with a GVWR less than 11,793 kg (26,000 lb.), but there were 209 in
buses with a GVWR greater than 11,793 kg (26,000 lb.).
    Moreover, MAP-21, which was enacted after publication of the NPRM,
requires the Secretary to consider requiring ESC systems on certain
large buses if the Secretary determines that such a requirement is
consistent with the requirements of the Motor Vehicle Safety Act. We
believe that mandating ESC systems on the buses covered by the NPRM,
subject to some minor changes discussed below, is consistent with those
requirements. That is, this standard is practicable, meets the need for
motor vehicle safety, and may be stated in objective terms. We believe
that ESC systems are currently available for must buses covered by this
final rule and can be developed for the others. Moreover, the safety
problem discussed in Section IV.D above highlights the rollover problem
in buses with a GVWR greater than 11,793 kg (26,000 lb.).
    NHTSA has decided to adopt the proposal to require all buses with a
GVWR over 11,793 kg (26,000 lb.), subject to some modified exclusions
for school buses, transit buses, and perimeter seating buses. In
Section V.B.1 of the NPRM, NHTSA mentioned the rationale for not
including a requirement for ESC on single-unit trucks with a GVWR over
4,536 kilograms (10,000 pounds) at this time.\36\ The rationale was
primarily based on the differences between truck tractors and single-
unit trucks; it was not intended and did not mention the differences
between buses built on truck chassis and buses built with monocoque
construction. Although the NPRM stated that single-unit trucks as a
whole are more complex and diverse than truck tractors, this does not
necessarily apply to buses built on truck chassis. Among the different
bodies that could be assembled on a truck chassis, a bus body presents
a degree of complexity and diversity that is substantially less than
the other truck bodies. For example, a bus body presents a scenario
where center-of-gravity height and cargo type are more easily
calculated because the bus is limited to transporting people and their
luggage rather than varied cargo. The chassis supplier for a bus would
be more likely to have knowledge of critical vehicle design parameters
that affect ESC calibration.
---------------------------------------------------------------------------

    \36\ 77 FR 30789.
---------------------------------------------------------------------------

    NHTSA reviewed various definitions used in motorcoach safety
legislation including the ``over-the-road bus'' definition in TEA-21
that was referenced in MAP-21. Similar to the final rule requiring seat
belts on certain buses, we are not limiting the applicability of the
ESC requirement to TEA-21's definition of over-the-road buses.\37\ We
believe that the definitions referring to over-the-road buses or over-
the-road bus service are too narrow, because a number of intercity
transport buses involved in fatal crashes were body-on-chassis buses
that lacked an elevated passenger deck over a baggage compartment.
Further, definitions based on the intended use of the vehicle could
pose difficulties for manufacturers and dealers, because the intended
use of a vehicle might not be known at the time of vehicle manufacture
or sale. We want to make sure as reasonably possible that the buses we
most wanted to affect (high-capacity buses associated with known
fatality and injury risks) would meet the ``motorcoach'' safety
standards, without having to depend on the state of knowledge of
persons in the manufacturing and distribution chain about the
prospective use of the bus.
---------------------------------------------------------------------------

    \37\ 78 FR 70429.
---------------------------------------------------------------------------

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

definition for motorcoach in its regulations, but it considers a
``motorcoach'' to be an over-the-road bus. As noted above, over-the-
road buses are a subset of the buses NHTSA believes should be regulated
as ``motorcoaches,'' encompassing a part of but not enough of the heavy
bus safety problem we seek to address.
    We reviewed the underlying chassis structure of high-occupancy
vehicles involved in fatal crashes. Some had a monocoque structure with
a luggage compartment under the elevated passenger deck (``over-the-
road buses''). However, an elevated passenger deck over a baggage
compartment was not an element common to the buses involved in fatal
intercity transport. In FARS data for buses with a GVWR greater than
11,793 kg (26,000 lb.), 36 percent of the fatalities were in the other
bus and unknown bus categories, i.e., not in the over-the-road bus
category. Some buses were built using body-on-chassis configurations.
    We believe that body-on-chassis configurations are newer entrants
into the motorcoach services market. They appear to be increasing in
number. A cursory review of the types of buses being used in the
Washington, DC area for motorcoach services showed that traditional
motorcoaches are generally used for fixed-route services between major
metropolitan areas. However, for charter, tour, and commuter
transportation from outlying areas, many bus types are used. Some are
of monocoque structure, while others are of body-on-chassis structure.
    The agency tested Class 8 buses, those with a GVWR greater than
14,969 kg (33,000 lb.), because these buses have larger dimensions and
masses than Class 7 buses, and it places them on the most severe end of
the spectrum. The performance criteria were created based on the
testing of the larger Class 8 buses, and the agency has made a reasoned
determination that the criteria are applicable for Class 7 buses, as
well. If a Class 8 bus with a larger GVWR can pass the minimum
performance criteria for ESC systems, a Class 7 bus with a smaller GVWR
can reasonably be required to meet the same criteria.
    Despite the fact that some of these buses are built in multiple
stages by multiple manufacturers, the agency does not agree that
compliance with the ESC standard will be very difficult. Presently,
manufacturers building buses in various stages must provide an
incomplete vehicle document (49 CFR part 568) to subsequent
manufacturers listing each standard that applies. One example of a
standard that must be documented is FMVSS No. 121, Air Brake Systems. A
number of factors such as GVWR, GAWR, and any other specific conditions
given by the manufacturer must be considered when determining if a bus
will be compliant with the braking requirements after it is built.
Likewise, the agency expects manufacturers to give similar conditions
of final manufacture under which the manufacturer specifies that the
completed vehicle will conform to the ESC standard. The agency
considers that burden of bus manufacturers to comply with the ESC rule
will not be more difficult than the current burden of complying with
the air brake requirements in FMVSS No. 121.
3. Hydraulic-Braked Buses
    In the NPRM, we requested comment on manners in which hydraulic-
braked buses may be differentiated, such as by exclusion or a different
phase-in period for the ESC rule. Six commenters provided statements
about hydraulic-braked buses and how they should be excluded.
Specifically, Blue Bird opposes an ESC mandate on hydraulic-braked
buses with a GVWR of 36,200 pounds and less. It also commented that the
agency should wait until ESC systems are developed and fully evaluated
for hydraulic-braked medium or heavy buses and not include hydraulic-
braked buses as part of the ESC rule at this time. Blue Bird, Daimler,
Meritor WABCO, Navistar, and EMA all commented that they are not aware
of any ESC systems available for hydraulic-braked buses covered by the
NPRM. Meritor WABCO recommended that NHTSA exclude vehicles that are
not ``pneumatically braked.'' Finally, both Daimler and EMA stated that
they want the ESC regulation to extend only to motorcoaches over 33,000
pounds.
    NHTSA has no convincing evidence to exclude hydraulic-braked buses
from this ESC rule. The NPRM proposed to require ESC on both Class 7
and Class 8 buses. The mandate in the Motorcoach Enhanced Safety Act
makes no differentiation between Class 7 and Class 8 buses. In order to
address the rollover and loss-of-control safety problems with these
classes of buses, the ESC rule must include both air and hydraulic
brakes.
    Based on feedback received from the commenters, we recognize that
Class 7 buses are composed of both air- and hydraulic-braked vehicles.
We recognize that manufacturers who produce large buses equipped with
hydraulic-powered brakes might need extra time to ensure the proper
integration between the ESC system and the vehicle's chassis, engine,
and braking system. Rather than exclude hydraulic-braked buses from the
rule entirely, NHTSA will extend the compliance date for buses that may
be equipped with hydraulic brakes. NHTSA acknowledges that ESC systems
are still in development for large buses with hydraulic-braked buses,
and therefore, manufacturers and suppliers need additional time to
implement this new technology. However, whether the bus is equipped
with air brakes or hydraulic brakes, we expect the performance
requirements to apply because they are based on the stability of the
bus as defined by its attributes such as geometry, mass, inertia, and
center-of-gravity height. There is a negligible change in these
attributes between an air-braked and a hydraulic-braked bus.
4. School Buses
    Six commenters recommended that NHTSA include a requirement that
school buses be equipped with ESC systems in the final rule. Consumers
Union commented that ESC technology should be required for school buses
in order to set a precedent for future crash avoidance technologies.
Martec recommended that ESC be required on all buses because it claims
that ``large school buses satisfy multiple criteria described by NHTSA
in its 2011-2013 Rulemaking and Research Priority Plan: the addition of
ESC/RSC to school buses would offer large safety benefits, would apply
to high-occupancy vehicles, and would apply to a vulnerable
population--children.'' Skagit, NTSB, and IIHS all want ESC to be
mandated on all buses greater than 10,000 lb., including school buses.
    Conversely, Daimler and NSTA both agreed that NHTSA not include
school buses in a final rule mandating ESC systems on large buses. NSTA
asserted that, if school buses were subject to an ESC mandate, the
costs to purchase school buses would increase. NSTA is concerned that
the added costs would reduce the number of school buses on the road,
and, consequently, reduce the number of children riding buses to
school. NTSA claims that students riding school buses are eight times
safer than riding in the family vehicle because school buses travel at
lower speeds and largely in residential areas.
    As in the NPRM, we are excluding school buses from the ESC
requirement. Each NHTSA rulemaking must address a present safety need
and be justified by present safety benefits. We cannot accept Consumers
Union's recommendation to do rulemaking now based on speculative
benefits of ESC systems on school buses. According to FARS data between
2000 and 2009, among the large buses, more than 70%

