provisions are met. The result is much greater braking efficiency and
shorter stopping distances compared to driver-modulated stops. This is
evident by reviewing the VRTC test data for tractors tested in the
unloaded condition. Compared to the FMVSS No. 121 requirement of
stopping within 335 feet (unloaded condition), typical bobtail tractor
stopping distances for tractors with improved foundation brake systems
are approximately 180 feet, or 46 percent lower than the current 335-
foot requirement. As an example, VRTC tests of the tractors equipped
with hybrid disc/drum brakes and all-disc brakes resulted in unloaded
stopping distances ranging from 176 to 183 feet (six tests), meeting a
target stopping distance of 235 feet (a 30 percent reduction from the
current stopping distance requirement) with margins of compliance
ranging from 25 to 22 percent.
    It is likely that even current standard drum brakes have the
necessary torque to permit a substantial reduction in tractor stopping
distance in the lightly-loaded condition. VRTC tests of the 6x4 severe
service truck (used as a surrogate example of a severe service tractor)
with all disc brakes (224-foot loaded-to-GVWR stopping distance)
stopped in the lightly-loaded condition in 172 feet, meeting a target
distance of 235 feet with a 27 percent margin of compliance. Even when
tested with brake configurations that did worse in the loaded-to-GVWR
test condition (all drum brakes and disc/drum brake hybrid
configurations), the unloaded stopping distances were 172 feet and 178
feet. This indicates that stopping performance in the unloaded
condition for this severe service vehicle was not significantly
sensitive to the configuration of the foundation brakes, since any
combination of foundation brakes could fully utilize the available
tire-road friction of the vehicle in its light weight condition.
Further, it demonstrates that the ABS system (6S/6M on this vehicle)
delivered good efficiency in keeping the braking force near the peak of
available tire-road friction.
    Very few comments were received on the agency's proposal to reduce
the stopping distance in the lightly-loaded condition by 20-30 percent.
No test data were submitted on stopping performance of tractors
equipped with improved braking systems tested in the lightly-loaded
condition. Several commenters made recommendations. TMA and
ArvinMeritor recommended 25 percent reductions in lightly-loaded
stopping distances, and IIHS recommended a 30 percent reduction, but no
data were provided to support these recommendations. TMA stated that
currently under unloaded conditions, tractors experience some wheel
slip at brake applications of 30 psi, and that if the steer axle brake
is improved to meet a 30 percent reduction in stopping distance, rear
wheel slip might be experienced at as little as 20 psi. However,
considering that TMA is recommending a 25 percent decrease in stopping
distance in the unloaded condition, we believe the shorter stopping
distance achieved more than compensates for the slight increase in ABS
activations under these conditions.
    Based on the available data, the agency believes that a longer
lightly-loaded stopping distance is not necessary for the highest-GVWR
severe service tractors. Those vehicles have been provided with some
relief (310-foot loaded-to-GVWR stopping distance requirement, as
opposed to 250 feet) for tests in the loaded condition because of the
torque-generating limitations of foundation brakes. However, the agency
does not believe that any relief is needed for these tractors when
tested in the lightly-loaded condition. The definition of a ``truck
tractor'' in 49 CFR 571.3 specifies that it is ``primarily for drawing
other motor vehicles and not so constructed as to carry a load other
than a part of the weight of the vehicle and the load so drawn.''
Therefore, tractors in the lightly-loaded condition have extremely
light load weights relative to their GVWR since they do not have any
load-carrying capability outside of trailer towing. Tractors in the
lightly-loaded condition, including the heaviest GVWR severe service
tractors, can therefore achieve braking performance similar to each
other.
    In this final rule, the agency is setting the heavy truck stopping
distance requirement in the lightly-loaded condition at 235 feet, a 30
percent reduction from the existing FMVSS No. 121 requirement. The
available test data, while limited in terms of the number of tests
conducted, indicate that margins of compliance of 20 percent or more
can readily be attained. Severe service tractors that have lift axles
would be expected to perform similarly, as the lift axles would be in
the raised position during this test. To the agency's knowledge, severe
service tractors

equipped with improved brake systems that have non-liftable auxiliary
axles, or tridem drive axles, have not been tested, but are expected to
perform similarly or with only slightly longer stopping distances
(e.g., due to driveline and axle interactions on a tridem drive system,
or slightly lower tire traction due to aggressive off-road tread
patterns). However, due to the large margins of compliance already
achieved, the agency believes that the 235-foot requirement is
practicable for tractors that might have slightly longer stopping
distances than the typical examples tested.
    One minor issue that the agency is addressing is the lack of a fuel
tank fill specification in FMVSS No. 121. Vehicle curb weight is
measured with all fluid levels and reservoirs (e.g., antifreeze,
windshield washer fluid) at the recommended levels (i.e., filled to
capacity or other designated fill levels). The agency reviewed FMVSS
No. 121 for a specification on the vehicle's fuel tank fill level
during road tests and found that this is not addressed. In contrast,
FMVSS No. 135, Light Vehicle Brake Systems, specifies under the vehicle
test conditions in paragraph S6.3.2 that the fuel tank shall be filled
to 100 percent of capacity at the beginning of testing and that it may
not be less than 75 percent of capacity during any part of the testing.
    The agency is adding a similar requirement to FMVSS No. 121 in this
final rule. The lack of a fuel tank fill specification adds a possible
source of test variability, such as when testing in the lightly-loaded
condition where the additional weight of the fuel may be advantageous,
in that it may increase the tractor test weight and thus provide
additional tire friction at the drive axles. Therefore, by specifying
that the fuel tank(s) must remain at least 75 percent full during all
portions of the brake testing, test variability is reduced. Test
severity is not increased as a result of providing this specification.
We note that for the loaded-to-GVWR tests, this specification permits
up to 25 percent of the fuel to be used over the course of testing
without continually adding ballast or refueling the vehicle.
5. Emergency Braking Performance of Tractors With Improved Brake
Systems
a. Background Information on the Emergency Braking Performance
Requirement
    In the NPRM, the agency proposed to reduce the emergency braking
stopping distance in FMVSS No. 121 by 20 percent to 30 percent, from
the current 720 feet to a value between 580 feet and 504 feet. However,
in light of concerns raised in the comments, NHTSA has decided against
adoption of any change in the standard's emergency brake stopping
distance performance requirements.
    The emergency brake system requirements in FMVSS No. 121 are tested
by inducing a single failure in the service brake system of a part
designed to contain compressed air, excluding specific components
(i.e., a common valve, manifold, brake fluid housing, or brake chamber
housing).
    Test data from VRTC tests of tractors in the emergency braking mode
were provided in Table II of the NPRM. These tests were conducted with
failed primary systems, and, therefore, the data represent the
performance of tractors stopping using only the steer axle brakes. The
longest stops measured were with standard, 15'' x 4'' S-cam drum brakes
(636 feet for one tractor and 432 feet for the other tractor). As steer
axle brake improvements were made, emergency stopping distance also
improved. The best stops were with disc brakes on the steer axle (four
tests), which ranged from 276 to 303 feet, demonstrating very good
margins of compliance against the 720-foot FMVSS No. 121 requirement.
Thus, the agency's proposed requirements of 504 feet to 580 feet for
emergency brake stopping distance appeared to be achievable with
improved brake systems.
b. Commenters' Responses to Proposed Emergency Braking Performance
Requirement
    Several commenters (Bendix, ArvinMeritor, International)
recommended that the agency leave the standard's emergency brake
stopping distance requirements unchanged. Bendix argued that increasing
the torque output on foundation brakes would have a corresponding
decrease in emergency brake stopping distance, but only if the improved
brakes are used in the emergency stopping test. Thus, a tractor that
has had its steer axle brake improved to meet a 30 percent reduction in
stopping distance would exhibit no enhancement in emergency braking
performance if the front brake circuit (secondary air system) were
disabled. This would potentially cause the vehicle to fail that portion
of the emergency brake stopping distance test, even with improved
foundation brakes. Bendix stated that the agency has not provided
evidence of a need for improved emergency braking system performance in
its analysis. ArvinMeritor commented that emergency braking performance
in the failed secondary air system test (i.e., using only the drive
axle brakes, which have a very low weight when measured in the unloaded
condition) is already limited by tire-road adhesion today, thus making
further improvements impossible due to wheel lockup.
    In its comments, TMA stated that the emergency braking performance
of tractors with improved brake systems could lead to more aggressive
lockup of wheels on the drive axle(s) during emergency braking.
According to TMA, increased use of ABS could cause the emergency
braking performance with improved drive axle brakes to be worse than
with current foundation brakes. TMA stated that truck manufacturers
would need to modify the ABS algorithms to allow more drive wheel
lockup to meet the agency's proposed emergency brake stopping distance
requirements, and that this would be detrimental to vehicle stability
and control. Further, TMA commented that the likelihood of a crash-
imminent situation occurring at the same time as a failure in either
the primary or secondary air systems is immeasurably small.
    Although somewhat counterintuitive, the agency acknowledges that
the failed secondary system braking performance of tractors might be
negatively impacted by improved brake systems, as suggested by the
commenters. Accordingly, we have decided that not to make any changes
in the emergency brake system stopping distance requirements at this
time. Maintaining the status quo for emergency brake stopping
requirements is not expected to have any negative effect on achieving
the estimated safety benefits of the overall heavy truck stopping
distance rulemaking, because tractors operating in bobtail mode and
experiencing an emergency braking situation are not significant
contributors to the crash problem that has been identified.
ii. Ancillary Issues Arising From Improved Brake Systems
1. Stability and Control of Tractors With Improved Brake Systems
    Several commenters (TMA, HDBMC, ATA) brought up a number of issues
relating to the stability and control of tractors that arise from
installation of improved brake systems pursuant to the agency's
proposal to improve heavy truck stopping distance performance
requirements by 30 percent. These issues included potential problems
with lateral stability (especially in two-axle, short wheelbase
tractors), excessive steering wheel pull, and excessive steer axle
suspension jounce (compression). Commenters stated that these problems

