet, giving it a 12 percent
margin of compliance. A fourth tractor with enhanced S-cam drum brakes
at all wheel positions had a shortest stop of approximately 248 feet,
and thus a marginal compliance with a 30 percent stopping distance
reduction. Three tractors tested, when tested with standard drum
brakes, could not meet a 250-foot stopping distance.
---------------------------------------------------------------------------

    \28\ Docket  NHTSA-2005-21462-35.
---------------------------------------------------------------------------

    Bendix also provided data indicating that two-axle tractors could
meet the 30 percent stopping distance reduction.\29\ Bendix provided
test data on the disc/drum hybrid configuration (two tests; and the
drive axle drum brake sizes were not specified). In those tests, the
average stopping distances for both tractors would meet the proposed
250-foot requirement with a margin of compliance of 12 percent for one
vehicle and nine percent for the other. Using the best of six stops for
the poorer performing vehicle (225 feet, rather than the average
stopping distance of 228 feet), the margin of compliance

increases to 10 percent. Bendix test data on all-disc brake two-axle
tractors (two tests) indicated that both vehicles would meet a 250-foot
stopping distance requirement and that the margins of compliance were
19 and 14 percent based on the average of six stops in each test. The
GAWRs for all two-axle tractor tests were 22,999 pounds or less on the
drive axle and 12,000 pounds or less on the steer axle (i.e., they were
not severe service two-axle tractors).
---------------------------------------------------------------------------

    \29\ Docket  NHTSA-2005-21462-24-0001, p. 9.
---------------------------------------------------------------------------

    Finally, in its original comments, TMA stated that drive axle brake
torque would need to be reduced to prevent wheel lockup (a condition
which would prove hazardous during normal road braking situations).
However, we believe ABS, which has been required on all new truck
tractors manufactured on or after March 1, 1997, prevents wheel lockup.
Hence, this comment is not persuasive.
    Based on the testing data accumulated by NHTSA and provided by the
commenters, the agency has concluded that meeting a 30 percent
reduction in stopping distance is achievable for currently-produced
two-axle tractors with at least a 10 percent margin of compliance with
all-disc configurations. To a lesser extent, the hybrid disc/drum
configurations (some of which had good margins of compliance, and some
of which had poor margins) may also be able to achieve the 30 percent
reduction in stopping distance.
3. Braking Performance of Severe Service Tractors With Improved Brake
Systems in the Loaded-to-GVWR Condition
a. Definition of Severe Service Tractor and Specific Safety Benefits
    With the exception of certain vehicles with extremely high GVWRs or
GAWRs that are excluded from the requirements of Standard No. 121, the
reduced stopping distance requirements proposed in the NPRM were to
apply to all severe service tractors. For purposes of this document,
NHTSA is using TMA's definition of a three-axle severe service tractor,
as a three-axle tractor having a steer axle GAWR greater than 14,600
pounds and tandem drive axles with a total GAWR greater than 45,000
pounds. In addition, severe service tractors include those tractors
with twin steer axles, auxiliary axles (e.g., lift axles), and/or
tridem drive axles. Chassis configurations include 6x4, 8x4, 8x6, 10x6,
and 14x4 layouts. Based on comments from TMA and Freightliner, the GVWR
of severe service tractors is greater than 59,600 pounds and can exceed
100,000 pounds. The commenters explained that severe service tractors
are used in special purpose applications such as oil field service,
extreme heavy hauling, transporting earth moving equipment, and
logging. The commenters further stated that operation is both on-road
and off-road, and in some cases, on-road use is at relatively low
speeds with the tractor-trailer combinations being accompanied by
escort vehicles.
    Freightliner \30\ stated that severe service tractors comprise
approximately seven percent of tractor production and are involved in
5.6 percent of fatal tractor crashes, according to the UMTRI report on
Class 8 tractors involved in fatal crashes (included with TMA's
comments).\31\ To the extent possible, the agency compares fatal crash
involvement rates of vehicle types based upon fatalities per 100
million vehicle miles traveled (VMT) (see Section II of the NPRM). As
described in the NPRM, tractors have a lower overall crash rate per 100
million VMT compared to light vehicles (passenger cars, light trucks,
and SUVs), but are over-represented in fatal crashes. The UMTRI report
submitted by TMA \32\ did not analyze tractor crash data for the three
types of tractors studied (typical three-axle, two-axle, and severe
service tractors) based upon VMT exposure, and the agency is not aware
such VMT exposure data being available from the known crash data
sources. Based upon the comments received, it appears that the on-road
mileage exposure for severe service tractors is lower than for typical
three-axle or two-axle tractors.\33\ Nonetheless, the 5.6 percent
fatality involvement rate does not indicate that severe service
tractors are underrepresented in fatal crashes to an extent that the
agency should consider excluding them from this final rule. Given the
potential safety benefits, we believe the deciding factor in
determining the loaded-to-GVWR stopping distance requirements for
severe service tractors under this final rule should be dependent on
the best performance that can be achieved using the available improved
brake systems.
---------------------------------------------------------------------------