of fatalities on large buses with a GVWR greater than 11,793 kg (26,000
lb.) were related to cross-country intercity bus crashes. Similarly, we
stated in the NPRM that FMCSA's Large Truck and Bus Crash Facts 2008
indicates that most of the school bus crashes are not rollover or loss-
of-control crashes that ESC systems are capable of preventing. For
these reasons, we will not require school buses to be equipped with ESC
at this time.
    Navistar, EMA, and Daimler requested that the school bus exclusion
extend into its line of school bus derivatives. Navistar and EMA
reasoned that some commercial buses are built on truck chassis. Because
of their similarities to school buses, they reasoned that those buses
should be exempted from the ESC rule. According to Daimler, school bus
derivatives are vehicles built with hydraulic brakes, and no ESC system
is available on these types of hydraulic brakes in the market today.
    We disagree with Daimler, EMA, and Navistar that the school bus
exception should extend to other buses that are similar or
``derivatives'' as Daimler stated. If the commenters' reasoning was
adopted, any manufacturer could offer a school bus version of a
particular bus model and claim that the school bus exception should
apply because of the artificially created similarities. This would
create an unintended loophole for the ESC requirement and potentially
undermine the rule.
5. Transit Buses
    The NPRM proposed to exclude from the ESC system requirements urban
transit buses sold for operation in urban transportation along a fixed
route with frequent stops. EMA and Volvo suggested that we exclude
certain buses based on the intended use of the vehicle in public
transit. Volvo requested that the agency base the exclusion on the
Federal Transit Administration's (FTA) bus procurement guidelines.
Volvo suggested excluding ``urban transit buses which may be used on
suburban express service and general service on urban arterial streets
along a fixed route with frequent stops.'' Similarly, EMA suggested
adding to the exclusion for transit buses ``urban transit buses used in
suburban express service.'' Conversely, Volvo stated during the public
hearing that it was practical and technologically feasible to equip its
urban buses with ESC, but it did not want to do so because it did not
perceive a safety need.
    The Motorcoach Enhanced Safety Act excludes from its mandate to
consider requiring ESC systems on large buses a bus used in public
transportation provided by, or on behalf of, a public transportation
agency. However, as we explained in the previous section regarding
school buses, an exclusion based on the intended use of the vehicle
could pose difficulties for manufacturers and dealers, because the
intended use of a vehicle might not be known at the time of vehicle
manufacture or sale. Consequently, we will not adopt the recommendation
suggested by EMA and Volvo to exclude urban transit buses used in
suburban express service.
    The final rule requiring seat belts at all passenger seating
position on certain buses noted that commenters on that NPRM were
troubled that the proposed transit bus exclusion was not sufficiently
clear. To make the definition more clear, the final rule made
clarifications that we believe are also warranted in this final rule
requiring ESC systems on certain buses.\38\ First, we made the
regulatory text clearer in describing a ``transit bus'' by referring to
a structural feature (a stop-request system) that buses must have to be
a ``transit bus.'' A ``stop-request system'' means a vehicle-integrated
system for passenger use to signal to a vehicle operator that a stop is
requested. Second, we expanded the description of a transit bus by
recognizing that a transit bus could be sold for public transportation
provided not only by, but also on behalf of, a State or local
government, for example, by a contractor.
---------------------------------------------------------------------------

    \38\ 78 FR 70438.
---------------------------------------------------------------------------

    Finally, we made clear that over-the-road buses, as defined by TEA-
21, do not qualify as ``transit buses,'' even if the over-the-road bus
has a stop-request system or is sold for public transportation provided
by or on behalf of a State or local government. This final
clarification ensures both that a manufacturer cannot integrate a
simple stop-request system on any bus and make it subject to the
transit bus exclusion. We recognize that any over-the-road bus used for
public transportation provided by or on behalf of a State or local
government is likely to be used as a commuter express bus that would
carry large numbers of passengers over long distances at highway
speeds. However, this use case is similar to the use of over-the-road
buses by private companies in intercity service.
6. Minimum Seating Capacity and Seating Configuration
    The NPRM also excluded buses that had fewer than 16 designated
seating positions (DSPs), including the driver. This reference was
included in the seat belt NPRM based on FMCSA's definition of a
``commercial motor vehicle,'' for purposes of FMCSA's commercial
driver's license requirements.\39\ In the final rule, however, NHTSA
noted that FMCSA's regulations state that buses with a GVWR greater
than 11,793 kg (26,000 lb.) are commercial vehicles under the
commercial driver's license regulations, regardless of the number of
DSPs. Accordingly, that exclusion was removed from the final rule.\40\
---------------------------------------------------------------------------

    \39\ 75 FR 50969.
    \40\ 78 FR 70433.
---------------------------------------------------------------------------

    EMA and Daimler suggested that the rule exclude all buses with
fewer than 40 passenger seats, which they imply would exclude buses
that are not considered ``motorcoaches.'' However, neither EMA nor
Daimler included any explanation for why 40 passenger seats is an
appropriate cutoff for an ESC system requirement, and we can perceive
none. We do not believe that a minimum number of passenger seats would
serve to include or exclude buses that are being driven at long
distances or at highway speeds.
    The NPRM also proposed to exclude buses with fewer than two rows of
passenger seats that are rearward of the driver's seating position and
are forward-facing or can convert to forward-facing without the use of
tools. This reference was included in the large bus seat belt NPRM to
distinguish buses with perimeter seating such as those used to
transport passengers in airports between the terminal and locations
such as a rental car facility or long term parking.\41\ These buses
typically have a single forward-facing row of seats in the back of the
vehicle and seats along one or both sides of the bus. These buses
typically carry people for a relatively short period, often transport
standees, generally accommodate baggage and other items, and are
designed for rapid boarding and alighting. These buses were excluded
because we believed they would be used for relatively short distances
on set routes, which are not widely exposed to general traffic.
---------------------------------------------------------------------------

    \41\ 78 FR 70434.
---------------------------------------------------------------------------

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

applicable to the FMVSSs and it was difficult to define ``row'' for the
purpose of excluding perimeter-seating buses using plain language.
Thus, we defined a ``perimeter-seating bus'' as a bus with 7 or fewer
DSPs rearward of the driver's seating position that are forward-facing
or can convert to forward-facing without the use of tools, and excluded
perimeter-seating buses from the seat belt requirement.\42\
---------------------------------------------------------------------------

    \42\ See 78 FR 70434-35.
---------------------------------------------------------------------------