would be expected to apply to all tractors, but commenters expressed
their opinion that such problems would likely be especially acute in
two-axle tractors, particularly in those with a shorter wheelbase.
    In a meeting held with NHTSA on March 29, 2006, representatives of
TMA, HDBMC, and ATA discussed several issues involving tractors with
improved brake systems that were included in presentation materials
available for review in the DOT docket.\44\ One issue raised in that
presentation involved computer simulations provided by TMA which were
conducted by Freightliner of two tractors in a braking-in-a-curve
maneuver (see Slide 10). In that maneuver, the tractor with more
powerful foundation brakes (a hybrid configuration of front disc brakes
and rear drum brakes) experienced a jackknife loss-of-control, while
the tractor with standard drum brakes remained stable. According to
TMA's comments, this indicated that installing more powerful foundation
brakes to improve performance in the straight-line stopping distance
test could have the unintended consequence of inducing stability
problems in some on-road driving situations. Thus, TMA raised concerns
about the stability and control of short-wheelbase two-axle tractors
when more powerful foundation brakes are applied. Although not depicted
in the presentation slides, the following were the test conditions for
the above scenario, as described by TMA at the meeting:
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    \44\ See Docket No. NHTSA-2005-21462-33.
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     The curve has a radius of 500 feet and was a high-friction
dry surface (0.9 peak coefficient of friction).
     The entry speed of the tractor was 48 mph.
     The tractor was connected to a tandem-axle trailer, and
the trailer was rear-loaded to 34,000 pounds weight on the trailer
axles.
     The trailer was unbraked.
     A full-treadle brake application was used.
    While the maneuver described by TMA has some similarities to the
FMVSS No. 121 stability and control test requirement that is used as a
pass-fail measure to assess the performance of a tractor's ABS (see
S<IP_ADDRESS>), the agency does not believe that the TMA test is appropriate
for assessing the vehicle's stability, due to vital differences in the
test procedures, as explained below. In the FMVSS No. 121 test, the
road surface is wetted and slippery (0.5 peak coefficient of friction
as opposed to 0.9), and the entry speed is typically between 30 and 34
mph, as opposed to 48 mph. The loading condition of the trailer in the
FMVSS test is also different. Although an unbraked FMVSS No. 121
control trailer is used, in the FMVSS test, the trailer is front loaded
(i.e., loaded over the kingpin at the front of the trailer) in order to
load the tractor to its GVWR. In contrast, in the TMA test, the trailer
was rear loaded, which puts the majority of the weight on the unbraked
trailer axles rather than the tractor's drive axles. This maneuver
deprives the drive axles of braking traction, and constitutes a worst-
case braking scenario.
    At the March 29, 2006 meeting, the agency questioned whether TMA's
simulation is representative of a real-world driving situation. As
explained below, the simulation appeared to the agency to be a
combination of several worst-case scenarios, the first of which
involves the high entry speed of the tractor that, for this curve,
approaches the rollover threshold of some high-center-of-gravity
tractor-trailer combinations. Second, the trailer is rear-loaded, which
is not a safe operating practice. (In general, trailers should not be
rear-loaded because the tractor drive axles will be too light during
braking and/or acceleration.) Third, the trailer brake system was
deactivated. Finally, the test assumes a full-brake application which,
on the highway, represents a panic braking situation. As a result, the
agency is not convinced by TMA's comment that improving the steer axle
brakes will have a negative impact on lateral stability.
    The agency has further reason to doubt TMA's assertion that lateral
stability will be negatively impacted by improving the tractor's
foundation brakes. In its comments, TMA referred to a Society of
Automotive Engineers (SAE) technical paper, A Study of Jackknife
Stability of Class VIII Vehicles with Multiple Trailers with ABS Disc/
Drum Brakes (SAE 2004-01-1741). TMA stated that this study, consisting
of vehicle simulation modeling to evaluate the stability of two-axle
tractors towing double trailers, found that two-axle tractors with more
aggressive brakes either jackknifed or ran off the road under various
combinations of conditions. However, based upon the agency's review,
that study seems to indicate that more powerful foundation brakes were
not a cause of the jackknifing, but rather that the cause was a lack of
tractor ABS. In analyzing this SAE report, the agency notes that only
when the tractor ABS was disabled did instability occur, and it
occurred regardless of whether the tractor was equipped with S-cam drum
brakes or disc brakes. However, the type of instability exhibited
varied depending upon the types of foundation brakes installed on the
tractor; specifically, tractors with all drum brakes went into a
jackknife (oversteer), while the tractors with disc brakes tended to
plow out of the curve (understeer).
    The only benefit of less powerful brakes indicated by the tractor
simulations with inoperative ABS was that the lane departure occurred
sooner in the maneuver when the tractor was equipped with disc brakes.
We do not believe that this argument justifies a requirement that would
result in installation of weaker foundation brakes. Instead, we believe
that this study is more indicative of the importance to fleets in
maintaining ABS on tractors, trailers, and converter dollies. It is
also important to note TMA's comment that 4 to 16 percent of tractors
and 8 to 26 percent of trailers in service have non-functioning ABS or
ABS warning lamps. While this rulemaking does not relate to in-service
maintenance issues (issues which generally fall under FMCSA's
jurisdiction), proper maintenance is very important.
    The agency conducted an additional investigation to determine the
validity of the TMA testing regarding lateral instability. To further
investigate suggestions regarding the potential for increased lateral
instability, the agency held a meeting with the TMA at the VRTC in East
Liberty, Ohio, on July 11, 2006.\45\ At that meeting, the agency
presented results of several braking-in-a-curve simulations performed
at VRTC using its Truck Sim vehicle dynamics modeling software to
estimate the scope of potential vehicle instability problems for two-
axle tractors. In a high-friction (i.e., 0.9 coefficient of friction,
or mu), 500-foot radius curve braking test with a rear-loaded, unbraked
trailer, a two-axle tractor with a very short wheelbase of 130 inches
experienced a jackknife condition. Two other tractors with short
wheelbases (142 and 148 inches) were marginally stable, meaning they
were not under full control, but did stay within the 12-foot-wide lane.
For comparison purposes, we note that a three-axle tractor with a 190-
inch wheelbase remained stable during this maneuver. The agency also
performed slippery-surface (low friction) tests at 45 mph, and found
that a short-wheelbase tractor (148 inches) spun out both with standard
drum brakes and with disc brakes. This test also caused a standard
three-axle tractor (with drum brakes) to spin out. For a final
comparison, we

note that during a previous track test, even a high-performance sports
car spun out during this maneuver at 45 mph. Again, these results
demonstrated to the agency that the TMA test was too rigorous for any
typical vehicle to be able to navigate the curve.
---------------------------------------------------------------------------

    \45\ Docket No. NHTSA-2005-21462-36.
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    Further, we note that in its supplemental comments from October
2006, TMA submitted information about tests on four two-axle tractors
that showed substantially fewer problems of lateral instability than
had been suggested earlier. The results of these tests showed that two-
axle tractors are capable of maintaining a high degree of lateral
stability when equipped with improved foundation brakes. TMA
acknowledged that these vehicles did not exhibit controllability or
handling problems. Nonetheless, TMA suggested in its supplemental
comments that due to the relatively large amount of testing and
validation required for issues such as brake lining, brake chamber
sizes, slack adjuster lengths, tire properties, ABS algorithms, and
potentially electronic stability control (ESC) systems, additional lead
time for two-axle tractors may be required.
    In the end, after considering all of the available information on
stability and control that affects shorter wheelbase, two-axle
tractors, the agency has decided that an allowance for longer stopping
distances is unnecessary. Only under the most severe conditions was
instability found to be an issue, and rarely did it correlate with the
improved braking systems. Nonetheless, the agency is aware that there
is a greater need for additional design efforts and validation on two-
axle tractors, so in this final rule, we are providing more lead time
for manufacturers to achieve compliance with the new stopping distance
requirements for these tractors, thereby providing manufacturers with
more time to identify and remedy potential problems. (The issue of the
compliance date is addressed in further detail below in Section III, c,
viii.)
2. Brake Balance Issues on Tractors With Improved Brake Systems
    Because the main factor in generating the additional brake torque
to achieve a reduced stopping distance is the addition of more powerful
steer axle brakes, the effects of more powerful steer axle brakes are
raised by this rulemaking. These issues involve the balance of braking
power generated by different tires, as well as concern that the new
designs could engender off-balance brake systems. Two issues raised
included the difference in brake torque generated by the steer and
drive axles, and the potential for increased steering wheel pull
resulting from more powerful steer axle brakes. The agency addresses
each of those concerns below.
    Several commenters asserted that the mandate to decrease stopping
distance would necessitate less powerful drive axle brakes on two-axle
tractors, because dynamic loading would cause the weight on the drive
axle to be substantially less during hard braking.\46\ Freightliner
commented that because 31 percent of the rear axle load will transfer
to the steer axle during hard braking, two-axle tractors will require
less powerful drive axle brakes than they currently have. While
Freightliner did not provide a rationale for this in its comment, it is
presumed that this would be to improve brake balance at maximum
braking, without having to cycle the ABS on the drive axle. Similarly,
ATA commented that it may be necessary to reduce drive axle brake power
on two-axle tractors to compensate for the weight transfer to the steer
axle. In its original comments, TMA also argued that decreasing the
drive axle torque by 20 percent would be necessary to prevent ABS
activation, which could result in even longer stopping distances. All
of these commenters argued that the combination of much more powerful
steer axle brakes and less powerful drive axle brakes would result in a
vehicle that would perform poorly under real-world conditions, arguing
that the agency should not consider the issue of stopping distance in
isolation.
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    \46\ ``Dynamic Loading'' refers to the temporary redistribution
of downward force during a hard braking incident. During rapid
deceleration, proportionally more weight is borne by the front of
the tractor (the steer axle) and less is borne by the rear (the
drive axle and the trailer axle). In two-axle tractors, where
proportionally more weight is borne by the steer axle than in other
designs, the concern is that during hard braking, too little weight
will be borne by the drive axles, and the available tire-road
friction will not be high enough to allow them to utilize all of the
available brake torque. In these situations, the ABS would be
activated, lessening those brakes' effectiveness.
---------------------------------------------------------------------------

    The agency's test data, however, do not fit with these statements.
The agency's data indicate that a reduction in drive axle torque would
not be necessary to improve stopping distances in hard-braking
situations. Test data from VRTC \47\ tests on a two-axle tractor showed
that after installing more powerful steer axle disc brakes, installing
more powerful drive axle brakes only served to shorten overall stopping
distance. The agency also notes that this improvement occurred without
stability or control problems when tested both in the lightly-loaded
and loaded-to-GVWR conditions as specified in the FMVSS No. 121
braking-in-a-curve test. In nearly every test, whether using two-axle,
three-axle, or severe service tractors, the tractors that achieved the
shortest stopping distances were those equipped with more powerful disc
brakes at all wheel positions. In all tests, the ABS was found to
perform very efficiently in limiting wheel lockup and allowing tractors
with improved braking systems to maintain good stability in both
straight line and braking-in-a-curve tests.
---------------------------------------------------------------------------

    \47\ Docket NHTSA-2005-21462-39, p. 25.
---------------------------------------------------------------------------

    On a related topic, TMA also commented that more powerful steer
axle brakes could contribute to instability through steering wheel
pull. Steering wheel pull can occur when the steer axle brake on one
side of the vehicle is able to produce more braking power than the
brake on the other side. This is an issue that affects all tractors
with enhanced steer axle brakes, not just two-axle tractors. TMA stated
that on ``split-mu surfaces,'' i.e., ones where one side of the road
has less friction than the other (such as transitional surfaces, or
when one side of the road is wet), imbalances in steer axle brakes are
magnified and drivers must provide sufficiently more frequent and
aggressive steering wheel input to keep the vehicle on its intended
path. TMA argued that if the power of the steer axle brakes were
increased, the potential effects of side-to-side imbalance would also
increase.
    The agency believes that disc brakes, in general, will provide
better steer axle brake balance than current standard drum brakes do.
This is because for any given air pressure, the torque output of drum
brakes can vary by 30 percent due to hysteresis,\48\ lining variations,
brake adjustment, and drum condition (e.g., eccentricity and being out-
of-round). In comparison, for any given air pressure, disc brakes
typically do not have variations in torque output exceeding 10 percent.
Thus, in a tractor with two disc brakes on the steer axle under
braking, there would typically be less steering wheel pull during
braking, as compared to a tractor using drum brakes. However, the
agency is aware that if a manufacturer chose to upgrade the steer
brakes to enhanced S-cam drum brakes, there is a potential for more
steering wheel pull than with standard S-cam drum brakes.
---------------------------------------------------------------------------

    \48\ Hysteresis refers to friction in the foundation brake
components.
---------------------------------------------------------------------------

    Steering wheel pull on split-mu road surfaces is a potential
problem with any type of brake (although most significantly with
enhanced drum brakes), but there are various steps that