    \30\ Docket  NHTSA-2005-21462-25.
    \31\ Docket  NHTSA-2005-21462-26.
    \32\ Docket No. NHTSA-2005-21462-26; see attachment, p. 16.
    \33\ Docket  NHTSA-2005-21462-26, p. 11.
---------------------------------------------------------------------------

    In its comments, TMA delineated several broad categories of severe
service tractors that the agency believes comprise highly relevant
categories. The first is three-axle severe service tractors with GVWRs
ranging from approximately 60,000-70,000 pounds. These tractors have a
steer axle GAWR in the 13,000-14,500-pound range and tandem drive axles
rated in the approximate range of 46,000-55,000 pounds (as depicted in
Figure 5 in TMA's April 2006 comments, which shows a three-axle tractor
towing double trailers.) The second category of severe service tractors
described by TMA are three-axle severe service tractors with GVWRs
above 70,000 pounds. Finally, there are severe service tractors in 8x4,
8x6, 10x6, 14x4, and other configurations. This group of vehicles is
used in special purpose or extreme heavy haul applications (as depicted
in Figure 6 of TMA's comments, which shows a 10x6, twin-steer tractor
with tridem drive axles.) Based upon the information provided to the
agency in several ex parte meetings that have been held since the
publication of the NPRM,\34\ the typical weight ratings for the 10x6
tractor photographed would be 14,500 pounds GAWR for each steer axle
and 20,000 pounds for each drive axle, yielding a GVWR of 89,000
pounds. This tractor would not be excluded from FMVSS No. 121 based on
its axle ratings. Other unusual tractor configurations would also tend
to have high GVWRs over 70,000 pounds and still be subject to FMVSS No.
121.
---------------------------------------------------------------------------

    \34\ Memorandums of ex-parte meetings provided in Docket No.
NHTSA-2005-21462-36.
---------------------------------------------------------------------------

b. Three-Axle Severe Service Tractors With a GVWR Under 70,000 Pounds
    Based on the agency's testing, as well as test data provided by the
commenters, NHTSA believes that severe service three-axle tractors with
a GVWR under 70,000 pounds can meet a 250-foot stopping distance
requirement using enhanced foundation brake systems. VRTC test results
and commenter data lead the agency to believe that three-axle severe
service tractors with a GVWR between 60,000 and 70,000 pounds are
capable of meeting the 30 percent reduction in stopping distance using
available enhanced braking systems.
    NHTSA's testing indicated that lower-GVWR three-axle severe service
tractors will be able to meet a 250-foot stopping distance requirement.
Here, NHTSA refers to the Peterbilt truck, tested by the VRTC, which is
very similar to three-axle severe service tractors of the 60,000-70,000
pounds GVWR category. As stated above, the VRTC testing used a single-
unit truck with comparable braking performance to a severe-service
three-axle truck tractor. This tractor, when equipped with disc brakes
and tested at a GVWR of 62,000 pounds, was able to meet the 250-foot
stopping distance requirement with a 10 percent margin of
compliance.\35\ Therefore, the

agency believes that it is practicable to require similarly-configured
tractors to achieve similar braking performance.
---------------------------------------------------------------------------

    \35\ Docket  NHTSA-2005-21462-40.
---------------------------------------------------------------------------

    TMA's supplemental comments include data that enhance NHTSA's
confidence in the practicability of this requirement. The data indicate
that for lower GVWR three-axle severe service tractors, a 250-foot
stopping distance and a ten percent margin of compliance can be
achieved for three-axle, all-disc braked tractors of 62,000 and 66,000
pounds GVWR.\36\ Both VRTC and TMA test data show that three-axle
severe service tractors under 70,000 pounds GVWR are capable of meeting
the reduced stopping distance with improved foundation brakes and can
also achieve a 10 percent margin of compliance.
---------------------------------------------------------------------------