    We believe that this exclusion is similarly applicable to the ESC
system requirement, and we are adopting in this final rule the
simplified language used in the seat belt final rule. A perimeter-
seating bus typically carries people for short distances on set routes
and is often less exposed to general traffic than transit buses.
However, consistent with the Motorcoach Enhanced Safety Act, we are not
excluding from the ESC system requirement perimeter-seating buses that
are also over-the-road buses. Some of these buses may include vehicles
often referred to as ``limo buses'' or ``party buses.'' These vehicles
may also be used as touring or entertainment buses with eating and
sleeping accommodations that are used by celebrities and entertainers
on tour. We expect that these types of buses will be used for intercity
travel and driven at highway speeds.
7. Compliance Dates
    The NPRM proposed that buses meet the ESC system requirements two
years after publication of a final rule implementing the proposal.
Although we did not receive any comments specifically addressing the
compliance date for large buses, the Motorcoach Enhanced Safety Act
specifically states that a stability enhancing requirement shall apply
to all motorcoaches manufactured more than 3 years after the date on
which the regulation is published as a final rule. Based on the
Congressional determination that any enhancing stability technology
rulemaking shall apply to all over-the-road buses manufactured more
than 3 years after the final rule is published, we will allow bus
manufacturers that amount of time inasmuch as a three-year lead time is
practical.
    With respect to Class 7 buses, the agency has determined that a
three-year compliance date is not practical. The scope of this final
rule includes buses that are hydraulic-braked. We recognize the
manufacturers of hydraulic-braked buses will likely require extra time
to ensure system availability and that the ESC system is properly
integrated with the vehicle. Based on the comments received from the
bus industry, Class 7 buses are equipped with both air and hydraulic
brakes. Rather than differentiate between brake systems of the Class 7
buses, we believe it would be better to base the compliance date
requirements on GVWR. This will also address the concerns of
manufacturers of buses built on truck chassis, for which ESC systems
may not currently be equipped. We believe that at least four years of
lead time are necessary to ensure that suppliers have ESC systems
available for hydraulic-braked large buses. Accordingly, this final
rule allows Class 7 bus manufacturers four years of lead time before
the requirements of this final rule become applicable.
8. Class 3 Through 6 Buses
    Some of the commenters recommended that we expand the scope to
include mid-size buses which are typically built on single-unit truck
frames. Skagit, NTSB, IIHS, NAPT, Advocates, and Bosch all suggested
that ESC should be mandated on all buses greater than 10,000 pounds.
The NTSB estimated that 11,600 mid-size buses (buses with a GVWR
between 10,000 pounds and 26,000 pounds) are produced each year.
Advocates recommended that NHTSA should consider the NTSB
recommendation that all buses over 10,000 pounds GVWR should be
equipped with stability control systems. Bosch stated that the agency
should develop a performance standard to cover vehicles in Classes 3
through 7 with hydraulic brakes because this segment accounts for a
large number of commercial and load bearing vehicles on the U.S. roads.
Bosch claims that a standard with a phase-in period is needed to
facilitate industry development of ESC systems for these vehicles.
Bosch also cites Annex 21 of UN ECE Regulation 13, which requires ESC
on buses operating in the European Union.
    We are not expanding the scope of this rule to include vehicles
with a GVWR of 11,793 kilograms (26,000 pounds) or less. After
publishing the NPRM, we began a research program to study the safety
benefits and performance criteria of ESC systems on single-unit trucks,
which includes mid-size buses. The research is not yet complete on
single-unit trucks or smaller buses. However, we believe including
buses with hydraulic brakes in this final rule will spur development of
ESC systems for other hydraulic-braked vehicles, including buses with a
GVWR of greater than 4,536 kilograms (10,000 pounds) but not more than
11,793 kilograms (26,000 pounds).

C. Retrofitting

    NHTSA considered proposing to require retrofitting of in-service
truck tractors, trailers, and large buses with stability control
systems. The Secretary has the statutory authority to promulgate safety
standards for ``commercial motor vehicles and equipment subsequent to
initial manufacture.'' \43\ The Secretary has delegated authority to
NHTSA to promulgate safety standards for commercial motor vehicles and
equipment subsequent to initial manufacture when the standards are
based upon and similar to an FMVSS promulgated, either simultaneously
or previously, under chapter 301 of title 49, U.S.C.\44\ Additionally,
the Federal Motor Carrier Safety Administration (FMCSA) is authorized
to promulgate and enforce vehicle safety regulations, including those
aimed at maintaining commercial motor vehicles so they continue to
comply with the safety standards applicable to commercial motor
vehicles at the time they were manufactured.
---------------------------------------------------------------------------

    \43\ See Motor Carrier Safety Improvement Act of 1999, section
101(f), Pub. L. 106-159 (Dec. 9, 1999).
    \44\ See 49 CFR 1.50(n).
---------------------------------------------------------------------------

    Although the NPRM did not propose requiring truck tractors,
trailers, or large buses to be equipped with stability control systems
``subsequent to initial manufacture,'' we requested public comment on
several issues related to retrofitting in-service truck tractors,
trailers, and buses:
     The extent to which a proposal to retrofit in-service
vehicles with stability control systems would be complex and costly
because of the integration between a stability control system and the
vehicle's chassis, engine, and braking systems.
     The changes necessary to an originally manufactured
vehicle's systems that interface with a stability control system, such
as plumbing for new air brake valves and lines and a new electronic
control unit for a revised antilock brake system.
     The additional requirements that would have to be
established to ensure that stability control components are at an
acceptable level of performance for a compliance test, given the
uniqueness of the maintenance condition for vehicles in service,
particularly for items such as tires and brake components that are
important for ESC performance.
     The original manufacture date of vehicles that should be
subject to any retrofitting requirements.

     Whether the performance requirements for retrofitted
vehicles should be less stringent or equally stringent as for new
vehicles, and, if less stringent, the appropriate level of stringency.
     The cost of retrofitting a stability control system on a
vehicle, which we believe would exceed the cost of including stability
control on a new vehicle.
    Several commenters addressed issues related to retrofitting in-
service vehicles with ESC systems. We received comments both favoring
and opposing retrofitting.
    Road Safe America, NTSB, and Advocates supported a requirement for
ESC to be retrofitted to existing heavy vehicles. Road Safe America
recommended that RSC systems be retrofitted on all existing truck
trailers. NTSB cited its recommendation that RSC systems be retrofitted
on in-use cargo tank trailers. In its comments, Advocates said that
there should be a retrofit requirement to install ESC systems on all
in-service vehicles. Advocates stated that the failure to require
retrofitting could significantly delay fleet penetration of ESC systems
because of the extended service life of the affected vehicles.
    Many more commenters were opposed to a retrofit requirement for ESC
systems. IIHS stated that ESC systems should not be required to be
retrofitted at this time, but that the agency should explore the
feasibility creating a requirement in the future. American Highway
Users requested that there should be no retrofit requirements for
existing vehicles in order to incorporate ESC systems and would oppose
any efforts to implement a retrofit requirement. In its comment, ATA
did not support a retrofit requirement for ESC systems because it
claims there is an average of a 4-5 year turnover for a majority of
Class 7 and Class 8 tractors. Volvo commented that there should not be
a retrofit of trucks because the changes to the vehicle are too
significant, and there is no way to assure the quality of the retrofit.
    Meritor WABCO stated that there should not be a retrofit of
vehicles because, as a system supplier, it does not offer an ESC system
retrofit option. Meritor WABCO also specified that ESC systems must be
engineered and validated for each vehicle model and parts must be
added, which would be difficult to do on in-service vehicles. Meritor
WABCO further stated that an ESC system requires a steering wheel angle
sensor, which is difficult to design for in-service vehicles. Meritor
WABCO also expressed concern about the possibility of incomplete or
incorrect retrofit installations if retrofits are required.
    The National Ready Mix Concrete Association argued that there
should not be an ESC system retrofit requirement on single-unit trucks
or truck tractors because retrofit costs will be higher on existing
trucks than installations on new trucks. They further stated that a
variety of improvised techniques are needed when doing retrofit
installations, and these techniques result in higher maintenance costs.
They were also concerned that a retrofitted system would not work on
some older trucks because of unworkable truck designs and interference
with safety and electronic features.
    HDBMC stated that there should be no retrofit requirement because
retrofitting of ESC systems is impractical and difficult. HDBMC cited
the challenges of ESC system retrofitting, which include: (1)
Compatibility of the vehicle; (2) computer hardware and software
issues; (3) issues with new component installation; (4) vehicle
downtime to make the conversion; (5) testing and validation; and (6)
further unknown variables.
    EMA asserted that it would be unsafe to implement a retrofit
requirement because ESC systems are not currently installed over
existing components. EMA also believes that aftermarket facilities do
not have the capability to design, test, and implement ESC systems. EMA
stated that rotational sensors, yaw rate, and lateral accelerometers
must be mounted close to the vehicle's center of yaw rotation, or
complex calculations must be used to compensate for any deviations in
the mounting. Finally, EMA commented that the necessary components for
an ESC system do not exist for older vehicle models.
    Bendix commented that it had, for the purposes of research and
development, retrofitted ESC to more than 25 vehicles. Bendix estimated
that retrofitting in-service vehicles would take between 80 and 120
person-hours for installation because each installation would have to
be customized and there would be little or no OEM support.
    After considering the public comments, NHTSA has decided not to
include a retrofit requirement in this final rule. NHTSA recognizes
that the costs and safety risks of mandating an ESC system retrofit may
exceed the benefits. Those commenters supporting an ESC system retrofit
did not provide any information to mitigate issues such as: (1) The
complexity and cost to retrofit in-service vehicles with ESC systems;
(2) the changes necessary to integrate the ESC system to the vehicle's
chassis, engine, and braking system; (3) the changes necessary on the
in-service vehicle to interface with the ESC system such as plumbing
for new air brake valves and lines and a new electronic control unit
for the ABS system; and (4) the additional requirements for in-service
vehicles considering the uniqueness of the maintenance condition of the
tire and brake components. Considering that the potential safety risks
and certain high costs associated with a requirement to retrofit in-
service vehicles with ESC systems greatly exceed the benefits, NHTSA
has not included a retrofit requirement in this final rule.