manufacturers can take to ameliorate the problem. One approach is to
utilize a modified individual wheel ABS control strategy to reduce the
pressure to both steer axle brakes in the event the wheel on the low-
friction surface approaches lockup. In its comments, Meritor Wabco
stated that most of today's antilock systems use Modified Individual
Regulation (MIR) on the steer axle to reduce the yaw moment produced
when different levels of torque are generated by the steer axle brakes,
a situation that typically occurs during braking on split-mu surfaces.
According to the commenter, after a short amount of time, the pressure
can be adjusted to match the friction at each wheel. This action can
result in steering wheel pull, but it is added incrementally, so it
does not surprise the driver. This method of ABS control ensures that
the driver is able to easily control the vehicle during the maneuver,
and it also produces a shorter stopping distance by taking advantage of
the higher braking forces generated by the wheel on the high friction
surface. Thus, the agency believes that the potential for additional
steering wheel pull is small, and when combined with advancements in
ABS and the use of disc brakes, we have decided that this is not a
reason to adopt a less stringent stopping distance requirement.
3. Brake Balance and Trailer Compatibility Issues for Tractors With
Improved Brake Systems
a. Brake Balance Between the Steer and Drive Axles
    ``Brake balance'' refers to the concept that brakes on the steer
axle and drive axle(s) should provide approximately equal shares of the
retardation force in response to the dynamic loads placed on them
during hard braking. Currently, the drive axle brakes of many tractors
produce a large percentage of the total brake torque during heavy
braking, as steer axle brakes are designed for long life. When
addressing the issue of good brake balance on a tractor that is loaded
to its GVWR and subjected to a full treadle brake application, the
agency must take into account that the vertical load on the steer axle
can increase by up to 50 percent or more. It is therefore expected that
manufacturers will meet the reduced stopping distance requirements in
this rulemaking primarily by improving the brake torque of steer axle
brakes, thus allowing good brake balance during hard braking
applications.
    The agency notes that a bobtail tractor (i.e., with no trailer)
will generally have poor brake balance. This is because the drive axles
have a very low vertical loading, while the steer axle is typically
closer to its rated capacity. In that case, a tractor is reliant on its
ABS to prevent drive axle wheel lockup during moderate and hard brake
applications. This rulemaking will not have a substantial effect on the
brake balance of tractors operated in the bobtail condition.
    Achieving the desired loaded-to-GVWR, limit-of-performance stopping
distance reduction, as well as brake balance, will generally require
upgrades to both the steer and drive axles of a truck tractor. The
benefits of this rulemaking will primarily be achieved by increasing
the steer axle brake power on tractors. As previously discussed, small
improvements are also likely to be needed on tractor drive axles, as
test data show there were no tractors complying with 30 percent
reductions in stopping distance, with good margins of compliance, using
standard-sized 16.5'' x 7'' drive axle S-cam drum brakes. Agency
testing has shown that increasing the drive axle brake power allows
better utilization of the available tire friction and reduces brake
fade during a single high-speed stop and also during repetitive stops
at all speeds.
    Several organizations commented on the issue of brake balance
between the steer and drive axles. HDMBC stated that improvements in
brake torque will mainly be on the steer axles of tractors, and this
will result in the steer axle doing a larger share of combination
braking work that could affect brake wear balance. However, HDBMC did
not recommend that NHTSA take any particular regulatory action in light
of this. Haldex stated that more evaluation will be needed to determine
the effects of improved braking systems on brake balance.
    The agency agrees that the majority of improvements in tractor
braking performance will be gained by significant increases in steer
axle brake torque. The agency believes that this will result in
improvements in the tractor's brake balance during maximum effort
braking, as under current conditions, standard steer axle brakes do not
have the same power as drive axle brakes. The agency also believes that
modest increases in tractor drive axle brake torque will be necessary
for most tractors, but we do not think that this will cause significant
brake balance issues, as some commenters argued. In reaching this
conclusion, the agency notes that the available test data show that one
of the best-performing three-axle tractors (used in the Radlinski
tests) was a tractor currently used in regular fleet service, so we
presume that this vehicle exhibited acceptable brake balance in terms
of both performance and maintenance costs. We also note that the
enhanced drive axle drum brakes on this tractor (16.5'' x 8.625'') were
primarily designed for long service life. This is achieved by operating
at lower temperatures during low-pressure braking, thereby reducing
lining wear that is temperature-sensitive.
    In its comments, ArvinMeritor argued that reductions in stopping
distance of over 25 percent would adversely impact brake balance and
would likely result in significant dissatisfaction on the part of end
users. ArvinMeritor stated that these concerns specifically include
brake lining life reductions, brake drum durability problems, more
frequent maintenance, and reduced vehicle uptime as a result of these
issues. ArvinMeritor also stated that tractor-trailer compatibility
will be a significant issue if the standard were to require stopping
distance to be reduced by more than 25 percent from current levels. The
commenter claimed that the mixing of new truck tractors with either new
or old trailers would represent a real and disruptive issue for the
trucking industry, although it failed to state why it would cause
disruption.
    Without any supporting data for ArvinMeritor's comment, the agency
cannot accept its above-stated position, particularly given the
substantial evidence in the record that tractor-trailer compatibility
will not be negatively affected by the improved foundation brake
systems on new truck tractors. Although the agency is not aware of any
published reports on the compatibility issue of tractors with improved
brake systems being used with the existing trailer fleet, we note that
the tests conducted by Radlinski (using a three-axle tractor with
enhanced S-cam drum brakes on both the steer and drive axles) were with
a production vehicle used in regular fleet service. Those tests were
conducted in 2003, and tractors such as the one tested have been in use
since at least that time, with no indications of brake balance or
trailer compatibility problems of which the agency is aware. Further,
in 2004, the agency (in concert with other government agencies and
private industry partners under cooperative agreement contract)
completed field tests of 50 Volvo three-axle tractors equipped with
disc brakes in regular fleet service.\49\ The disc brakes were one
component of several crash avoidance enhancement systems installed on
these tractors. No compatibility or brake

balance issues were found among these vehicles during extensive
operation with trailers equipped with standard, 16.5'' x 7'' S-cam drum
brakes. Brake lining wear rates on the tractors were lower than those
of standard drum brake components, and similar to the wear rates of
extended life (enhanced) S-cam drum brakes.
---------------------------------------------------------------------------

    \49\ See http://www.itsdocs.fhwa.dot.gov/JPODOCS/REPTS_TE/
14349.htm.
---------------------------------------------------------------------------

b. Tractor-Trailer Compatibility
    ``Tractor-trailer compatibility'' is closely related to brake
balance and has a similar definition. Traditionally, that term has been
defined to mean equal truck tractor drive axle brake operating
conditions and life relative to trailer axle brake operating conditions
and life. The compatibility issue is important for end-users of
tractor-trailers, as they desire even wear on trailer and tractor drive
axle brakes. One commenter, ArvinMeritor, stated that typically,
tractor-trailer compatibility does not include steer axle brakes, due
to comparatively lower torque output and resulting longer life compared
to the other brakes in the combination. The agency understands that
under the current stopping distance requirements, typical steer axle
drum brakes (15'' x 4'') have comparatively low torque output and long
life compared to brakes at other wheel positions.
    Several commenters argued that the majority of braking takes place
at pressures between 10 psi and 15 psi, as opposed to full treadle
brake applications. HDMBC commented that at these pressures, balanced
brake wear is expected between the truck tractor and trailer by the end
user. HDBMC stated that further evaluation may be needed in light of
the increased percentage of braking contributed by the truck tractor.
    Similarly, many commenters discussed how the improved stopping
distance requirements in the agency's proposal would require the
tractor to take on an increased percentage of the total braking of the
truck tractor/trailer combination. Haldex and HDBMC both raised this
issue, although neither recommended that NHTSA take any particular
regulatory action in light of this issue. HDBMC stated that its purpose
in commenting on this issue was to highlight the impact that reduced
stopping distance requirements will have on maintenance costs and end-
user acceptance of new vehicles, while Haldex merely stated that brake
balance will require more evaluation.
    ATA commented that tractor-trailer compatibility should not be an
issue if stopping distance were reduced by only 20 percent. However,
ATA did not comment on potential compatibility issues for a 30 percent
reduction. ATA stated that in the case of two-axle and severe service
tractors, there could be operational or safety issues associated with
the reduced stopping distance proposal, and, therefore, a delay in the
implementation of new requirements for those vehicles would be needed
to overcome these issues.
    ArvinMeritor relayed significant concerns regarding tractor-trailer
compatibility in its comments. ArvinMeritor stated that reductions in
stopping distance of up to 25 percent can be achieved without
sacrificing brake balance or tractor-trailer compatibility. It stated
that this is because that level of reduced stopping distance can be
achieved by only increasing steer axle brake torque. However, it stated
that for reductions of over 25 percent, increases in tractor drive axle
torque will be necessary, and that this will adversely impact brake
compatibility and result in more frequent brake maintenance and reduced
vehicle uptime. Arvin Meritor stated that it does not have enough
information on the compatibility of tractors with air disc brakes when
operated with the existing trailer fleet to provide more specific
comments.
    NHTSA does have testing information on disc brakes, and after
evaluating that data, the agency believes that disc brakes installed on
a typical three-axle tractor's drive axles would not have detrimental
brake balance issues during braking. Dynamometer testing was performed
at VRTC on two brands of 16.5'' x 7'' S-cam drum brakes and two brands
of air disc brakes (one 16.93'' rotor diameter x 1.77'' rotor
thickness, the other 16.90'' x 1.77'') \50\ to quantify such
characteristics. In one comparison of an S-cam drum brake to a disc
brake, similar torque outputs were produced when each brake was stopped
on the dynamometer from an initial speed of 30 mph. However, when
stopped from a high speed of 70 mph, the S-cam drum brake lost 42
percent of its maximum effectiveness while the disc brake lost only 24
percent of its maximum effectiveness. Such a disc brake, when installed
on a typical tractor drive axle, would not be expected to have
detrimental brake balance issues under normal, low-pressure braking
because the torque output is similar to the drum brake. In addition, it
provides much shorter stopping distance when under hard braking from
highway speeds because of reduced brake fade.
---------------------------------------------------------------------------

    \50\ SAE Technical Report, Comparison of Heavy Truck Foundation
Brake Performance Measured with an Inertia Brake Dynamometer and
Analyses of Brake Output Responses to Dynamic Pressure Inputs (SAE
Report No. 2005-01-3611, Hoover and Zagorski, Transportation
Research Center, Inc.). Available from SAE, and the report is
available for review at NHTSA's Technical Reference Division.
---------------------------------------------------------------------------

    There is also the possibility that the drive axle can be equipped
with an enhanced S-cam drum brake instead of an air disc brake, as it
would be in a hybrid or all-drum brake configuration. While the agency
has not completed sufficient testing of enhanced drive/trailer axle S-
cam drum brakes (either 16.5'' x 8'' or 16.5'' x 8.625'') under its
dynamometer test program at VRTC to determine the reasons for improved
torque generation, it is likely that the wider brake drum has increased
thermal capacity. This is because the total friction between the lining
and the drum would take place spread out over a larger area. Therefore,
during a single, 60 mph stop, experience has shown that there would be
less fade than for a standard 16.5'' x 7'' axle brake. The agency may
conduct future dynamometer testing at VRTC to determine in further
detail the characteristics of the enhanced S-cam tractor drive axle
drum brake. Currently, however, the agency refers back to the use of
the in-service truck tractor used in the Radlinski tests (which used
enhanced drum brakes) as evidence of the lack of significant brake
balance issues using enhanced S-cam drive axle drum brakes.
c. Brake Balance and Trailer Compatibility Issues for Two-Axle and
Severe Service Tractors
    NHTSA does not believe that two-axle or severe service tractors
will have problems with regard to brake balance and trailer
compatibility.
    There were no comments regarding tractor-trailer compatibility for
two-axle tractors, although Freightliner expressed concern that two-
axle tractors may suffer from tractor-trailer compatibility problems of
reduced balance when used with existing trailer brakes. The agency is
aware of little data on the brake balance and trailer compatibility
issues for two-axle tractors with improved brake systems, and most of
the comments on two-axle tractors were concerns with stability and
control rather than issues of balance between steer and drive or
tractor and trailer brakes. NHTSA is aware that some two-axle tractors
are already being equipped with larger 16.5'' x 5'' steer axle S-cam
brakes, and presumably these brakes are providing satisfactory brake
balance trailer compatibility in fleet service. While test data cited
above shows that two-axle tractors can attain the reduced stopping
distances using disc brakes on the steer and drive axles, that data did
not consider compatibility with existing