    \36\ TMA comment of October 2006, docket  NHTSA-2005-
21462-35.
---------------------------------------------------------------------------

    In its original comments,\37\ TMA also stated that building a
severe service tractor with improved brakes would result in production
of a vehicle that is not commercially viable. TMA argued that such a
vehicle would have far too aggressive brake linings, which would result
in chatter and frequent failures of various brake components. TMA
stated that this would be a commercially non-viable product. NHTSA
notes that in its later comments submitted on October 2006, TMA tested
a severe service tractor with disc brakes that was able to meet the
proposed reduced stopping distance, and the organization did not
further discuss these problems. NHTSA also notes that when equipped
with modern enhanced braking systems, similarly-configured vehicles can
meet the proposed requirements without the problems that TMA foresaw in
its April 2006 comments. Therefore, the agency believes that the
problems TMA described are obviated by the use of disc brakes.
---------------------------------------------------------------------------

    \37\ Docket  NHTSA-2005-21462-26.
---------------------------------------------------------------------------

    In October 2006, TMA submitted supplemental comments that included
additional information on severe service tractor stopping distance
performance. The TMA testing included six drum and six disc brake
configurations, performed on vehicles with three different drive axle
GAWRs. TMA stated that the disc brakes used in these tests were
prototype models that had not been fully tested for production (as
dynamometer and other test data were not yet available). The agency
assumes that these would be the largest practical disc brakes that
would work within the available wheel and suspension envelope.
    TMA's test results are discussed below, but the result we believe
to be most noteworthy is that the TMA testing indicated that the
proposed 30 percent reduction in stopping distance could be achieved
using disc brakes. To summarize the TMA test results, when tested at a
steer axle weight of 20,000 pounds and a tandem drive axle weight of
46,000 pounds, yielding a GVWR of 66,000 pounds, the baseline all-drum
brake configuration (it was not specified whether the drum brakes were
standard or larger sized) had a stopping distance of 262 feet. Testing
of a hybrid configuration using the prototype disc brakes on the steer
axle yielded a stopping distance of 229 feet, thus meeting the target
with an eight percent margin of compliance. Finally, when tested with
disc brakes at all wheel positions; the stopping distance was 223 feet,
yielding an 11 percent margin of compliance. We note that the data for
the all-disc brake test are consistent with the performance obtained by
VRTC in its tests of the Peterbilt truck with a 62,000 pounds GVWR.
c. Three-Axle Severe Service Tractors With GVWR Over 70,000 Pounds
    In contrast to three-axle tractors with a GVWR between 59,600-
70,000 pounds, agency testing and commenters' data indicate that it is
not practicable at this time for higher-GVWR three-axle severe service
tractors to meet a 250-foot stopping distance requirement. In making
this determination, the agency carefully considered its own data, as
well as the data on high-GVWR three-axle truck tractors provided by the
TMA in its comments. Nonetheless, NHTSA believes that improvements in
stopping distance for these vehicles should be pursued, albeit at a
level less than a 30 percent reduction. TMA's supplemental comments
indicate that tractors with very high GVWRs (with regard to three-axle
tractors, these have single axle weight ratings of 26,000 pounds or
more, or tandem axle weight ratings of 52,000 pounds or more) make up
less than one percent of annual tractor production.
    The agency believes that severe service tractors over 70,000-pound
GVWR can meet the stopping distance requirements for similar vehicles
that are configured as single-unit trucks rather than tractors, because
similarly-configured single unit trucks are currently being
manufactured in compliance with FMVSS No. 121. As the service brake
stopping distance requirement for single-unit trucks is 310 feet in the
loaded-to-GVWR condition, the agency believes that specifying this
standard on severe service tractors of similar weight is a practicable
alternative to a 30 percent reduction in stopping distance.
    TMA provided simulation test data for hybrid and all-disc
foundation brake configurations of three-axle severe service tractors
with a GVWR over 70,000 pounds.\38\ The data that TMA used in its
comments were based upon unspecified simulations, presumably similar to
the Truck Sim work performed by VRTC. A footnote in the supplemental
TMA submission indicates that one all-drum brake configuration at
72,000 pounds GVWR was verified by actual vehicle testing. The
simulation results for a 72,000-pound GVWR tractor (20,000-pound steer
axle load and 52,000-pound tandem drive axle load) estimated that the
hybrid configuration would achieve a 248-foot stopping distance (within
the 30 percent reduction target, but with little margin of compliance).
When equipped with disc brakes at all wheel positions, the stopping
distance was estimated at 242 feet, which would meet a 30 percent
reduction in stopping distance with a three percent margin of
compliance. The configuration with drum brakes \39\ at all wheel
positions was road tested at 72,000 pounds GVWR and had a stopping
distance of 285 feet, above the 250-foot limit. TMA also stated that it
is unclear what technologies would be needed to achieve high levels of
braking performance improvements for tractors in this weight category.
---------------------------------------------------------------------------