X. Performance Testing

A. NHTSA's Proposed Performance Tests

    The agency's research initially focused on a variety of maneuvers
that we could use to evaluate the roll stability performance and the
yaw stability performance of truck tractors and large buses. Several of
these maneuvers were also tested by industry and some of them are
allowed for use in testing for compliance to the UN ECE stability
control regulation. The agency's goal was to develop one or more
maneuvers that showed the most promise as repeatable and reproducible
roll and yaw performance tests for which objective pass/fail criteria
could be developed. Based on the agency's own testing and the results
from industry-provided test data, two stability performance tests were
proposed to evaluate ESC systems on truck tractors and large buses--the
SIS test and the SWD test.
1. Characterization Test--SIS
    The agency proposed using the slowly increasing steer maneuver
(SIS) as a characterization test to determine the unique dynamic
characteristics of a vehicle. This maneuver would allow the agency to
determine the relationship between the steering wheel angle and lateral
acceleration of a vehicle. Also as part of the SIS characterization
test, the ability of the ESC system to reduce engine torque is
determined. During each of the SIS maneuvers, ESC activation is
confirmed by verifying that the system automatically reduces the driver
requested engine torque output. The NPRM proposed that, for each of the
SIS maneuver test runs, the commanded engine torque and the driver
requested torque signals must diverge at least 10 percent for 1.5
seconds after the beginning of ESC system activation. This test

demonstrates that the ESC system has the capability to reduce engine
torque, as required in the functional definition. The vehicles that the
agency tested were all able to meet this proposed performance level.
2. Roll and Yaw Stability Test--SWD
    In the NPRM, we proposed using the sine with dwell maneuver (SWD)
to test the ability of an ESC system to mitigate conditions that would
lead to rollover or loss of control. Conceptually, the steering profile
of this maneuver is similar to that expected to be used by real drivers
during some crash avoidance maneuvers. As the agency found in the light
vehicle ESC research program, the severity of the SWD maneuver makes it
a rigorous test, while maintaining steering rates within the
capabilities of human drivers. We believed that the maneuver is severe
enough to produce rollover or vehicle loss-of-control without a
functioning ESC system on the vehicle.
    The agency's test program was able to develop test parameters for
the SWD maneuver so that both roll stability and yaw stability could be
evaluated using a single loading condition and test maneuver.
Previously, the SWD maneuver had typically been used to evaluate only
the yaw instability of a vehicle. NHTSA evaluated several loading
conditions and found that a loading condition of 80 percent of the
tractor's GVWR enabled us to evaluate both the yaw and roll stability
control of the ESC system.
    For a truck tractor, the agency would conduct the SWD test with the
truck tractor coupled to an unbraked control trailer and loaded with
ballast directly over the kingpin. The combination vehicle would be
loaded to 80 percent of the tractor's GVWR. For a bus, the vehicle is
loaded with a 68 kilogram (150 pound) ballast in each of the vehicle's
designated seating positions, which would bring the vehicle's weight to
less than its GVWR. The test vehicles were proposed to be equipped with
outriggers to prevent the trailer from rolling over in case the ESC
system does not function properly.
    The SWD test would be conducted at a speed of 72 km/h (45 mph). An
automated steering machine would be used to initiate the steering
maneuver. Each vehicle is subjected to two series of test runs. One
series uses counterclockwise steering for the first half-cycle, and the
other series uses clockwise steering for the first half-cycle. The
steering amplitude for the initial run of each series is 0.3A, where A
is the steering wheel angle determined from the SIS maneuver. In each
of the successive test runs, the steering amplitude would be increased
by increments of 0.1A until a steering amplitude of 1.3A or 400
degrees, whichever is less, is achieved. Upon completion the test runs,
the agency would conduct post-processing of the yaw rate and lateral
acceleration data to determine the lateral acceleration ratio, yaw rate
ratio, and lateral displacement, as discussed below.
    The lateral acceleration ratio (LAR) is a performance metric
developed to evaluate the ability of a vehicle's ESC system to prevent
rollovers. Lateral acceleration is measured on a bus or a tractor and
corrected for the vehicle's roll angle. As a performance metric, the
lateral acceleration value is normalized by dividing it by the maximum
lateral acceleration that was determined at any time between 1.0
seconds after the beginning of steering and the completion of steering.
The two proposed performance criteria are described below:
     A vehicle must have a LAR of 30 percent or less 0.75
seconds after completion of steer.
     A vehicle must have a LAR of 10 percent or less at 1.5
seconds after completion of steer.
    The yaw rate ratio (YRR) is a performance metric used to evaluate
the ability of a vehicle's ESC system to prevent yaw instability. The
YRR expresses the lateral stability criteria for the sine with dwell
test to measure how quickly the vehicle stops turning, or rotating
about its vertical axis, after the steering wheel is returned to the
straight-ahead position. The lateral stability criterion, expressed in
terms of YRR, is the percent of peak yaw rate that is present at
designated times after completion of steer. This performance metric is
identical to the metric used in the light vehicle ESC system
performance requirement in FMVSS No. 126. The two proposed performance
criteria are described below:
     A vehicle must have a YRR of 40 percent or less 0.75
seconds after completion of steer.
     A vehicle must have a YRR of 15 percent or less at 1.5
seconds after completion of steer.
3. Lateral Displacement
    Lateral displacement is a performance metric used to evaluate the
responsiveness of a vehicle, which relates to its ability to steer
around objects. Stability control intervention has the potential to
significantly increase the stability of the vehicle in which it is
installed. However, we believe that these improvements in vehicle
stability should not come at the expense of poor lateral displacement
in response to the driver's steering input.
    A hypothetical way to pass a stability control performance test
would be to make either the vehicle or its stability control system
intervene simply by making the vehicle poorly responsive to the speed
and steering inputs required by the test. An extreme example of this
potential lack of responsiveness would occur if an ESC system locked
both front wheels as the driver begins a severe avoidance maneuver that
might lead to vehicle rollover. Front wheel lockup would create an
understeer condition in the vehicle, which would result in the vehicle
plowing straight ahead and colliding with an object the driver was
trying to avoid. It is very likely that front wheel lockup would reduce
the roll instability of the vehicle since the lateral acceleration
would be reduced. This is clearly, however, not a desirable compromise.
    Because a vehicle that simply responds poorly to steering commands
may be able to meet the stability criteria proposed in the NPRM, a
minimum responsiveness criterion was also proposed for the SWD test.
The proposed lateral displacement criterion was that a truck tractor
equipped with stability control must have a lateral displacement of
2.13 meters (7 feet) or more at 1.5 seconds from the beginning of
steer, measured during the sine with dwell maneuver. For a bus, the
proposed performance criterion is a lateral displacement of 1.52 meters
(5 feet) or more at 1.5 seconds after the beginning of steer. The
lateral displacement criteria is less for a bus because a large bus has
a longer wheelbase than a truck tractor and higher steering ratio,
which makes it less responsive than a truck tractor.