trailers (and converter dollies, as two-axle tractors are often used in
double- or triple-trailer combinations).
    Given the lack of data or other evidence of a problem, we think
that Freightliner's arguments in this context involve speculative
concerns; consequently, the agency currently has no reason to believe
that two-axle tractors with improved brake systems will have
compatibility issues. Nonetheless, considering the complexity of brake
system interactions and the current lack of available data (as well as
for many other reasons, discussed at length below), the agency has
decided to provide longer lead time for the requirements of this final
rule for two-axle and severe service tractors so as to provide four
years of lead time. This will provide truck manufacturers time to
develop designs that do not have problems in this area.
    The agency similarly received few comments regarding trailer
compatibility for severe service tractors. However, both TMA and
Freightliner stated that some heavier severe service tractors are
limited to low speeds when fully loaded, and if such a tractor were
required to comply with shorter stopping distances from 60 mph, the
brakes would be over-designed (i.e., be too powerful for their typical
usages). At highway speeds with light loads, this could result in
excessive wheel lockup.
    The agency has already partially addressed this issue by providing
a longer, 310-foot stopping distance requirement for high-GVWR severe
service tractors. We understand that many of the severe service
tractors that require escort vehicles and low speeds when loaded to
GVWR fall into this category, or have a GAWR over 29,000 pounds, and
thus are excluded from FMVSS No. 121 entirely. In addition, because the
overall brake balance problem for the widely-varying loading condition
already exists for these vehicles, we believe that installation of
improved brake systems on severe service tractors would have only an
incremental (and minimal) effect on brake balance and trailer
compatibility.
iii. Cargo Securement
    A comment from OOIDA stated concern that the proposed requirement
of shorter stopping distances would increase the g-forces acting upon a
truck's load to the point where such forces exceed the conditions
specified in standards for cargo securement under Federal Motor Carrier
Safety Administration (FMCSA) regulations. Under the relevant
provisions of FMCSA's cargo securement requirements, 49 CFR
393.102(a)(1) provides that tiedown assemblies (including chains, wire
rope, steel strapping, synthetic webbing, and cordage) and other
attachment or fastening devices must be designed, installed, and
maintained to ensure that the maximum forces acting on the devices do
not exceed the manufacturer's breaking strength under a 0.8g
deceleration in the forward direction. These requirements were adopted
in a September 27, 2002 final rule (67 FR 61212) and became effective
on January 1, 2004. The purpose of this FMCSA requirement is to reduce
crashes caused by incidents of shifting and falling cargo.
    In response to OOIDA's comment, the agency reviewed deceleration
rates from tractor tests with improved brake systems to determine
whether the cargo securement limits had been reached. Agency testing
indicated that under FMVSS No. 121 testing in the loaded-to-GVWR
condition with an unbraked control trailer, deceleration rates of
approximately 0.65g were typical. However, as noted by Freightliner in
its comments, such a tractor is capable of higher deceleration rates
when operating with a normal load on a braked trailer. Freightliner
stated that tests of such a combination vehicle showed that it was able
to stop in 187 feet from a speed of 60 mph, but did not provide
deceleration data for this test.
    After reviewing the previously-discussed data from VRTC, NHTSA
believes that trailers will not exceed FCMSA's cargo securement
requirement. The agency analyzed stopping data for a two-axle tractor
equipped with disc brakes at all wheel positions, towing a 53-foot van
trailer which was also equipped with disc brakes. The tractor and
trailer had normal ABS control of all wheels, and had the shortest
measured stopping distance of all tractor-trailer combination tests at
VRTC. In the test, the tractor steer axle was loaded to 11,000 pounds;
its drive axle was loaded to 22,700 pounds, and the tandem trailer
axles were loaded to 34,000 pounds (loaded-to-highway weight). This
combination stopped from 60 mph in a distance of 186 feet. NHTSA
reviewed the deceleration rate during the stop and notes that
deceleration was fairly constant at approximately 0.8g once steady-
state deceleration was achieved (approximately 0.6 seconds after the
full treadle application).\51\ We do note that there were momentary
spikes of higher and lower deceleration (typical for data traces of
this type), with the highest peak at 0.89g for a very short duration.
However, the accelerometer was mounted on the tractor frame, and it is
NHTSA's belief that the acceleration peaks were anomalies likely due to
vibration, as it would not possible for a massive object such as a
loaded tractor or trailer to have acceleration rate changes indicated
by the peaks. Therefore, the agency has concluded that the highest
deceleration rate by a tractor with improved brakes was slightly below
0.8g, thus remaining under the deceleration specified by FMCSA's cargo
securement requirement.
---------------------------------------------------------------------------

    \51\ Docket  NHTSA-2005-21462-39, p. 28.
---------------------------------------------------------------------------

    The agency also reviewed deceleration data for the VRTC test
tractor in the unloaded condition, and we arrived at similar
conclusions. The unloaded stopping distance for this tractor-trailer
combination was 191 feet (a longer stopping distance than 187 feet, and
thus producing even less g-forces on deceleration), which indicates
that both in the loaded and unloaded condition the limits of tire
adhesion have been reached. The slightly longer stopping distance in
the unloaded condition is likely due to additional cycling of the ABS
on both the tractor and trailer compared to the loaded-to-highway
weight testing.
iv. Testing Procedures
1. Brake Burnish Issues for Tractors With Improved Brake Systems
    As discussed in this section, brake burnishing is the process of
wearing in the friction components of foundation brakes (brake linings
and brake drums or disc rotors), which is necessary to allow the
friction surfaces to reach a close-to-normal operating condition prior
to conducting stopping distance and grade-holding tests. Currently, in
FMVSS No. 121, the burnish procedure is specified in S6.1.8. This
procedure involves subjecting a tractor to a series of 500 brake
``snubs'' (i.e., applications of the brake) from an initial speed of 40
mph to a final speed of 20 mph. Virtually all heavy vehicles (trucks,
tractors, and buses) use this burnish procedure. Prior to September 1,
1993, vehicle manufacturers were able to use an alternate burnish
procedure, which conducted the snubs from higher initial speeds.\52\
The primary difference between these two procedures is the temperature
at which the brake operates during the burnish. The current procedure
is frequently referred to as a ``cold burnish,'' because the brake
temperatures typically reach only 300-400 degrees Fahrenheit (F),
whereas the

old procedure is known as a ``hot burnish,'' as the temperatures
typically reached 500 degrees F or more. The reason the agency changed
from the hot to cold burnish procedure is that when heavy vehicles are
operated on the road under normal conditions, the brakes may never
reach the same temperatures that are reached under the hot burnish
procedure. Therefore, the real world brake performance may have been
lower than that tested under FMVSS No. 121 before September 1993.
---------------------------------------------------------------------------

    \52\ See 53 FR 8190.
---------------------------------------------------------------------------

    In the March 14, 1988 final rule establishing the brake burnish
procedures, NHTSA stated that given ``consistent research findings
about the temperatures to which drum brakes are subjected during normal
driving, the agency concludes that a burnish that subjects drum brakes
to significantly higher temperatures cannot be said to be
representative of normal driving conditions. By allowing the drum
brakes to be heated to temperatures well in excess of those encountered
during normal driving, the burnish procedures would ideally condition
the drum brakes. However, the agency is more interested in the braking
capability of vehicles when the brakes are in the condition they are
most likely to be when used on the roads than in the maximum braking
capability of a braking system if the brakes are ideally conditioned.''
See 53 FR 8194.
    Several commenters recommended that changes to the burnish
procedure be made in relation to the agency's overall efforts to
achieve a reduction in stopping distances for truck tractors.
Specifically, comments on this issue were raised by HDBMC, which
recommended changes to the current burnish procedure that would allow
the brake linings to be burnished at higher temperatures than the
current burnish procedure produces (essentially a return to the pre-
1993 requirements). While the agency has considered the comments
relating to burnish procedure, it has decided not to make any changes
to that procedure in this rulemaking, for the reasons that follow.
    HDBMC recommended in its comments that NHTSA reinstate an optional
temperature in FMVSS No. 121, as permitted prior to September 1, 1993,
to use the hot burnish procedure. HDBMC stated that in order to achieve
the proposed reduction in stopping distance, many tractors will be
equipped with higher torque steer axle brakes. In addition, the
commenter stated that there tractors will also likely be equipped with
wider rear axle brakes (arguing that because NHTSA is mandating a 30-
percent reduction in stopping distance, most vehicles will be using
wider drive axle drum brakes or disc brakes). As a result, the
commenter reasoned that steer axle brakes will do more of the work
during burnish, thus lowering the temperature on the drive axle brakes.
If wider drive axle drum brakes are used, HDBMC continued, this will
result in further lowering of the drive axle brake temperatures. These
lower temperatures could result in insufficient brake burnishing on the
drive axle brakes. If this were the case, higher friction brake linings
on the drive axle brakes may be required, likely resulting in higher
maintenance costs and less end-user satisfaction.\53\ Further, HDBMC
indicated that the decreased lining contact on the drive axles may
negatively impact parking brake drawbar pull performance. HDBMC
provided an example where a tractor with standard (15'' x 4'') steer
axle drum brakes was able to achieve 8,800 pounds of parking brake
force, while with enhanced (16.5'' x 5'') steer axle drum brakes it
produced only 8,000 pounds of force.
---------------------------------------------------------------------------

    \53\ According to comments by TMA, aggressive high friction
brake linings designed to meet strict performance criteria can
produce unsatisfactory results when used in real-world applications.
For example, in one scenario, TMA suggested that overly aggressive
brake linings could glaze over under normal use conditions. This
could lead to brake chatter and the subsequent failure of numerous
components. TMA Comment from April 14, 2006, available at NHTSA-
2005-21462-34).
---------------------------------------------------------------------------

    According to HDBMC, therefore, if NHTSA required the improved
stopping distances without altering the burnish procedure to provide
better burnishing, vehicle manufacturers would have to provide highly
unsatisfactory brake linings in order to meet the standard, which would
be unfit then for on-road use. Therefore, HDBMC suggests that the
burnish procedure be altered.
    As discussed in the rulemaking cited above concerning burnish, the
agency believes it is appropriate to test the braking capability of
vehicles when the brakes are in the condition they are most likely to
be when used on the roads. For this reason, we do not believe it would
be appropriate to modify the burnish procedure so that it is less
reflective of the conditioning experienced by brakes in the real world.
However, we have analyzed whether the proposed reduced stopping
distance requirements, coupled with the ``cold burnish'' procedure,
would result in the problems suggested by HDBMC. For reasons discussed
below, we believe these problems will not occur.
    NHTSA has reviewed the agency's data from the Radlinski testing in
order to consider this issue. This test used the current cold burnish
procedure in preparation for testing a typical three-axle tractor with
enhanced S-cam drum brakes at all wheel positions, and that vehicle
achieved a 30-percent reduction in stopping distance with a good margin
of compliance. Based on the review of all of the test data for this
vehicle, as well as the simple fact that the vehicle was able to
achieve the required stopping distances using the cold burnish
procedure, the agency concluded that the current procedure adequately
conditioned the foundation brakes in preparation for conducting the
remainder of the FMVSS No. 121 test sequence.
    A review of the three-axle tractor tests conducted by Radlinski
provides insight into the brake lining condition and temperatures of
improved braking systems during and after the cold burnish procedure.
Comparing two tests using the same brake lining (Spicer EES 420 linings
on the steer and drive axles, with ArvinMeritor cast iron drums) at two
drive axle GAWRs (34,000 and 40,000 pounds) showed that the lining
contact patterns on the drive axle brakes (the percentage of the lining
surface that is in full contact with the brake drum) after burnish
appeared to be slightly better at the higher 40,000-pound GAWR. Steer
axle burnish contact patterns for the two test conditions were
approximately the same. Drive axle lining temperatures for the two test
conditions throughout the burnish showed slightly higher temperatures
for the 40,000-pound GAWR test (average approximately 400 degrees F)
than for the 36,000-pound GAWR test (average approximately 380 degrees
F), with the highest temperatures occurring at the end of the burnish
sequence. Steer axle burnish temperatures were approximately the same
for both test conditions and averaged around 280 degrees F.
    Parking brake force was also adequate using the current burnish
procedure. The average parking brake force (forward and rearward
drawbar pulls, four tests with one-quarter wheel revolution per test,
with parking brakes on the forward drive axle only) slightly favored
the lower drive axle GAWRs. Although lining contact patterns were about
the same for the front drive axle (which is not the one equipped with
the parking brakes), overall, the tests at the higher GAWR had slightly
more lining contact among both drive axles, which is consistent with
the slightly higher burnish temperatures. Parking brake performance
measured by the drawbar method \54\ showed that with the tests
conducted at 36,000 pounds GAWR, the margin of compliance was

approximately 35 percent. The margin of compliance for the tests with
the drive axles rated at 40,000 pounds GAWR was approximately 20
percent.
---------------------------------------------------------------------------