    \38\ Docket  NHTSA-2005-21462-34.
    \39\ TMA did not provide dimensions for these brakes, but
described them as the highest available performance brakes.
---------------------------------------------------------------------------

    In addition, TMA simulated a test condition with a tractor at
78,000 pounds GVWR, with a 20,000-pound steer axle load and a 58,000-
pound tandem drive axle load. This tractor was not able to meet a 250-
foot stopping distance with any brake combination, although it must be
noted that a vehicle with a 58,000-pound tandem rating (29,000-pound
GAWR per axle) is exempt from FMVSS No. 121 under Section 3,
Applicability, paragraph (b). The stopping distance simulation results
for this vehicle were 307 feet for the drum/drum configuration, 268
feet for the hybrid configuration, and 261 feet for the all-disc
configuration. Despite the fact that the specific vehicle tested here
would not be subject to the requirements of FMVSS No. 121, it does
represent the upper edge of the GVWR range regulated under the FMVSS
No. 121 requirements, and therefore the agency believes the TMA data
are useful in setting stopping distance

requirements for severe service tractors as part of this final rule.
    In its October 2006 comments, TMA presented testing that indicated
trucks with a GVWR over 70,000 pounds are incapable of meeting a 250-
foot stopping distance requirement. In one example, a 72,000-pound GVWR
tractor equipped with all disc brakes only achieved a three percent
margin of compliance, which the agency does not consider to be enough
for manufacturers to reliably build tractors with assured compliance to
FMVSS No. 121. Similarly, a 78,000-pound GVWR three-axle tractor
equipped with all disc brakes stopped in 261 feet, thus it did not meet
a 250-foot stopping distance requirement. Because all-disc brake
configurations generally produce the best available braking
performance, it is not clear what advancements could be used to bring
trucks of this weight within a 250-foot stopping distance. The agency
therefore concludes that three-axle tractors with a GVWR greater than
70,000 pounds should be provided with a longer stopping distance
requirement.
    The agency has considered all of the available data and comments
regarding severe service tractors to determine appropriate loaded-to-
GVWR stopping distance requirements for these vehicles. The agency
agrees with TMA that, based on all available information, foundation
brakes that could provide loaded-to-GVWR stopping distance performance
in the 250-foot range at 60 mph are not available for three-axle severe
service tractors with a GVWR over 70,000 pounds. There are little or no
test data available for tractors with a GVWR over 70,000 fitted with
the largest available disc brakes to demonstrate that they would be
able to meet a 30 percent reduction in stopping distance. In making
this statement, the agency notes the TMA supplemental comments, which
discuss the lack of extensive testing of prototype disc brakes.\40\
Therefore, the agency does not believe it is practicable at this time
to require three-axle severe service tractors over 70,000 pounds GVWR
to meet the 30 percent reduction in stopping distance.
---------------------------------------------------------------------------

    \40\ TMA comments of October 12, 2006. Docket No. NHTSA-2005-
21462-34.
---------------------------------------------------------------------------