B. Comments on SIS and SWD Maneuvers

    The agency received many comments, particularly from
representatives of ESC system, truck tractor, and bus manufacturers
specifically addressing the slowly increasing steer and sine with dwell
maneuvers proposed in the NPRM. The comments raised issues regarding
the relevance of the SWD and SIS tests, the amount of space required to
perform the test, and the automated steering machine.
    Daimler Trucks North America (DTNA), the ATA, and Navistar claimed
the SWD was not representative of a real-world maneuver. EMA stated the
no manufacturer to date was using the SWD maneuver to test and validate
an ESC system. Navistar claimed the standard width of a highway lane
does not allow room for the SWD maneuver

to be completed. EMA shared Navistar's belief that a driver of a truck
tractor would require 6 to 8 lanes of road width to perform a SWD
maneuver on a roadway, and the SWD test is unlike any maneuver likely
to occur on public roads.
    DTNA asserted that the SWD test fails to provide adequate pass/fail
criteria as an ESC performance test. Similarly, Volvo stated that the
SWD performance test criteria is impractical and unnecessary because
there are established validation test methods available and in use.
    DTNA, Navistar, and EMA suggested that tuning the ESC system to
pass the SWD test could compromise the system performance. Navistar
reasoned that focusing on the SWD test would diminish the amount of
design work done to optimize ESC performance for other conditions.
Navistar also speculated that some ESC systems may not comply with the
SWD test and may require a lengthy research and development plan to
redesign the systems. On the other hand, Bendix Commercial Vehicle
Systems (Bendix) assured the agency that tractors equipped with the
current Bendix ESC systems could pass the proposed SWD and SIS tests.
    DTNA and EMA alleged that there would be additional burdens and
restrictions on manufacturers caused by a SWD performance test. DTNA
stated that manufacturers have a burden to conduct extensive ESC
testing because of the lack of experience with the SWD test. EMA
claimed that heavy vehicle options would be restricted to ensure
compliance with the SWD test. Neither commenter provided details to
support its claims.
    We also received comments on the amount of space required to
conduct SIS and SWD tests. According to Navistar, EMA, and Bendix, the
SWD and SIS tests require a large area in order to perform the tests.
Navistar, EMA, DTNA, Volvo, and the HDBMC claimed that the
Transportation Research Center (TRC) in Ohio is the only test facility
large enough to perform the SWD and SIS tests. Based on this belief,
they assume an increase in the number of manufacturers using TRC will
limit the test facility availability. Bendix provided data and
calculations to support its recommendation for the test area dimensions
needed to safely perform the SIS and SWD tests. According to Bendix,
the SIS test needs an area of 176 m (563.2 ft.) by 151 m (483.2 ft.),
and the SWD test needs a smaller area of 112 m (358.4 ft.) by 58 m
(185.6 ft.). Bendix further argued that the ESC performance tests
should be portable, meaning that any test facility that can run FMVSS
No. 121 tests should be able to run FMVSS No. 136 tests.
    In the NPRM, we proposed using a steering machine to provide the
steering wheel inputs for the vehicles during the SIS and SWD tests.
Advocates recommended that the SWD and SIS tests should be required
along with an automated steering machine. However, Bendix, Volvo, and
EMA expressed concern regarding the steering machine and the
capabilities of a vehicle's steering system to perform the SWD
maneuver. Bendix stated that the steering robot specified in the NPRM
is inadequate and suggested that more research needs to be done to find
a steering controller more suited for large vehicles. According to
Volvo, the same steering machine requirements as those found in FMVSS
No. 126 would not be sufficient for heavy vehicles. EMA and Bendix
expressed concerns that the SWD requires steering inputs that approach
the limit of what a human being can accomplish. EMA also claims the SWD
test exceeds the capacity of power steering systems on some tractors,
which affects the results of the SWD and exposes the driver to safety
risks.
    Commenters also addressed the costs of conducting the proposed SIS
and SWD tests. ATA and EMA stated that the proposed SWD test would be
costly because of the logistics and preparation costs to test at TRC.
Navistar said that a new facility would need to be built to conduct the
SWD tests at an estimated cost of $4 to 6 million plus additional costs
for maintenance and repair of the facility.
    Meritor WABCO, EMA, and Volvo provided estimates regarding the
costs and burden of conducting the SWD test. Meritor WABCO commented
that the tests are too costly and estimated the costs to be in excess
of $28,000 per tractor. EMA claimed the SWD is too expensive because
heavy vehicles have many variations, small volumes, and typically
testing is performed on saleable vehicles. EMA estimated that each
truck tractor manufacturer would need to run 50 to 80 tests for its 6x4
tractors causing a high cost for the SWD testing, which is spread out
over a low production volume of heavy vehicles. EMA further commented
that manufacturers might have to redesign steering systems to comply in
order to perform the SWD tests, which would further increase the costs.
Additionally, EMA claims NHTSA did not test any severe service tractors
using SWD testing, and the sample of truck tractors NHTSA tested was
too narrow to support the proposal. Further EMA criticized NHTSA's test
program for using only one control trailer and one test facility. Volvo
alleged that the proposed performance tests could potentially damage
test vehicles, and some manufacturers conduct assurance tests on
customer vehicles.

C. Alternative Maneuvers Considered in the NPRM

    We considered other test maneuvers besides the SIS and SWD tests in
the NPRM. The SWD maneuver was chosen in the NPRM over other maneuvers
because our research demonstrated that it has the most optimal set of
characteristics, including the severity of the test, repeatability and
reproducibility of results, and the ability to address rollover,
lateral stability, and responsiveness. However, we left within the
scope of the NPRM several other test maneuvers that could be used to
test an ESC system's ability to mitigate instability.
    With respect to rollover instability mitigation, we discussed the
ramp steer maneuver (RSM) and J-turn maneuver. The two tests are
similar in that both maneuvers require the tested vehicle to be driven
at a constant speed and then the vehicle is turned in one direction for
a certain period of time. The test speed and the severity of the turn
are designed to cause a test vehicle to approach or exceed its roll
stability threshold such that, without a stability control system, the
vehicle would exhibit signs of roll instability. Both tests would be
performed with the tractor loaded to its GVWR. Furthermore, we do not
expect a vehicle that could pass one test to fail the other.
    The most notable difference between the J-turn and the RSM
maneuvers is that the J-turn is a path-following maneuver. That is, it
is performed on a fixed path curve. In contrast, the RSM maneuver is a
non-path-following maneuver that is performed with a fixed steering
wheel input determined for each vehicle. For example, during the
agency's and EMA's testing, the J-turn maneuver was performed on a 150-
foot radius curve. In contrast, the RSM is performed based on a
steering wheel angle derived from the SIS test. We expect that, with
the RSM, the radius of the curve would be close to the fixed radius
used in the J-turn maneuver. However, in the RSM, the vehicle would be
steered with a steering controller and the driver would not have to
make adjustments and corrections to steering to maintain the fixed
path.
    We included both maneuvers in our roll stability testing. We also
included possible performance metrics. For the

RSM, these performance metrics were included in the preamble to the
NPRM. For the J-turn maneuver, the performance metrics were included in
materials supporting the NPRM that were placed in the docket.\45\
---------------------------------------------------------------------------

    \45\ See ``Tractor Semi-Trailer Stability Objective Performance
Test Research--Roll Stability,'' Docket No. NHTSA-2010-0034-0009
Pages xiv, 18, 22-27, 35.
---------------------------------------------------------------------------