    \54\ FMVSS No. 121, S5.6.
---------------------------------------------------------------------------

    During the loaded-to-GVWR service brake stops from 60 mph, the
tests with the drive axles at 36,000 pounds GAWR and Type 20 brake
chambers on the steer axle showed that steer axle brake temperatures
were typically 30 to 40 degrees F lower than the drive axle lining
temperatures (that averaged around 180 degrees F) during the first half
of the stop. However, the steer axle temperatures during the second
half of the stop increased to approximately the same temperatures as
the drive axle brakes. When tested with Type 24 brake chambers on the
steer axle, temperature trends during the stop were similar, except
that the steer axle brakes were approximately 20 degrees F hotter than
for the tests with Type 20 steer axle brake chambers. In both cases,
the steer axle brake temperatures increased more than the drive axle
temperatures over the duration of the stops.
    The agency has concluded from reviewing the brake temperatures
during the burnish, and the brake temperatures and stopping distance
data during the loaded-to-GVWR tests, that under the various
combinations of drive axle GAWRs, brake chamber sizes, and slack
adjusters that were reviewed, the vehicle appeared to perform optimally
in all regards. The parking brake drawbar test margins of compliance
were also good, with the tests at the lower GAWR having slightly better
compliance margins. In sum, the test results revealed that the current
burnish procedure provided adequate burnishing for tractors with
improved braking systems to meet both service brake stopping distance
requirements as well as parking brake requirements.
    The agency also recognizes that the results from tests conducted by
Radlinski may not be as applicable to two-axle or severe service
tractors. However, agency stopping distance testing on these tractors
indicated that installation of disc brakes generally would be required
in order to meet the improved stopping distance requirements. Agency
tests with disc brakes showed that there were no apparent brake burnish
problems, and disc brakes are generally less sensitive to the burnish
procedure because of the geometry of the linings and rotors. Disc
brakes' linings and rotors are manufactured with flat friction surfaces
that mate well when assembled on the vehicle. Thus, there is little
wear-in necessary to achieve full lining to rotor contact, and the
brakes readily achieve full torque-generating capability under the
existing FMVSS No. 121 burnish procedure.
    VRTC testing of two-axle and severe service tractors demonstrated
that these vehicles are able to achieve the required stopping distances
using the cold burnish procedure. VRTC tests on a two-axle tractor with
a 148-inch wheelbase, using all disc brakes, yielded a 200-foot
stopping distance and good parking brake performance. Tests on the same
tractor with a hybrid braking system yielded a 223-foot stopping
distance.\55\ Preliminary tests of the three-axle severe service
surrogate tractor (i.e., a single-unit truck) with a hybrid brake
configuration (disc brakes on the steer axle and standard 16.5'' x 7''
drum brakes on the drive axles) showed mixed results. After the burnish
procedure, the drive axle brakes showed less contact area after
burnishing than when the truck was tested with drum brakes on the steer
axle, supporting HDBMC's argument. However, the test results for the
hybrid configuration showed higher parking brake drawbar forces on the
drive axles when compared to tests of the all-drum brake vehicle that
had more drive axle lining contact area after burnish.\56\ Based on the
test results, it is evident that the current FMVSS No. 121 brake
burnish procedure provides adequate burnishing to conduct the required
tests for stopping distance and parking brake pull.
---------------------------------------------------------------------------

    \55\ VRTC testing of the two-axle tractor with all drum brakes
revealed problems with replacement brake linings, but the agency has
yet to determine how much of the problem is due to burnish procedure
versus lining properties. This test yielded two different stopping
distances (241 feet versus 332 feet) with original and replacement
brake linings. When the replacement linings were machined to better
match the curvature of the drums, they achieved similar stopping
distances, leading NHTSA to believe that the cause is related to the
lining properties, and not the burnish procedure. Regardless,
neither lining was able to achieve a 30 percent reduction in
stopping distance with a 10 percent margin of compliance.
    \56\ Currently, additional brake research is underway on this
vehicle to determine stopping distance and brake burnish effect
interactions with enhanced drum brakes.
---------------------------------------------------------------------------

    In summary, based upon available data, NHTSA has decided to
maintain its prior rulemaking decision amending FMVSS No. 121 to
require the use of the cold burnish procedure. The agency is not aware
of an actual problem with the burnish procedure for typical three-axle
tractors. The agency's testing revealed that all types of tractors were
able to meet the required stopping distances using the existing cold
burnish procedure. Furthermore, there is no evidence that the current
burnish procedure is not indicative of real-world braking conditions.
Therefore, we see no need to make any changes to the burnish
requirements of FMVSS No. 121.
2. Brake Dynamometer Test Requirements
    In the NPRM, the agency requested recommendations on potential
modifications to the brake dynamometer requirements of FMVSS No. 121.
These requirements test brake retardation force, power, and recovery
under strict conditions. The agency received a variety of responses to
this request. The majority of commenters stated that they recommend no
changes to the dynamometer requirements. However, NHTSA received one
comment (ArvinMeritor), suggesting the addition of an optional
dynamometer procedure. For the reasons discussed below, the agency has
considered the comments, and has decided that no action is necessary or
appropriate at this time.
    Currently, the requirements of paragraph S5.4.2, Brake power, apply
to all foundation brakes for all air-braked vehicles covered under
FMVSS No. 121. Under the standard, after burnishing, the fade portion
of the test specifies ten consecutive snubs from 50 to 15 mph at a
deceleration rate of 9 ft/sec\2\, followed by a hot stop from 20 mph at
a deceleration rate of 14 ft/sec\2\. After the hot stop, 20 brake
recovery stops from 30 mph at a deceleration rate of 12 ft/sec \2\ at
one minute intervals are made.\57\ Brake pressure limits are placed on
the fade and recovery requirements, while the hot stop does not have an
upper air pressure limitation.
---------------------------------------------------------------------------

    \57\ These requirements do not apply to the steer axle of
tractors.
---------------------------------------------------------------------------

    ArvinMeritor requested that NHTSA modify the dynamometer test
procedure to allow the option of conducting a series of six 60 mph 100
psi stops at the conclusion of the 350 degree F dynamometer
burnish.\58\ ArvinMeritor stated that it believes the torque data
obtained from these stops would be closer to the brake torques obtained
during the vehicle stopping distance test and, therefore, would provide
a more accurate stopping distance calculation. Currently, it states,
because the temperatures in the dynamometer tests significantly exceed
those generated during the stopping distance tests, the dynamometer
performance data do not always correlate directly with the actual
vehicle test results. According to ArvinMeritor, the optional stops,
conducted before the brakes are burnished at the high temperatures,\59\

would provide data that better correlate with data from the actual
tests, where the brakes have undergone similar lower-temperature
burnishing.
---------------------------------------------------------------------------

    \58\ See S6.2.6.
    \59\ Subsequent to this procedure, the brakes are burnished at a
temperature between 450[deg] and 550[deg].
---------------------------------------------------------------------------

    While there is some cause to believe that allowing an additional
six stops from 60 mph would provide useful information for modeling
purposes, as ArvinMeritor asserts, NHTSA does not have enough
information to adopt this recommendation. ArvinMeritor did not describe
what the test conditions would be for these optional stops (such as the
initial brake temperature or intervals between stops), but we assume
they would be conducted with an initial brake temperature between 150
and 200 degrees F, with a cool-down to that initial temperature between
stops. If so, the optional stops would probably not have much influence
on the remainder of the dynamometer test requirements, since those
stops occur in much higher temperature ranges. However, such stops
could have an influence on the brake retardation force requirements in
S5.4.1, if the 60 mph optional stops resulted in additional higher
temperature burnishing beyond the required burnish procedure. The
agency would need more information on the potential benefits and
ramifications of this procedure prior to amending the standard to
specify a manufacturer option in this area.
    Two commenters (HDBMC and Haldex) recommended that there be no
changes made to the current dynamometer requirements. Both stated that
the current requirements do not limit the amount of steer axle brake
torque. (Haldex also mentioned that there is no limit in drive axle
brake torque.) As the increases in stopping distance will largely be
achieved through increasing steer axle brake torque, both commenters
stated that this aspect of the requirements should not be changed. A
third commenter (Bendix) stated that it is conducting dynamometer
testing and would be willing to provide this information to NHTSA on a
confidential basis upon completion of its testing program, although
this information has not been received.
    TMA commented that the agency could not make any changes to the
dynamometer requirements without first issuing a separate NPRM, as no
specific changes to these requirements were proposed in the NPRM for
this rule. TMA stated that if the agency did go through with a separate
rulemaking to modify the dynamometer requirements, it would likely need
to have a different effective date than the one mandated in this final
rule. In that case, according to TMA, the effect would be to undo all
the work TMA member companies will need to do to respond to the current
final rule, since designs will have been tailored to meet the
currently-proposed requirements. TMA stated that any component change
can greatly influence performance of the braking system, and as a
result, TMA members require a 10-year stability period between
rulemakings that affect brake system design in order to amortize
development and investment costs. While this comment does not
substantively address the issue of possible changes to the dynamometer
requirements, the agency has taken TMA's concerns into consideration.
    Based on the comments received and our assessment of this issue,
the agency has decided not to modify the dynamometer test requirements.
TMA's concerns notwithstanding, the agency believes that, if necessary,
it would be better to consider revisions to the dynamometer
requirements in a future rulemaking effort separate from the current
tractor stopping distance rulemaking.
v. Stopping Distances at Reduced Initial Test Speeds
    HDBMC and Bendix commented that in the NPRM, the 20 percent and 30
percent stopping distance reduction values in Table II of FMVSS No. 121
for test speeds below 60 mph did not take into account the brake system
reaction time and average deceleration. Thus, under the agency's
proposed stopping distance requirements for a 30 percent reduction in
stopping distance from an initial speed of 20 mph, the commenters
stated that an average deceleration as high as 0.95 g would be
necessary (with an allowance for a 10 percent margin of compliance in
stopping distance). According to the commenters, this deceleration rate
is not achievable with existing truck braking and tire technology.
    The agency has reviewed the tables of stopping distances provided
by HDBMC and Bendix in their respective comments. In the case of HDBMC,
it did not indicate what equations or methods it used to derive their
recommended tables. For example, the agency could not determine what
was occurring during the brake system reaction time (for 0.36, 0.45,
and 0.54 second reaction times). Bendix provided similar
recommendations but again it did not describe how its recommended
tables of stopping distance were derived. The agency believes that
because both commenters recommended stopping distances at reduced test
speeds that are much longer than what the agency had proposed, the
commenters' recommendations are not accounting for the buildup in
deceleration that the agency's data indicate does occur during the
initial brake pressure increase during typical stopping distance tests
using a full treadle valve brake application. Nevertheless, after
consideration of this issue the agency is providing the following
analysis and revised stopping distance stables for tests conducted at
reduced test speeds.\60\
---------------------------------------------------------------------------

    \60\ We note that the neither the notice of proposed rulemaking,
nor the previous rulemaking on this issue (53 FR 8190), contained
detailed information on how the stopping distances for reduced
initial test speeds were derived.
---------------------------------------------------------------------------

    For this analysis, we are using the stopping distance equation that
was derived by researchers at the VRTC. The equation is as follows:

St = (\1/2\ Vo tr) + ((\1/2\)
Vo\2\;/af)--((1/24) af
tr\2\;)

Where:
St = Total stopping distance in feet
Vo = Initial Speed in ft/sec
tr = Air pressure rise time in seconds
af = Steady state deceleration in ft/sec[sup2]

    The complete derivation of this equation is included in the
docket.\61\ For the final rule, we selected an air pressure rise time
of 0.45 seconds that is equal to the brake actuation timing requirement
in S5.3.3. This requirement specifies that for a truck (including a
truck-tractor), the air pressure in the brake chambers must reach at
least 60 psi within 0.45 seconds.
---------------------------------------------------------------------------

    \61\ See Docket No. 2005-21462-39, p. 18.
---------------------------------------------------------------------------

    The agency reviewed three test plots of deceleration versus time
for tractor tests it conducted at VRTC to determine if the plot
characteristics matched the stopping distance equation and the pressure
rise time selected for this final rule. The three plots are included in
the docket.\62\ The first plot is for the Sterling 4x2 tractor equipped
with disc brakes at all wheel positions and coupled to a braked 53-foot
van trailer with tandem axles also equipped with disc brakes. The
vehicle was loaded to typical highway weight (i.e., steer axle 11,000
pounds; drive axle 22,700 pounds, tandem trailer axles 34,000 pounds)
that is slightly below the GVWR for each vehicle. This combination
represents the best-performing unit that was tested at VRTC, and it had
a 60 mph stopping distance of 186 feet. As the plot shows, the steady-
state deceleration was slightly less than 0.8g for the duration of the
stop. The 0.8g deceleration was reached within approximately 0.5
seconds from the point of brake application. This deceleration and
stopping distance are believed to be the best obtainable for a tractor-
trailer

combination vehicle using all production equipment (tires, antilock
braking system, air disc brakes, etc.) available at the present time.
---------------------------------------------------------------------------

    \62\ See Docket No. 2005-21462-39, p. 28.
---------------------------------------------------------------------------