    However, for three-axle tractors with a GVWR over 70,000 pounds, a
310-foot stopping distance requirement is an achievable goal. This
represents a 13 percent reduction in stopping distance from the current
355-foot requirement. Based upon this requirement, and assuming a 10
percent margin of compliance, the 78,000-pound GVWR three-axle tractor,
discussed in the TMA comments of October 2006, could meet the
requirement with an adequate margin of compliance in a hybrid or all-
disc brake configuration. Further, the 72,000-pound GVWR three-axle
tractor would achieve an eight percent margin of compliance with an
all-drum brake configuration. In that case, either slight improvements
in the drum brakes or the installation of disc brakes on the steer axle
would allow the tractor to achieve a ten percent margin of compliance.
The agency believes that in both cases safety benefits will be obtained
because of these improvements, but whether these benefits would be the
same or smaller than for typical (non-severe service) three-axle
tractors is unknown. We also note that for vehicles with a drive axle
GAWR of 29,000 pounds or more, FMVSS No. 121 is not applicable, so that
typically three-axle tractors with a GVWR of 78,000 pounds or more will
be exempt from this requirement.
    As previously discussed, the tests at VRTC of a severe service
truck (used as a surrogate severe service tractor), loaded to a GVWR of
76,000 pounds and equipped with all disc brakes, had an average
stopping distance of 254 feet. This represents an 18 percent margin of
compliance to the 310-foot stopping distance requirement implemented
under this final rule.

d. Severe Service Tractors With Four or More Axles

    For severe service tractors with more than three axles, there is a
similar distinction to be made between lower-GVWR tractors and higher-
GVWR tractors. While the NPRM proposed reducing the stopping distance
for all tractors uniformly, commenters and agency testing have
indicated that a distinction should be made, similar to the distinction
within severe service three-axle tractors. With regard to severe
service tractors with four or more axles, we believe there are some
tractor configurations that, even though they are in the severe service
category, can comply with a 250-foot stopping distance requirement when
most or all of the brakes are upgraded to disc brakes. A small
percentage of these tractors, however, will not be able to currently
comply with this requirement, and thus necessitate a different
approach.
    Some extra-axle tractors are based on, and perform very similarly
to, severe service three-axle truck tractors. One example of this is a
severe service three-axle tractor that has an auxiliary axle installed
by either the truck manufacturer or by a vehicle alterer. The agency
believes that its testing of a single-unit truck at VRTC provides a
basis for determining the scope of this final rule with regard to
similarly configured tractors. Using the VRTC three-axle Peterbilt
truck as a guideline, which had GAWRs of 18,000 pounds for the steer
axle, 44,000 pounds for the tandem drive axles, and a total GVWR of
62,000 pounds, we considered the installation of a lift axle placed in
front of the drive axles with a GAWR of 20,000 pounds. We note that
this is on the upper end of axle weight ratings for lift axles; many
lower GAWR ratings for lift axles are also available. The GVWR would
now be increased to 82,000 pounds, and although the agency has no full
vehicle test data, the loaded-to-GVWR service braking performance of
the tractor would not be expected to decrease substantially from the
performance in the original three-axle configuration (this vehicle was
tested with three axles at 62,000 pounds GVWR and was able to stop in
224 feet when equipped with disc brakes at all wheel positions). We
make this assumption because of the auxiliary brake requirements FMVSS
No. 121, which mandate high levels of fade resistance and stopping
power requirements.
    Although the agency does not have data on the dynamic load
increases on lift axles under hard braking, we expect load transfer
increases (if any) to be minimal. This assumption is based on prior
analyses that show the greatest load transfer to be on the steer axle,
while drive axles (and trailer axles in the case of combination vehicle
tests) typically have small decreases in vertical load under hard
braking.\41\ Thus, it would not be expected that lift axle foundation
brakes would need to be substantially increased in size to provide the
needed retardation force to meet the new stopping distance
requirements.
---------------------------------------------------------------------------

    \41\ Docket No. 21462-2005-33 (see slide 8 of TMA's presentation
for typical load transfer of a tractor-trailer combination vehicle
during hard braking).
---------------------------------------------------------------------------

    TMA provided data that confirmed NHTSA's belief that lower-GVWR
severe service tractors with four or more axles are capable of meeting
a 250-foot stopping distance requirement, even when using drum brakes
on the drive axles. We note that the TMA supplemental data, supplied in
October 2006, for the 66,000-pound GVWR three-axle severe service
tractor showed that this tractor was able to achieve a stopping
distance of 229 feet in a hybrid configuration (disc brakes on steer
axle only), and its drive axles were rated at 23,000 pounds GAWR each.
Therefore, adequately performing drum brakes that