    When comparing the J-turn to the RSM in the NPRM, the agency
considered the RSM to be a preferable test maneuver because the RSM
maneuver can be performed with an automated steering wheel controller.
Because the J-turn is a path-following maneuver, a test driver must
constantly make adjustments to the steering input for the vehicle to
remain in the lane throughout the test maneuver. Moreover, driver
variability could be introduced from test to test based upon minor
variations in the timing of the initial steering input and the position
of the test vehicle in the lane.
    In addition, the RSM appeared to be more consistent because it
involves a fixed steering wheel angle rather than a fixed path. There
is negligible variability based on the timing of the initial steering
input because the test is designed to begin at the initiation of
steering input, rather than the vehicle's position on a track.
Moreover, an automated steering wheel controller can more precisely
maintain the required steering wheel input than a driver can.
Therefore, we tentatively concluded that the RSM is more consistent and
more repeatable than the J-turn, which is critical for agency
compliance testing purposes.
    Notwithstanding the above observations, we recognized that many
manufacturers perform NHTSA's compliance tests in order to certify that
their vehicles comply with NHTSA's safety standards. We also recognize
that, over time, manufacturers are likely to use other methods such as
simulation, modeling, etc., to determine compliance with Federal Motor
Vehicle Safety Standards. In this regard, we observed that, because the
J-turn and the ramp steer maneuvers are so similar, manufacturers may
be able to determine compliance with a stability control standard by
using the J-turn maneuver even if the agency ultimately decided to use
the RSM for compliance testing. Thus, if a manufacturer sought to
certify compliance based upon performance testing, a manufacturer would
not necessarily need to perform compliance testing with an automated
steering controller.
    The RSM would use a similar, but not identical lateral acceleration
ratio performance metric to evaluate roll stability. As with the SWD
maneuver, the LAR used in the RSM would indicate that the stability
control system is applying selective braking to lower lateral
acceleration experienced during the steering maneuver. In the SWD
maneuver, the LAR is the ratio of the lateral acceleration at a fixed
point in time to the peak lateral acceleration during the period from
one second after the beginning of steer to the completion of steer. In
contrast, the LAR metric we would use for the RSM would be the ratio of
the lateral acceleration at a fixed point in time to the lateral
acceleration at the end of ramp input, which is the moment at which the
steering wheel angle reaches the target steering wheel angle for the
test. Also, in contrast to the SWD maneuver, the LAR measurements for
the RSM would be taken at a time when the steering wheel is still
turned. This means that, although the SWD maneuver is a more dynamic
steering maneuver, the LAR criteria for the RSM would be greater than
the LAR criteria for the SWD maneuver. The performance criteria for the
RSM would depend on whether fixed-rate steering or fixed-time steering
input is used.
    In a March 2012 submission given to the agency prior to the
publication of the NPRM, which was revised with additional details in
April 2012, EMA suggested that NHTSA use different test speeds and
performance criteria for the J-turn maneuver.\46\ EMA suggested that a
test speed that is 30 percent greater than the minimum speed at which
the ESC system intervenes with engine, engine brake, or service brake
control. Instead of measuring LAR, EMA suggested that, during three out
of four runs, the vehicle would be required to decelerate at a minimum
deceleration rate. NHTSA has conducted testing on variations of this
EMA maneuver, and we suggested that we would conduct further testing.
We requested comments on EMA's suggested test procedure and performance
criteria for the J-turn maneuver.
---------------------------------------------------------------------------

    \46\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040.
---------------------------------------------------------------------------

    After evaluating several maneuvers on different surfaces, the
agency was unable to develop any alternative performance-based dynamic
yaw test maneuvers that were repeatable enough for compliance testing
purposes. Bendix described two maneuvers intended to evaluate the yaw
stability of tractors.\47\ However, neither of these test maneuvers was
developed to a level that would make them suitable for the agency to
consider using as yaw performance tests.
---------------------------------------------------------------------------

    \47\ These tests are discussed in section IV.E.3. See Docket No.
NHTSA-2010-0034-0037 and Docket No. NHTSA-2010-0034-0038.
---------------------------------------------------------------------------

    In July 2009, EMA provided research information on several yaw
stability test maneuvers.\48\ One of these maneuvers was the SWD on dry
pavement that is similar to what was proposed in the NPRM. The second
maneuver was a SWD maneuver conducted on wet Jennite. The third
maneuver was a ramp with dwell maneuver on wet Jennite.\49\ EMA did not
provide any test data on the last two maneuvers. Thus, we considered
them to be concepts rather than fully developed maneuvers that we could
consider using for yaw stability testing.
---------------------------------------------------------------------------

    \48\ Docket No. NHTSA-2010-0034-0035.
    \49\ This ramp with dwell maneuver is the same one identified by
Bendix referenced in the prior paragraph and in section IV.E.3.
---------------------------------------------------------------------------

    We received no other alternative yaw performance tests from
industry until EMA's submission of data in late 2010.\50\ EMA suggested
using a wet Jennite drive through test maneuver demonstrated yaw
performance in a curve on a low friction surface. The maneuver is based
upon a maneuver the agency currently conducts on heavy vehicles to
verify stability and control of antilock braking systems while braking
in a curve. As part of the test, a vehicle is driven into a 500-foot
radius curve with a low-friction wet Jennite surface at increasing
speeds to determine the maximum drive-through speed at which the driver
can keep the vehicle within a 12-foot lane. As with the J-turn, we are
concerned about the repeatability of this test maneuver because of
variability in the wet Jennite test surface and the drivers' difficulty
in maintaining a constant speed and steering input in the curve.
---------------------------------------------------------------------------

    \50\ Docket No. NHTSA-2010-0034-0022; Docket No. NHTSA-2010-
0034-0023.
---------------------------------------------------------------------------

    In a March 2012 submission, which was revised with additional
details in April 2012, EMA provided information about another yaw
stability test along with additional information on the J-turn
maneuver.\51\ This maneuver simulates a single lane change on a wet
roadway surface. It is be conducted within a 3.7 meter (12 foot) wide
path. The roadway condition is be a wet, low friction surface such as
wet Jennite with a peak coefficient of friction of 0.5. The other test
conditions (i.e., road conditions, burnish procedure, liftable axle
position, and initial brake temperatures) are similar to those proposed
in the NPRM. In this

maneuver, the truck enters the path at progressively higher speeds to
establish the minimum speed at which the ESC system intervenes and
applies the tractor's brakes. The maneuver is then be repeated four
times at that speed with the vehicle remaining within the lane at all
times during the maneuver. EMA suggests, as a performance criterion,
that during at least three of the four runs, the ESC system must
provide a minimum level (presently unspecified) of differential
braking. At the NPRM phase, the agency had not had an opportunity to
conduct testing of this maneuver, but we expressed an intention to
determine whether this is a viable alternative yaw stability test. The
agency requested comment on all aspects of EMA's yaw stability test
discussed in its March and April 2012 submissions, including the test
conditions, test procedure, and possible performance criteria that
would allow the agency to test both trucks and buses with this
maneuver.
---------------------------------------------------------------------------

    \51\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040.
---------------------------------------------------------------------------