    The next two plots included from VRTC tests are for tractors that
achieved a stopping distance of approximately 250 feet. These were used
to determine the steady-state deceleration required to achieve this
stopping distance. The second plot \63\ is for a Volvo 6x4 tractor
equipped with disc brakes on the steer axle and S-cam drum brakes on
the drive axles, and it was coupled to an unbraked control trailer. The
tractor was loaded to GVWR and was also braking the extra 4,500 pounds
on the control trailer axle. The stopping distance for this vehicle
from 60 mph was 249 feet and the steady state deceleration was
approximately 0.45g. The plot shows that this tractor achieved the
0.45g deceleration rate at approximately 0.4 seconds.
---------------------------------------------------------------------------

    \63\ Docket No. 2005-21462-39, p. 29.
---------------------------------------------------------------------------

    The third plot is for a Peterbilt 6x4 tractor equipped with
enhanced S-cam drum brakes on the steer axle and standard S-cam drum
brakes on the drive axles, loaded to GVWR with an unbraked control
trailer. The 60-mph stopping distance was 250 feet, and the
deceleration varied slightly from approximately 0.48g at the midpoint
of the stop to approximately 0.56g near the end of the stop. The
deceleration during the stop was not exactly stead state since the
deceleration rate increased towards the end of the stop. The rate at
0.45 seconds was approximately 0.36g.
    The plots for the second and third tests, the Volvo and Peterbilt
tractors respectively, demonstrate that for a 250-foot stopping
distance requirement, deceleration rates in the range of 0.45g to 0.56g
would be achieved by actual vehicles. It appears that the Volvo had a
slightly faster application timing, and thus had a lower steady-state
deceleration rate than the Peterbilt while attaining approximately the
same stopping distance.
    Using the VRTC equation for stopping distance, we derived the
following three tables of stopping distance for three requirements in
this final rule: (1) Standard service tractors loaded to GVWR plus
4,500 pounds on the unbraked control trailer axle; (2) severe service
tractors loaded to GVWR plus 4,500 pounds on the unbraked control
trailer axle; and (3) all tractors tested in the lightly-loaded vehicle
condition. Note that the table for severe service tractors contains the
same values currently in FMVSS No. 121 for single-unit trucks loaded to
GVWR, but we are reproducing this table here to show the estimated
deceleration levels with a 0.45-second pressure rise time.

 Table I--Stopping Distance Calculations for Two- and Three-Axle Tractors With a GVWR of 70,000 Pounds or Less,
   and Tractors With Four or More Axles and a GVWR of 85,000 Pounds of Less, in the Loaded-to-GVWR Condition.
                                  (Brake System Reaction Time is 0.45 Seconds)
----------------------------------------------------------------------------------------------------------------
            Initial vehicle speed                       Steady-state deceleration             Stopping distance
----------------------------------------------------------------------------------------------------------------
        (mph)                 (ft/sec)             (ft/sec\2\)               (g's)                  (ft)
----------------------------------------------------------------------------------------------------------------
                20                   29.3                  18.00                   0.56                     30
                25                   36.7                  18.00                   0.56                     45
                30                   44.0                  17.50                   0.54                     65
                35                   51.3                  17.00                   0.53                     89
                40                   58.7                  17.00                   0.53                    114
                45                   66.0                  16.80                   0.52                    144
                50                   73.3                  16.80                   0.52                    176
                55                   80.7                  16.80                   0.52                    212
                60                   88.0                  16.80                   0.52                    250
----------------------------------------------------------------------------------------------------------------

  Table II--Stopping Distance Calculations 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, in the Loaded-to-GVWR Condition. (Brake
                                      System Reaction Time of 0.45 Seconds)
----------------------------------------------------------------------------------------------------------------
            Initial vehicle speed                       Steady-state deceleration             Stopping distance
----------------------------------------------------------------------------------------------------------------
        (mph)                 (ft/sec)             (ft/sec\2\)               (g's)                  (ft)
----------------------------------------------------------------------------------------------------------------
                20                   29.3                  15.00                   0.47                     35
                25                   36.7                  14.65                   0.45                     54
                30                   44.0                  14.15                   0.44                     78
                35                   51.3                  13.90                   0.43                    106
                40                   58.7                  13.75                   0.43                    138
                45                   66.0                  13.60                   0.42                    175
                50                   73.3                  13.45                   0.42                    216
                55                   80.7                  13.40                   0.42                    261
                60                   88.0                  13.35                   0.41                    310
----------------------------------------------------------------------------------------------------------------

Table III--Stopping Distance Calculation for All Tractors in the Unloaded Condition. (Brake System Reaction Time
                                                of 0.45 Seconds.)
----------------------------------------------------------------------------------------------------------------
            Initial vehicle speed                       Steady-state deceleration             Stopping distance
----------------------------------------------------------------------------------------------------------------
        (mph)                 (ft/sec)             (ft/sec\2\)               (g's)                  (ft)
----------------------------------------------------------------------------------------------------------------
                20                   29.3                  19.80                   0.61                     28
                25                   36.7                  19.40                   0.60                     43
                30                   44.0                  18.80                   0.58                     61
                35                   51.3                  18.10                   0.56                     84
                40                   58.7                  18.10                   0.56                    108
                45                   66.0                  17.95                   0.56                    136
                50                   73.3                  17.95                   0.56                    166
                55                   80.7                  17.95                   0.56                    199
                60                   88.0                  17.95                   0.56                    235
----------------------------------------------------------------------------------------------------------------

    We compared the calculated values for the 60 mph, 250-foot stopping
distance requirements in Table I for a typical tractor to those test
vehicles described above, in order to determine if the actual and
calculated decelerations are similar. The calculated steady-state
deceleration from the table with an initial test speed of 60 mph is
0.56g of deceleration, and this compares to 0.45g for the Volvo (that
had a quicker response time, and thus slightly lower steady-state
deceleration than the Peterbilt), and 0.48 to 0.52g for the Peterbilt
(which had a slower response time, and thus a slightly higher steady-
state deceleration than the Volvo). These values are similar to the
0.52g calculated in Table I, and therefore the agency believes the
equation used to calculate the stopping distances is valid. We did not
perform similar analyses for stopping distances conducted at other
initial test speeds, because we did not conduct any testing at reduced
test speeds. Only tests from an initial speed of 60 mph were conducted
at VRTC.
    We do not understand the basis for the concerns raised by HDBMC and
Bendix in their comments about the proposed stopping distances
requiring abnormally high deceleration levels. As shown in the tables
of calculated stopping distances, the maximum required deceleration for
an unloaded tractor at an initial speed of 20 mph is 0.61g. Even with a
ten percent added margin of compliance, the actual performance would
not appear to need to be greater than 0.67g. As described above, for
the tests on the Sterling tractor operated with a braked van trailer,
deceleration of almost 0.8g was attained at highway weight. Our tests
of unloaded tractors indicated that nearly similar stopping distance
performance was attained in the bobtail mode, an in each case a margin
of compliance substantially greater than 10 percent was achieved when
the vehicle was tested from an initial speed of 60 mph. It appears to
us that HDBMC and Bendix could be using a method such as a free-roll
during pressure rise that would assume no braking during the initial
pressure rise. However, these commenters did not provide enough detail
in the comments for the agency to thoroughly evaluate their claims. In
any event, for the reasons discussed above, we believe that the new
stopping distance calculations for the lower initial test speeds
properly take into account brake actuation periods, and do not require
excessive rates of deceleration.
vi. Comments Regarding Foreign Trade Agreements
    A comment from the government of the People's Republic of China
requested that Chinese manufacturers be given a longer transitional
period for implementation of improved stopping distance requirements,
citing the Agreement on Technical Barriers to Trade.\64\ China cited
clause 12.3 of the Agreement, which reads:
---------------------------------------------------------------------------

    \64\ A summary of the treaty on the Web site of the World Trade
Organization reads, ``[t]his agreement will extend and clarify the
Agreement on Technical Barriers to Trade reached in the Tokyo Round.
It seeks to ensure that technical negotiations and standards, as
well as testing and certification procedures, do not create
unnecessary obstacles to trade. However, it recognizes that
countries have the right to establish protection, at levels they
consider appropriate, for example for human, animal or plant life or
health or the environment, and should not be prevented from taking
measures necessary to ensure those levels of protection are met. The
agreement therefore encourages countries to use international
standards where these are appropriate, but it does not require them
to change their levels of protection as a result of
standardization.'' Available at http://www.wto.org/english/docs_e/
legal_e/ursum_e.htm#dAgreement.

    Members shall, in the preparation and application of technical
regulations, standards, and conformity assessment procedures, take
account of the special development, financial and trade needs of
developing country Members, with a view to ensuring that such
technical regulations, standards and conformity assessment
procedures do not create unnecessary obstacles to exports from
---------------------------------------------------------------------------
developing country Members.

    In its comment, China quoted the agency in stating in the NPRM that
``improvements in truck tractor stopping distance performance may
involve more than simply increasing the power of foundation brakes, as
changes might be required to suspensions and frames, etc., to handle
the higher braking torque without decreasing vehicle durability and
safety.'' Further, China noted that the requirements of the Chinese
National Standards on truck stopping distance (GB7258-2004 and GB12676-
1999) are significantly less stringent than the stopping distances
proposed by NHTSA. Finally, China cited the fact that disc brakes--
along with larger capacity drum brakes, electrically controlled braking
systems, and anti-lock braking systems--were only starting to be used
on a limited number of vehicles in China. All of these factors, China
stated, should be taken into consideration in a decision whether to
give Chinese manufacturers a longer transitional period for
implementation of the improved stopping distance requirements.
    We have carefully considered China's comments. In responding, we
begin by noting that, in the U.S., the applicable FMVSSs are the same
regardless of where a motor vehicle or item of motor vehicle equipment
is manufactured. Therefore, any extension of lead time would not be
limited to Chinese manufacturers but would be available to all
manufacturers irrespective of where they manufacture truck tractors for
the U.S. market. While we carefully consider the issue of necessary
lead time in establishing and amending FMVSSs, we also recognize that
extending lead time can also result in the delay of safety benefits.
    We note that while China highlighted substantial differences
between the Chinese and proposed U.S.