are typically installed on auxiliary axles should be available for a
20,000-pound auxiliary axle; in other words, it is not expected that
disc brakes would be needed on the auxiliary axles in order to achieve
satisfactory performance.
    Next, we turn to TMA comment that dynamic load transfer to the
steer axle may be an issue for some severe service tractors with four
or more axles, such as the twin-steer example described above with a
GVWR above 85,000 pounds. Using a 20,000-pound steer axle GAWR as an
example, the agency believes there is not an adequate installation
envelope to install a large enough disc brake to be able to meet a 250-
foot stopping distance requirement for these vehicles. There are a
number of constraints on the installation envelope that limit the
diameter of the disc rotor and caliper assembly that can be fit within
the inside diameter of the wheel rim, including: (1) The articulation
of the spindle and foundation brakes needed for adequate steering cut;
(2) vertical clearance with chassis components during dynamic steer
axle loading (compression during hard braking); and (3) the size of the
wheels. The agency agrees with TMA that, based on all available
information, foundation brakes that could provide loaded-to-GVWR
stopping distance performance in the 250-foot range are not available
for these tractors. Further, NHTSA is not aware of sufficient test data
available for such tractors fitted with the largest disc brakes to
confirm this (noted in the TMA supplemental comments citing tests of
prototype disc brakes that have not been tested extensively). Because
of these inherent limitations of the steer axle brakes, the agency has
decided to adopt requirements for stopping distance of tractors with
four or more axles and a GVWR greater than 85,000 pounds of 310 feet
(rather than 250 feet) along the lines of the requirements for single-
unit trucks of this size. The agency believes, for the same reasons as
discussed above, that tractor-trailers can achieve similar service
braking performance as similar single-unit trucks.

e. Two-Axle Severe Service Tractors

    We also respond to TMA's April 2006 comments regarding what it
identified as a distinct class of severe service two-axle tractors,
which TMA defined as a two-axle truck tractor having a drive axle GAWR
of 23,000 pounds or more. Based on our review of the commenters' data,
the agency does not believe that the commenters have provided
sufficient information to justify allowing these tractors to be subject
to a less rigorous stopping distance requirement than other two-axle
tractors, and that the proposed specifications for improved stopping
distances are practicable.
    Commenters' test data show that two-axle truck tractors with a
higher GVWR have similar braking performance to other two-axle
tractors. TMA provided test data for one severe service two-axle
tractor with standard 16.5'' x 5'' S-cam drum brakes on the steer axle
and standard 16.5'' x 7'' S-cam drum brakes on the drive axle.\42\ The
stopping distance for this tractor was approximately 315 feet, so this
brake configuration would not meet a 250-foot stopping distance
requirement. However, this test result does not make it necessary to
exclude severe service tractors from the improved stopping distance
requirement entirely.
---------------------------------------------------------------------------

    \42\ Docket  NHTSA-2005-21462-26.
---------------------------------------------------------------------------

    First, we note that the two-axle tractor cited by TMA is not a
typical severe service tractor because it does not have a GVWR in
excess of 59,600 pounds, thereby putting it outside the standard
definition of a severe service tractor.
    Second, of particular significance is the fact that this test
result does not show how this vehicle would perform with upgraded
brakes, specifically disc brakes. Disc brakes are the type of brakes
that have been demonstrated to typically provide the shortest stopping
distance. Therefore, the agency declines to use the TMA data on this
``severe service two-axle tractor'' in formulating the requirements of
this final rule.
    We do not have test data for this specific configuration of vehicle
equipped with disc brakes. However, considering that the achieved
stopping distance of the severe service two-axle tractor is roughly
equivalent to what many other two-axle tractors can achieve when
equipped with standard S-cam drum brakes at all wheel positions,\43\
NHTSA believes that ``severe service two-axle'' tractors will be able
to achieve similar enhancements using enhanced S-cam drum brakes or
disc brakes in lieu of standard S-cam drum brakes. Therefore, the
agency is not specifying a longer stopping distance for these vehicles.
However, for reasons discussed below, the agency is providing a longer
lead time for all two-axle tractors.
---------------------------------------------------------------------------

    \43\ Docket  NHTSA-2005-21462-26.
---------------------------------------------------------------------------

f. Summary of Severe Service Tractors

    Based upon the above analysis, the agency is setting the loaded-to-
GVWR stopping distance requirements for severe service tractors as
follows:
     A tractor with three axles and a GVWR of 70,000 pounds or
less must stop within 250 feet.
     A tractor with three axles and a GVWR greater than 70,000
pounds must stop within 310 feet.
     A tractor with four or more axles and a GVWR of 85,000
pounds or less must stop within 250 feet.
     A tractor with four or more axles and a GVWR greater than
85,000 pounds must stop within 310 feet.