D. Comments on Alternative Test Maneuvers

    Seven commenters (Daimler, Volvo, Meritor WABCO, Navistar, HDMA,
EMA, and Bendix) recommended that NHTSA adopt alternative dynamic
performance test maneuvers instead of the SIS and SWD. These
alternative maneuvers were either described in the NPRM or included in
comments submitted in response to the NPRM.
    EMA submitted a comment including general test conditions for a J-
turn maneuver to test roll stability and a single lane change on a wet
surface to test yaw stability. In a later submission, EMA provided
actual test information and suggested performance criteria based on
data gathered at two different test facilities using 10 different truck
tractors. Daimler, Meritor WABCO, HDMA, EMA, and Bendix supported
adopting EMA's J-turn test maneuver as the performance test requirement
for testing roll stability.
    The J-turn maneuver described in EMA's submissions uses a test
course with straight lane connected to a 45.7-meter (150-foot) radius,
a lane width of 3.7 meters (12 feet), and a surface coefficient of 0.9.
The test speed of the maneuver is determined by driving a vehicle on
the test course and identifying the minimum vehicle speed that causes
the ESC system to apply the service brakes. That speed is the reference
speed. The vehicle is then driven on the test course, entering the
curve at 1.3 times the reference speed. The deceleration rate is
determined from a time starting at when the ESC system activates the
service brakes. The brakes are considered to be activated when at least
35 kPa (5 psi) is observed at the service brakes. EMA recommended that
four test runs be performed and that the deceleration rate must be at
least 0.91 m/s\2\ (3.0 ft./s\2\) in three of the four test runs.
    With respect to the SWD test in the agency's proposal, EMA stated
that the SWD maneuver is nearly identical to the maneuver used in FMVSS
No. 126. However, in FMVSS No. 126, NHTSA stated that the maneuver was
only used to test yaw stability, not roll stability. EMA observed that
heavy vehicles are different from light vehicles because they have
higher centers of gravity and are more likely to roll over than to lose
directional control. Because the SWD test does not test roll stability
on light vehicles, EMA reasoned that the maneuver should not be used to
test roll stability on heavy vehicles.
    Regarding yaw testing, EMA disagreed with NHTSA's assessment in the
NPRM that low friction surfaces such as wet Jennite may be too variable
to conduct ESC testing, citing NHTSA's use of wet Jennite in testing
air brake performance in FMVSS No. 121. EMA recommended using a test
course with an overall length of 58.5 meters (192 feet). The vehicle
proceeds into the maneuver in a 3.1-meter (10-foot) wide entrance lane.
A steering maneuver is made within 28 meters (92 feet), and the vehicle
completes the maneuver by entering a second 3.7-meter (12-foot) wide
departure lane with a length of 15.2 meters (50 feet). The coefficient
of friction of the road surface is 0.5. The maneuver is similar to a
single lane change on a wet surface test. The test is conducted at a
speed that is 1.6 km/h (1 mph) greater than the reference speed
determined in the rollover maneuver. The vehicle is driven on the test
course for four test runs at the test speed and the brake pressure is
measured at opposite wheel ends. EMA recommended that a differential
brake pressure of at least 69 kPa (10 psi) in three of the four test
runs as a minimum performance requirement.
    Daimler, HDMA, EMA, and Bendix recommended that NHTSA adopt the
single lane change maneuver described in EMA's comment for testing yaw
stability, if the test is workable. Otherwise, they recommended
removing performance requirements related to yaw stability, leaving
only an equipment definition requiring yaw stability performance.
    Other commenters had similar views on yaw testing. For example,
Meritor WABCO recommended that NHTSA should wait to test yaw stability
until it could develop a new yaw stability test. Bendix submitted test
data and criteria using a ramp with dwell maneuver, which it suggested
could be used for testing both the roll and yaw stability of a vehicle.
IIHS did not endorse a particular performance test, but made a general
statement that there should be a requirement of performance tests for
ESC.
    Furthermore, EMA agrees with NHTSA's assessment that it is
difficult to test for understeer control. EMA believes that the
reasoning for not testing understeer control in FMVSS No. 126 can be
carried over to heavy vehicle ESC. In that rulemaking, NHTSA concluded
that the understeer prevention requirement that was included in the
system capability requirements was objective, even without a
performance test.\52\
---------------------------------------------------------------------------

    \52\ 72 FR 17261 (Apr. 6, 2007).
---------------------------------------------------------------------------

E. NHTSA Examination and Testing of EMA Maneuvers

    In response to the March and April 2012 submission from EMA and
additional data submitted to the agency in June 2012 and November 2012
after the issuance of the NPRM containing results of additional tests
discussed by EMA, the agency conducted its own testing in 2013 using
EMA's suggested rollover performance maneuver.\53\ The results of this
testing are summarized in the reports: (1) ``2013 Tractor Semitrailer
Stability Objective Performance Test Research--150-Foot Radius J-Turn
Test Track Research;'' (2) ``Stability Control System Test Track
Research with a 2014 Prevost X3-45 Passenger Motorcoach;'' and (3)
``Stability Control System Test Track Research with a 2014 Van Hool
CX45 Passenger Motorcoach.'' \54\ This section provides a summary of
these reports.
---------------------------------------------------------------------------

    \53\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040; Docket No. NHTSA-2012-0065-0059; Docket No. NHTSA-2012-
0065-0060.
    \54\ Docket No. NHTSA-2012-0065-0062; Docket No. NHTSA-2012-
0065-0063; Docket No. NHTSA-2012-0065-0064.
---------------------------------------------------------------------------

    These reports do not address the yaw stability performance maneuver
suggested by EMA to test yaw stability. EMA's lane change maneuver test
is performed on a wet level surface with a peak friction coefficient of
0.5. NHTSA's past test results with this test surface and similar
performance maneuvers has shown that ESC systems have the capability to
improve vehicle yaw and roll stability performance on low friction
surfaces. However, vehicle handling characteristics dictated the
performance of the vehicle on low friction surfaces. Test data revealed
that, depending on whether the tractor

understeered or oversteered with respect to the trailer, the ESC system
behavior changed. Under such varying behaviors, measures of performance
that were investigated could not be standardized to capture the
benefits of an ESC system over the whole range of vehicles tested. We
have concluded that objective performance tests for ESC using a low
friction surface requires additional data analysis, maneuver design,
and test procedure development, which would require further delaying
this final rule with no assurance that an acceptable maneuver on a low-
friction surface could be developed. Therefore, we have not further
tested EMA's suggested yaw performance maneuver. We may investigate
this maneuver in the future.
    The main objective of NHTSA's truck-tractor testing was to gain
additional experience with a the 150-foot radius J-turn maneuver
procedures suggested by EMA and to collect test track performance data
on air braked truck tractors equipped with stability control system.
The agency conducted tests on three class 8 air-braked truck tractors
and two control trailers. The three trucks used were a 2006
Freightliner 6x4 equipped with separate RSC and ESC systems, a 2006
Volvo 6x4 equipped with an ESC system, and a 2011 Mack 4x2 equipped
with an ESC system.
    The test procedures were derived from those EMA submitted in April
2012, which the agency placed in the docket with the NPRM.\55\ The test
course consisted of a 12-foot wide curved lane with a 150-foot radius
measured from the center of the lane and a peak surface friction
coefficient of 0.9. The curved lane formed a semicircle, and a straight
lane used for bringing the vehicle up to speed was oriented
tangentially at both ends of the curved lane. This allowed the same
test course to be used in both a clockwise and counterclockwise
orientation. The agency placed cones at every 11.25 degrees of arc
angle to mark the inner and outer lane boundaries.
---------------------------------------------------------------------------

    \55\ Docket No. NHTSA-2010-0034-0040.
---------------------------------------------------------------------------

    Prior to testing, the test tractors were loaded to the GVWR by
attaching them to one of the two unbraked control trailers used for
testing. The remaining test conditions (i.e., road surface friction,
ambient temperature conditions, burnish procedure, liftable axle
conditions) largely mirrored those specified in FMVSS No. 121 for
testing air brakes, which also generally mirrored the test conditions
set forth in the NPRM.
    The test driver maneuvered the test vehicle into the straight lane
and approached the curve, then traveled through the 180 degrees of arc
in the curve. The driver attempted to steer the vehicle in such a
manner that it stayed in the lane throughout the maneuver. The brake
pressure was measured at each wheel end and was monitored using a
computer. All maneuvers were conducted in one direction, and then the
entire procedure was completed in the opposite direction, so that
vehicles were tested both clockwise and counterclockwise independently.
The test sequence was repeated for each of the test vehicles and, for
the Freightliner, repeated separately with the ESC and RSC systems
enabled.
    Each test was conducted at a specified entrance speed, with a
tolerance of +/-1 mph, which the driver would reach and maintain prior
to entering the curve. The test driver released the throttle two or
more seconds after the stability control system intervened with either
torque reduction or brake application. However, it was discovered that
it was easier for the test driver to control speed if throttle was
maintained until the stability control system reduced the vehicle's
forward speed by 2 to 3 mph.
    Initially, vehicles were tested with an entrance speed of 20 mph.
Additional test runs were conducted at entrance speeds increased
incrementally by 1 mph until a reference speed could be determined. The
reference speed was the speed at which the stability system intervened
with at least 5 psi of service brake pressure. Additional tests were
conducted at speeds incremented by 1 mph until the target test speed
was reached, which was 130 percent of the reference speed. Four
additional test runs were conducted at the target test speed.
    Near the end of testing, the agency conducted four additional test
runs at the reference speed, during which the test driver fully
depressed the accelerator pedal after crossing the start gate. The
purpose of this testing was to evaluate the stability control system's
ability to reduce driver-commanded engine torque.
    Following this procedure, the agency determined reference speeds
and target test speeds for each test vehicle connected to each of the
control trailers and run in each direction. All vehicles tested had the
ESC systems intervene at entrance speeds not greater than 30 mph. The
results are summarized in the following table.