requirements regarding stopping distance requirements for heavy truck
tractors, it did not provide specific information explaining why
particular Chinese manufacturers would need additional time to comply
with the new stopping distance requirements. There are many other
substantial differences in vehicle safety regulation between the two
countries, and we believe that a manufacturer building vehicles
otherwise compliant to the U.S. FMVSSs would likely be capable of
making the relatively minor modifications in brake design required by
the upgraded performance requirements in this final rule, consistent
with the lead time provided in this final rule.
    With specific regard to extended lead time, we note that as
discussed above, the agency is providing longer lead time, relative to
that proposed in the NPRM, of four years for two-axle and severe
service tractors. This relates to the additional design and testing
work that must be done on these tractors to ensure that they can meet
the improved stopping distances while maintaining good stability and
control of the vehicles at issue. Therefore, Chinese manufacturers,
like other manufacturers, will have longer time to undertake the design
and testing necessary to meet the improved standards for these classes
of truck tractors.
    However, we believe that two years is adequate lead time for
manufacturers to design standard three-axle tractors that can meet the
improved stopping distance requirements. We note that standard three-
axle tractors that already comply with the 30 percent reduction in
required stopping distance are being manufactured and used on public
roads in this country already. NHTSA has determined that these tractors
can be improved to meet the enhanced requirements with relatively
little design work, as compared to other classes of heavy truck
tractors. We also believe that extending the lead time for these
vehicles would inappropriately delay the safety benefits of this final
rule.
vii. Miscellaneous Comments
    Several commenters expressed concerns regarding the current state
of heavy truck tractor maintenance. Brake Pro, Haldex, and HDBMC all
commented that current vehicle maintenance procedures in many cases do
not maintain braking systems at the same level as original equipment.
Brake Pro added that aftermarket and foreign-produced brake lining
material may be less efficient than materials included as original
equipment. While these may be valid concerns, they are outside the
scope of this rulemaking. This rulemaking addresses only new vehicles
and the equipment sold on new vehicles; it does not apply to
maintenance procedures once the vehicles are sold to end users.
    In-service performance requirements for brake systems on commercial
vehicles are covered under the Federal Motor Carrier Safety
Administration's (FMCSA's) Federal Motor Carrier Safety Regulations
(FMCSRs), as cited in the Code of Federal Regulations at Title 49, Part
393, Section 52, Brake Performance. That regulation sets service and
emergency brake stopping distance requirements for various categories
of passenger- and property-carrying commercial motor vehicles from an
initial speed of 20 mph. It also includes minimum vehicle deceleration
requirements for service brake systems. While it may be appropriate to
set new standards for tractors that will be required to comply with
shorter stopping distance requirements, it is not clear how that would
be done at the present time, given the influences of trailer braking
and operating weight versions the FMVSS No. 121 testing that is
performed at full GVWR using an unbraked control trailer. Presumably,
additional research or study would need to be conducted to derive
proposed revisions to the FMCSRs. However, that work has not yet been
performed.
    A comment from an individual (Mr. John Kegley) requested that the
new rule mandate that all Class 8 trucks have engine or exhaust brakes.
Similarly, a comment from Mr. Timothy Larrimore suggested that the
regulation should mandate that all trucks have four axles. Based on the
data presented above, it is our belief that modifying the stopping
distance requirements is the best way to achieve safety benefits, while
still permitting manufacturers to use their own discretion in how they
meet those requirements. We are not adopting these commenters'
suggestions.
    Finally, a comment from Mr. Roger Sauder suggested that instead of
mandating new stopping distance requirements, the agency should focus
on informing the public about proper driving techniques in the presence
of large vehicles. We are not adopting this suggestion. We note that
currently, such public education projects are already in place.
Further, the data presented above indicate that reducing the stopping
distance of heavy trucks will result in a substantial reduction in
injuries and property damage prevented.
viii. Costs and Benefits of Shorter Tractor Stopping Distances
1. Estimated Benefits of a 30 Percent Reduction in Stopping Distance
    In the Final Regulatory Impact Analysis (FRIA), the agency
estimates that substantially greater safety benefits will be attained
with a 30 percent reduction in required stopping distance compared to
the benefits for a 20 percent reduction. For the 30 percent reduction
scenario, the agency estimates that 227 fatalities and 300 serious
injuries (AIS 3-5) will be prevented by improving the stopping distance
requirement. For the 20 percent reduction scenario, the agency
estimates that only 91 fatalities and 127 serious injuries would be
prevented.\65\ The differential in estimated reduced property damage is
even greater, with approximately five times the property damage
prevented for the 30 percent case versus the 20 percent case ($205
million compared to $39 million).\66\ In estimating the numbers of
property damage-only (PDO) vehicle involvements, crashes, and injuries,
figures were derived from the agency's 2004-2006 GES database and the
number of fatalities was determined from the agency's 2004-2006 FARS
database. A more detailed comparison between the two alternatives,
using a 7% discount rate, is laid out in the table below: \67\
---------------------------------------------------------------------------

    \65\ See FRIA, at VI-6.
    \66\ See FRIA, at VI-7. We note that these figures in 2007
dollars discounted at 3%.
    \67\ See FRIA, at VI-13.

                      Annual Costs and Benefits in Millions of 2007 Dollars Discounted at 7% for 30% Reduction in Stopping Distance
--------------------------------------------------------------------------------------------------------------------------------------------------------
     Costs (in millions)           Benefits (in millions)              Net benefit                     Net cost                     Cost per ELS
--------------------------------------------------------------------------------------------------------------------------------------------------------
                       Most    Property                                             Most                            Most                          Most
   Low       High     likely    damage      ELS    Monetized     Low      High     likely      Low       High      likely      Low      High     likely
--------------------------------------------------------------------------------------------------------------------------------------------------------
     $27      $192       $54      $169       212     $1,293    $1,271    $2,872    $1,410    -$141.4     $22.9       \*\-       N/A      $0.1       N/A
                                                                                $115.1
--------------------------------------------------------------------------------------------------------------------------------------------------------
* The PDO benefits were greater than the costs, which resulted in a negative number.

                      Annual Costs and Benefits in Millions of 2007 Dollars Discounted at 7% for 20% Reduction in Stopping Distance
--------------------------------------------------------------------------------------------------------------------------------------------------------
      Costs (in millions)            Benefits (in millions)              Net benefit                    Net cost                    Cost per ELS
--------------------------------------------------------------------------------------------------------------------------------------------------------
                         Most    Property                                             Most                          Most                          Most
   Low        High      likely    damage      ELS    Monetized     Low      High     likely      Low      High     likely      Low      High     likely
--------------------------------------------------------------------------------------------------------------------------------------------------------
     $19       $134        $48       $32        87       $531      $426    $1,082      $512    -$12.9    $101.6     $15.4       N/A      $1.1      $0.2
--------------------------------------------------------------------------------------------------------------------------------------------------------

    The FRIA estimates there are 864 fatalities, 15,614 non-fatal
injuries and 17,621 PDO crashes occurring annually in which the front
of a braked truck tractor strikes another vehicle. It is estimated that
reducing the stopping distance of truck tractors will reduce the
following subsets of those crashes: (1) Rear-end, truck striking
passenger vehicle (4 percent of total passenger car occupant
fatalities); (2) passenger vehicle turned across path of truck (8
percent); and (3) straight path, truck into passenger vehicle
(generally side-impact crashes at roadway junctions; 14 percent). The
total percentage of all passenger vehicle occupant fatalities for these
crash types was 26 percent. In addition, it is possible that some of
the head-on collisions could be reduced in severity, since improvements
in the braking capability of large trucks could reduce impact
speeds.\68\
---------------------------------------------------------------------------

    \68\ See FRIA, at II-4.
---------------------------------------------------------------------------

    The reduction in required stopping distance also produces
substantial benefits in property damage reduction. Using a three
percent discount rate, the agency believes that $205 million of
property damage will be prevented annually (present value of property
damage savings over the lifetime of these vehicles) with the 30 percent
required reduction in stopping distance. Using a seven percent discount
rate, the resulting figure is $169 million in property damage
prevented.
    Some commenters (Advocates, IIHS) stated that the agency should
mandate not only the 30 percent reduction in required stopping
distance, but also mandate the use of disc brakes in truck tractors.
These commenters also stated that disc brakes have certain
characteristics (namely resistance to fading at high temperatures)
which would provide additional benefits that enhanced S-cam drum brakes
would not, even if they provided equivalent torque in the FMVSS No. 121
testing requirements. Accordingly, the commenters argued that these
benefits should be factored into the cost-benefit analysis.
    NHTSA, however, does not have data on the benefits of disc brakes
beyond the benefits of similar-performing drum brakes. We note that
FMVSS No. 121 is a performance-based standard, and any type of
foundation brake that can meet the stopping distance and other
requirements of the standard are permitted. Thus, it is not design-
restrictive with respect to the type of foundation brake used to meet
the requirements.
    In a comment, Freightliner and TMA suggested that two-axle tractors
present less of a need to reduce stopping distances than standard
three-axle tractors do. Freightliner and TMA stated that two-axle
tractors represent 10 percent of air-braked tractors produced annually,
but are only involved in 3.4 percent of fatal crashes involving
tractors. Because of this low fatality rate, the commenters claim,
these vehicles should not be included in the agency's rulemaking to
require shorter stopping distances. International also commented that
it believes two-axle tractors should be excluded from the rulemaking.
Although International did not cite the fatality involvement rates in
its comments, it stated that it was an active participant in the
preparation of TMA's comments.
    TMA included in its comments a report on Class 8 truck tractor
crash statistics performed by the University of Michigan Transportation
Research Institute (UMTRI) using its Trucks Involved in Fatal Accidents
database for the years 1999 through 2003.\69\ This submission presented
an alternative data set, which purportedly showed that the proportion
of fatalities from these types of accidents is only 21.2 percent. The
agency notes, however, that the UMTRI study was restricted to Class 8
(heavy truck tractors with a GVWR greater than 33,000 pounds) vehicle
crashes, which would account for the slight disparity between the
figures cited by TMA and NHTSA.
---------------------------------------------------------------------------

    \69\ See Docket No. NHTSA-2005-21462-26, TMA submission of April
14, 2006.
---------------------------------------------------------------------------

    Table 7 of the UMTRI report shows the type of road (interstate,
U.S. route, State route, county road, etc.) on which the Class 8
tractor fatal involvements occurred, as well as the tractor type. The
data indicate that two and three-axle tractors have similar crash
rates, and that they occur on different types of roads in similar
frequencies. According to this submission, two-axle tractor crash data
regarding road type for Class 8 tractors were quite similar to those
for typical three-axle tractors. Only slightly fewer fatal crashes
occurred among two-axle tractors on interstates (29 percent) compared
to three-axle tractor fatal crashed occurring on interstates (34
percents). Crashes among the two vehicle configurations were nearly the
same for U.S. and State routes, and slightly higher for two-axle
tractor crashes on county roads (seven percent) versus typical three-
axle tractors (five percent).
    The agency does not agree with TMA that two-axle tractors are
under-represented in fatal crashes to a degree that would warrant their
being excluded from this final rule. Table 3 of the UMTRI report
indicated that there were 724 Class 3 through 7 tractors in the sample
(most if not all of these would be two-axle Class 7 tractors with a
GVWR between 26,001 and 33,000 pounds, and would be in the lower
combination weight applications such as beverage delivery), compared to
the 534 crashes of Class 8 two-axle tractors