Further, the agency does not recognize a class of two-axle severe
service tractors, and notes that all two-axle tractors are required to
meet a 250-foot stopping distance requirement.
    The agency believes that these requirements will enhance vehicle
safety by ensuring that the vast majority of tractors (estimated to be
approximately 99 percent of annual tractor production) will meet a
requirement with a 30 percent reduction in stopping distance. The
remaining one percent of tractors, which are high-GVWR severe service
tractors, will be required to meet a requirement with a 13 percent
reduction in stopping distance, which is equal to the current required
stopping distance performance for single-unit trucks. Finally, those
tractors with any axle with GAWR of 29,000 pounds or greater will
continue to be excluded from the FMVSS No. 121 requirements.
4. Braking Performance of Tractors With Improved Brake Systems in the
Unloaded Weight Condition
    In the NPRM, the agency proposed to reduce the existing FMVSS No.
121 unloaded weight stopping distance for heavy truck tractors from 335
feet by 20 percent (i.e., to 268 feet) to 30 percent (i.e., to 235
feet). Testing in the unloaded weight condition (also known as lightly-
loaded vehicle weight or LLVW) is performed without any trailer
attached to the tractor (i.e., bobtail condition), plus up to an
additional 500 pounds allowed for the test driver and vehicle
instrumentation. In addition, up to 1,000 pounds is allowed for a roll
bar structure. The tractor is required to meet the unloaded stopping
distance requirement for at least one out of six test stops.
    One potential issue that arises when reducing stopping distance in
the lightly-loaded condition is the issue of wheel lockup, as there is
far less available tire-road friction than in the loaded-to-GVWR
condition. Requirements in FMVSS No. 121, S5.3.1, paragraphs (a)
through (d), specify allowances for wheel lockup during either a
service brake stopping distance test in the loaded or unloaded

condition, and applies to trucks, tractors, and buses. At speeds above
20 mph, wheel lockup on certain axles is only permitted to be momentary
(less than one second), while unlimited wheel lockup on auxiliary axles
is permitted. At speeds below 20 mph, unlimited wheel lockup is
permitted on any wheel. These wheel lockup provisions were necessary
before ABS was mandated, to ensure that the test driver could bring the
vehicle to a stop without loss of control due to unlimited wheel
lockup. In the case of a tractor in the unloaded condition, the drive
axle wheels are very easy to lock up, as there is little vertical load
on them. Prior to the advent of ABS, some tractors were equipped with
bobtail proportioning valves to reduce the brake pressure to the drive
axles in the unloaded condition and make it easier to stop the vehicle
within the required distance (using more steer axle brake power, where
a substantial vertical load exists), and also to improve the on-road
drivability of bobtail tractors.
    However, since March 1, 1997, all tractors have been required to be
equipped with ABS on at least one steer axle and one drive axle, which
has virtually eliminated wheel lockup in tractors. While the relevant
FMVSS No. 121 requirement states that only one rear axle of a tractor
needs to be equipped with ABS, most tractors also indirectly control
the wheels on the other rear axle in the case of tandem drive axles, or
they employ direct ABS control of both tandem drive axles. In the case
of a severe service truck or tractor with non-liftable auxiliary axles
mounted rearward of the tandem drive axles, an auxiliary ABS system may
be necessary on those auxiliary axles to meet the wheel lockup
provisions in S5.3.1, but trucks and tractors with liftable auxiliary
axles typically do not need to have ABS on those axles. In addition,
the braking-in-a-curve test in S5.3.6 was included in FMVSS No. 121 to
ensure that the ABS provides adequate vehicle control and stability
when in a curve on slippery pavement and subjected to a full-treadle
brake application. The braking-in-a-curve test ensures that the ABS is
regulating the braking forces at the wheels to keep the tires rolling,
so they can generate the lateral forces required for maintaining the
curve, and the vehicle does not plow out of the curve during braking.
    In addition, ABS systems can help greatly decrease the stopping
distances for lightly-loaded tractors. Since the addition of these ABS
requirements, conducting braking tests on trucks and buses in the
unloaded condition has been greatly simplified. Rather than requiring
the driver to modulate the brake treadle to try to achieve the required
stopping distance while staying within the wheel lockup provisions in
S5.3.1, the test driver can make a full treadle brake application at
the initiation of the stop and the ABS ensures that the wheel lockup