(GVWR greater than 33,000 pounds) in the sample that was used in its
analysis. Thus, more than half of the two-axle tractors involved in
fatal crashes are missing from UMTRI's analysis because they were not
Class 8 tractors (the report states that only Class 8 tractors were
used in the analysis). Therefore, we believe that the data indicate
that two-axle tractors are represented in fatal crashes to a similar
extent as three-axle tractors.
2. Cost of Improved Brake Systems
    Because the agency does not know the specific methods that truck
manufacturers would use to upgrade tractor brake systems to meet the
new requirements, in developing the NPRM the agency used an array of
foundation brake upgrades to estimate the increased costs for the brake
system improvements. The highest cost of complying with shorter
stopping distance requirements would be realized if all tractors were
equipped with disc brakes rather than the current S-cam drum brakes,
and the lowest cost would be realized if all tractors could meet the
new requirements if they were equipped with enhanced (larger) S-cam
drum brakes. Both methods have been demonstrated to provide sufficient
improvements in braking performance for typical three-axle tractors,
while agency testing and data completed after the publication of the
NPRM show that the disc brake approach would be required to meet the 30
percent reduction in required stopping distance for certain less common
configurations of tractors (i.e., severe service and two-axle
tractors).
    In quantifying the costs to comply with the reduced stopping
distance requirements, in the FRIA, the agency used as a basis the
costs of installing improved brake systems on new truck tractors. NHTSA
also determined that currently, approximately ten percent of tractors
have enhanced S-cam drum brakes installed on the steer axle, and three
percent of tractors have enhanced S-cam drum brakes installed on the
drive axles. Therefore, in determining the costs of upgrading to
improved brake systems, we calculated the costs of upgrading 90 percent
of all steer axles and 97 percent of all drive axles. Commenters also
indicated that approximately 82 percent of all tractors are typical
three-axle tractors (similar to the tractors from the Radlinski and
VRTC tests). TMA and Freightliner stated that typical three-axle
tractors comprise 82 percent of annual tractor production and ATA
stated that such tractors comprise 81 percent of production.
Freightliner commented that two-axle tractors comprise ten percent of
tractor production, and severe service tractors comprise seven percent
(although there may be a rounding error as Freightliner's statements on
total production for the three types of tractors add to 99 percent).
    With regard to standard three-axle tractors, based on the VRTC test
report and the three test reports \70\ from Federal Mogul and Motion
Control Industries, the 30 percent reduction in required stopping
distance could be met by using larger S-cam drum brakes or disc brakes
at all wheel positions on tractors. The agency believes that the cost
to install larger drum brakes would be much lower than the cost to
install air disc brakes, although we do not have specific cost
information on the various modifications to truck tractor braking
systems. In the PRIA, the agency estimated that the cost for larger S-
cam drum brakes is $75 for the steer axle \71\ and $50 for each drive
axle \72\ to meet the 30 percent reduction requirement. For typical
three-axle tractors, which make up about 82 percent of annual
production, we estimated $175 ($75steer + 2 x
$50drive = $175) for larger drum brakes. In its comments
regarding the PRIA, Freightliner stated that larger drum brakes at all
wheel positions would be $222. However, that manufacturer did not break
costs associated with steer and drive axles. Due to limited data, for
purposes of our cost estimates in the FRIA, we assumed that the cost
for larger S-cam drum brakes is $85 for the steer axle and $65 for each
drive axle ($215 for typical three-axle tractors).\73\ Although the
estimated $215 is lower than Freightliner's $222 cost (about three
percent lower), we would expect that when larger quantities of brakes
are produced the cost will be lower than the current $222.\74\ The
agency estimates that if manufacturers were to install enhanced drum
brakes at all wheel positions, the total cost of this rulemaking would
be $27 million ($211 \75\ per vehicle).\76\
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    \70\ Test Report Nos. RAI-FM-20, RAI-MC-04, AND RAI-FM-21.
    \71\ The size increases from 15'' x 4'' to 16.5'' x 5'' or
16.5'' x 6''.
    \72\ The size increases from 16.5'' x 7'' to 16.5'' x 8\5/8\''
or 16.5'' x 8''.
    \73\ We note that this figure is in 2005 dollars.
    \74\ FRIA, V-1.
    \75\ Figures for the estimated incremental cost per vehicle take
into consideration the fact that 10 percent of tractors currently in
production are equipped with larger drum brakes at the steer axle,
and 3 percent are equipped with larger drum brakes at the drive
axle. See FRIA [V-2]. Further, we note that this figure is in 2007
dollars.
    \76\ FRIA, E-4.
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    Costs for disc brakes are estimated to be higher than those for
enhanced S-cam drum brakes.\77\ The agency does not have specific cost
information on disc brakes, but assumes, based on the current average
pricing of disc brakes, that the cost would be $500 per axle (either
steer or drive axles). If all affected vehicles are equipped with disc
brakes to meet the requirement, the agency estimates that the
associated incremental cost would be about $192M (or $1,475 per truck
tractor, considering that approximately 82 percent of truck tractors
have three axles) to fit disc brakes at each wheel position of the
130,000 truck tractors manufactured each year.\78\ Freightliner also
provided comments on the cost of disc brakes, indicating that the
incremental costs of upgrading to disc brakes on all axles would be
$1,627 for three-axle tractors and $963 for two-axle tractors. These
figures are not significantly different from those used in the FRIA,
and again we would expect that if larger quantities of brakes are
produced the cost would be lower than the current $500 per axle, as
suggested by the IIHS in its comments.
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    \77\ FRIA, V-3.
    \78\ Some of the typical three-axle tractors may need disc
brakes on the steer axle only, and many of these tractors may be
able to comply by upgrading to enhanced drum brakes (the lowest-cost
option). Thus it is unlikely that the total cost to implement the
requirements would be close to the high-end cost estimate in the
FRIA (which was to install disc brakes on all tractors).
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    In its analysis, the agency also considered the cost of installing
hybrid brake systems on all truck tractors. If all applicable vehicles
are equipped with front disc and rear larger S-cam drum brakes, the
associated cost of the rulemaking would be about $80M (or $613 per
vehicle).\79\
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    \79\ FRIA, V-4.
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    Finally, in the FRIA, the agency provides a best estimate of the
incremental cost. This scenario assumes that for typical three-axle
tractors, manufacturers would comply with the reduced stopping distance
requirements through use of the least costly means available, i.e., the
use of enhanced drum brakes at all wheel positions. For two-axle and
severe service tractors, which make up approximately 18 percent of all
tractors, manufacturers would need to use disc brakes at all wheel
positions. The total cost of these improvements, which consist of
upgrading standard three-axle tractors to enhanced S-cam drum brake
configurations and upgrading two-axle and severe service tractors to
all-disc brake configurations, would be an average cost of $413 per
vehicle, or about $55.4 million total

annual costs. However, we also note that a small number of commercial
truck tractors (approximately three percent, all of which are standard
three-axle tractors) already comply with the 30 percent reduction in
required stopping distance. Subtracting the cost of those vehicles from
the total implementation cost of the rule yields a total incremental
cost of $53.7 million.\80\
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    \80\ See FRIA, at V-5.
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3. Additional Costs Incurred Resulting From Improved Brake Systems
    The NPRM also asked for information on tractor components other
than the foundation brakes (e.g., frames and suspension) that may need
to be modified to meet shorter stopping distance requirements of 20-30
percent. Specifically, the agency was seeking to identify additional
costs or weight penalties that might be required to meet the new
stopping distance requirements. While numerous commenters discussed
potential additional costs that could result from the use of improved
brake systems in truck tractors, relatively little specific information
was supplied on vehicle modifications that may be required to equip
tractors with more powerful foundation brakes. TMA cited chassis
structural analysis, design, and validation, but did not elaborate on
the costs or scope of these issues. TMA also stated that more powerful
brakes may require tuning with regard to brake noise, vibration, and
modifications to the ABS. Freightliner stated that if two-axle tractors
are fitted with disc brakes, electronic stability control systems may
be needed to reduce instability during hard braking events. Haldex
stated that routine vehicle modifications (e.g., tires, suspensions,
chassis structure) would be most effectively addressed by the vehicle
manufacturers.
    On the issue of weight penalties for improved brake systems, Bendix
provided data on drum brake weights versus disc brake weights. It
stated that the heaviest drum brakes weigh more than the lightest disc
brakes, while the heaviest disc brakes weigh more than the lightest
drum brakes. It stated that for a three-axle tractor equipped with all
disc brakes, total vehicle weight could increase by 212 pounds, or
could decrease by 134 pounds, compared to an all drum braked tractor,
depending on which disc or drum brakes are used for comparison.
ArvinMeritor stated in its comments that the new brakes will weigh
more, although it did not provide a specific value. WABCO, on the other
hand, stated that the weight of a disc brake is equivalent to the
weight of high performance drum brakes.
    After evaluating all comments and available data, we estimate that
the improved brakes may add a small amount of weight to the vehicle,
resulting in slight additional fuel consumption and possible loss of
revenue by displacing cargo-carrying capability, but that those costs
cannot be determined from the available data. Overall, however, we
believe those costs to be very small.
4. Summary of Costs and Benefits Estimates
    The FRIA calculates cost and benefits ratios for larger drum brake,
disc brake, and hybrid disc/drum brake tractor configurations. As part
of this analysis, the agency estimated Net Cost per Equivalent Life
Saved (NCELS) for such scenarios. A wide range of estimates are
provided because of the uncertainty in knowing in advance exactly which
brake system improvements will be employed to meet the new
requirements. The agency's estimates of costs and benefits are
summarized in tables presented below. We note, for reasons discussed
earlier, that while manufacturers can meet the upgraded requirements
with larger drum brakes for a significant majority of tractors, it is
likely that disc brakes will be needed for two-axle and severe axle
tractors (comprising approximately 18 percent of tractors).

                    Estimated Annual Safety Benefits
------------------------------------------------------------------------
  Percent reduction in                               Serious injuries
   stopping distance        Fatalities reduced            reduced
------------------------------------------------------------------------
               30%                      227                      300
------------------------------------------------------------------------

                        Property Damage Prevented
                              [In millions]
------------------------------------------------------------------------
  Percent reduction in
   stopping distance           3% Discount              7% Discount
------------------------------------------------------------------------
               30%                     $205                     $169
------------------------------------------------------------------------

                                                Incremental Costs
                                                 [2007 Dollars]
----------------------------------------------------------------------------------------------------------------
                                      Larger S-cam drum    Disc brakes at     Front disc and
  30% Percent reduction in stopping      at all wheel        all wheel      larger rear S-cam     Most likely
              distance                    positions          positions             drum           combination
----------------------------------------------------------------------------------------------------------------
Total Cost..........................               $27M              $192M               $80M               $54M
Cost Per Vehicle....................                211              1,475                613                413
----------------------------------------------------------------------------------------------------------------

                   Net Cost per Equivalent Life Saved
          [For 30% reduction in stopping distance, in millions]
------------------------------------------------------------------------
           Brake system                 3 Percent          7 Percent
------------------------------------------------------------------------
Larger S-Cam Brake................                 NB                 NB
All Disc Brake....................                 NB             $0.108

Front Disc and Larger Rear S-Cam                   NB                 NB
 Drum.............................
Most Likely Combination...........                 NB                 NB
------------------------------------------------------------------------
 NB = Net Benefits (Property damage benefits exceed the costs).

ix. Lead Time
    NHTSA is specifying differing compliance dates for typical three-
axle tractors on the one hand, and two-axle and severe service tractors
on the other. The agency has described the available test data for
typical three-axle tractors with improved brake systems, showing that
compliance with the new stopping distance requirements can be readily
achieved. Therefore, the agency is requiring a compliance date that is
about two years from the date of publication of this final rule for
typical three-axle tractors (i.e., three-axle truck tractors with a
GVWR less than or equal to 59,600 pounds).\81\
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    \81\ As stated above, ``typical three-axle tractors'' 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. Summing these
GAWRs yields a GVWR that is equal to or less than 59,600 pounds.
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    The lead time for all two-axle tractors, and severe service
tractors with a GVWR greater than 59,600 pounds, is approximately four
years from the date of publication of this final rule. As previously
described, available 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 to more fully
evaluate new brake systems 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,
particularly on shorter wheelbase two-axle tractors. For severe service
tractors, the agency described the available test data and analyses
indicating that vehicle improvements are available that would make the
new 250-foot and 310-foot loaded-to-GVWR stopping distance requirements
attainable. However, only limited development work relevant to reduced
stopping distance has been performed on these vehicles to date. As
several commenters indicated, additional lead time is needed for
complete testing and validation of new brake systems for these vehicles
to ensure that full compliance can be achieved, without compromising
control, stability, and comfort elements important to end users.

IV. Rulemaking Analyses and Notices

a. Vehicle Safety Act

    Under 49 U.S.C. Chapter 301, Motor Vehicle Safety (49 U.S.C. 30101
et seq.), the Secretary of Transportation is responsible for
prescribing motor vehicle safety standards that are practicable, meet
the need for motor vehicle safety, and are stated in objective
terms.\82\ These motor vehicle safety standards set the minimum level
of performance for a motor vehicle or motor vehicle equipment to be
considered safe.\83\ When prescribing such standards, the Secretary
must consider all relevant, available motor vehicle safety
information.\84\ The Secretary also must consider whether a proposed
standard is reasonable, practicable, and appropriate for the type of
motor vehicle or motor vehicle equipment for which it is prescribed and
the extent to which the standard will further the statutory purpose of
reducing traffic accidents and associated deaths.\85\ The
responsibility for promulgation of Federal motor vehicle safety
standards has been delegated to NHTSA.\86\
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    \82\ 49 U.S.C. 30111(a).
    \83\ 49 U.S.C. 30102(a)(9).
    \84\ 49 U.S.C. 30111(b).
    \85\ Id.
    \86\ 49 U.S.C. 105 and 322; delegation of authority at 49 CFR
1.50.
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    Based upon the agency's research, the agency determined that a
substantial number of fatalities and injuries result annually from
collisions between combination trucks (i.e., tractor trailers) and
light vehicles. The agency further determined that a 30 percent
reduction in heavy truck tractor stopping distance is both
technologically and financially achievable and could prevent a
substantial number of these identified fatalities and injuries. In
developing this final rule amending the relevant requirements of FMVSS
No. 121 to reduce heavy truck stopping distance, the agency carefully
considered the statutory requirements of 49 U.S.C. Chapter 301.
    First, this final rule reflects the agency's careful consideration
and analysis of all issues raised in public comments on the agency's
December 2005 notice of proposed rulemaking. In responding to the
issues raised in the comments, the agency considered all relevant motor
vehicle safety information. In preparing this document, the agency
carefully evaluated relevant, available research, testing results, and
other information related to various air brake technologies. In sum,
this document reflects our consideration of all relevant, available
motor vehicle safety information.
    Second, to ensure that the heavy truck stopping distance
requirements remain practicable, the agency evaluated the potential
impacts of the proposed requirements in light of the cost,
availability, and suitability of various air brake systems, consistent
with our safety objectives and the requirements of the Safety Act. As
explained in detail in the FRIA, this final rule adopts a 30 percent
reduction in stopping distance for the overwhelming majority of
tractors, which corresponds to the most stringent of the requirements
proposed in the NPRM. (For the remaining one percent (mostly severe
service tractors with high GVWRs), the final rule adopts a requirement
for a 13 percent reduction in stopping distance beyond the standard's
existing levels.) Our analysis of the available data and public
comments shows that it is practicable for the subject vehicles to
achieve the newly required reduction in stopping distance using
available technology. In sum, we believe that this final rule is
practicable and will increase the benefits of FMVSS No. 121, including