                         TABLE 3--Reference Speed, Target Test Speed, and Lane Violations Observed During 150-foot J-turn Tests
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                        Reference speed (mph)           Target test speed  (mph) [Reference    Lane violations observed
                                              ----------------------------------------              Speed x 1.3]                at or below the target
                                    Control                                           ---------------------------------------         test speed
            Tractor                 trailer                                                                                ---------------------------
                                               Counter- clockwise       Clockwise      Counter- clockwise      Clockwise        Counter-
                                                                                clockwise     Clockwise
--------------------------------------------------------------------------------------------------------------------------------------------------------
Freightliner 6x4 ESC...........             1  28................  28................  36................  36...............             0             0
                                            2  27................  28................  35................  36...............             0             0
Freightliner 6x4 RSC...........             1  30................  26................  39................  34...............             2             0
                                            2  Not Tested........  Not Tested........  Not Tested........  Not Tested.......             -             -
Mack 4x2 ESC...................             1  25................  24................  33................  31...............             0             0
                                            2  25................  24................  33................  31...............             0             0
Volvo 6x4 ESC..................             1  26................  26................  34................  34...............             0             0
                                            2  26................  25................  34................  33...............             0             0
--------------------------------------------------------------------------------------------------------------------------------------------------------

    EMA suggested, as the performance metric, that the ESC system
decelerate the vehicle at a rate greater than 3 ft./s \2\ during three
of four test runs at an entrance speed of 130 percent of the reference
speed. In addition to evaluating EMA's suggested performance metric,
the agency considered additional performance

metrics for evaluating roll stability performance. In its roll
stability test development, the agency had considered lateral
acceleration and forward speed as possible roll stability performance
metrics.\56\
---------------------------------------------------------------------------

    \56\ See Docket No. NHTSA-2010-0034-0009.
---------------------------------------------------------------------------

    NHTSA's past test track research showed that tractors pulling
trailers with high centers-of-gravity have a high probability of
rolling over in a 150-foot radius curve when speeds exceeded 30
mph.\57\ Tractors equipped with ESC systems, driven under the same
scenario, were slowed down by the ESC systems and consequently, roll
instability was mitigated. These observations guided comparisons in
performance and allowed the agency to develop speed-based performance
metrics relative to the entrance to the 150-foot curve. Specific speed
thresholds can be established as a performance metric.
---------------------------------------------------------------------------

    \57\ See ``Tractor Semi-Trailer Stability Objective Performance
Test Research--Roll Stability,'' Docket No. NHTSA-2010-0034-0009;
``Tractor Semitrailer Stability Objective Performance Test
Research--Yaw Stability,'' Docket No. NHTSA-2010-0034-0046.
---------------------------------------------------------------------------

    In the agency's testing using a high center-of-gravity load, roll
instability (wheel lift) was first observed in tests generating
approximately 0.4g of lateral acceleration at the tractor's center of
gravity. Wheel lift was generally observed between 3 and 4 seconds
after the steering input, which is when 0.4g of lateral acceleration
was sustained. Based on these observations, the agency set the tractor
lateral acceleration thresholds for roll stability during the 150-foot
J-turn maneuver at a maximum of 0.375 g at 3.0 seconds after the
vehicle crossed the start gate and 0.350 g at 4.0 seconds after the
vehicle crossed the start gate.
    However, because the radius of the curved portion of the track is
fixed, these lateral acceleration thresholds can be related to speed
thresholds using the formula A=V\2\/R, where A is the lateral
acceleration, V is the vehicle's forward speed, and R is the radius of
the curve. Inserting the specified lateral acceleration levels and the
radius of the curve, the agency's lateral acceleration thresholds
converted to maximum speed thresholds of 29 mph and 28 mph at 3.0 and
4.0 seconds, respectively.
    Each tractor and stability control system tested exceeded EMA's
suggested 3 ft./s\2\ minimum deceleration test criteria. Each tractor
and stability control system tested also exceeded NHTSA's speed and
lateral acceleration thresholds.

F. Roll Stability Performance Test--J-Turn Test

1. Rationale for Using J-Turn Test
    NHTSA has decided to substitute the J-turn maneuver in place of the
SIS and SWD maneuvers as the performance test for an ESC system. The J-
turn test will be used to evaluate the roll stability of a vehicle.
Likewise, the J-turn will also be used to ensure that the ESC system
reduces engine torque to the wheels. Because the J-turn is conducted on
a fixed curve, longitudinal velocity (speed) directly correlates to
lateral acceleration. NHTSA has determined that the J-turn test is the
most cost-effective and least burdensome alternative that achieves the
objectives of the ESC rule. Moreover, the roll stability mitigation
performance requirements associated with the J-turn maneuver are
comparable to the minimum performance requirements associated with the
SWD maneuver proposed in the NPRM.
    To be clear, however, the agency rejects much of the criticism of
the SWD maneuver in the comments from truck manufacturers. Although we
are abandoning the proposed SIS and SWD maneuver in favor of a J-turn
maneuver to test roll stability in this final rule, NHTSA still
considers the SWD test to be a viable test to measure the minimum
performance of an ESC system on a heavy vehicle.
    We do not agree with the commenters' assertions about the relevance
of the SWD maneuver. The lack of voluntary adoption of the SWD test by
vehicle manufacturers does not, by itself, make the SWD test
irrelevant.
    Likewise, the comments regarding the width of public roads and how
the maneuver is not likely to occur on public roads are inapposite. The
purpose of the performance test is to determine the minimum performance
requirements of ESC systems using an objective and repeatable test. The
fact that the SWD test will not be performed on public roads and must
be performed on a test track, which can be 6 to 8 lanes of public road
width or larger, is not by itself a persuasive argument that the test
is irrelevant.
    Nor does the agency agree with the commenters suggesting that
additional design work would be necessary in order for vehicles to meet
SWD performance requirement. None of the commenters suggesting
additional design work was necessary submitted information to justify
the assertion. Moreover, Bendix, a system supplier, asserted that
current ESC systems could pass the proposed SWD test. NHTSA's own
testing using two typical 6x4 tractors each equipped with ESC systems
consistently met the proposed performance requirements using the SWD
test. In addition, no commenter submitted supporting information
describing any specific design compromises that would occur as a result
of complying with the SWD test.
    Likewise, the agency does not characterize the testing of saleable
vehicles as an unnecessary cost increase. Contrarily, performing the
tests on saleable vehicles, as opposed to manufacturing a vehicle
solely for testing purposes, reduces the amount of cost to a
manufacturer. The manufacturers have provided no basis for their
assertions that they could not resell vehicles after conducting SWD
tests. Although they have asserted that the vehicles may be damaged
during testing, NHTSA has not experienced any vehicle damage during its
own testing. In response to Volvo's claim of potential damage to
vehicles being tested, the agency recognizes that any performance test,
if done unsafely, could potentially damage the vehicle being tested.
    Nevertheless, NHTSA believes the J-turn test maneuver is more
efficient than the SWD test for assessing the roll instability
mitigation of ESC systems. The J-turn test can demonstrate roll
stability using only a single test. There is no need to analyze and
extrapolate data between two separate test maneuvers as there is using
the SIS and SWD tests. This will allow the agency to complete a
compliance test more quickly using the J-turn than using the SIS and
SWD tests.
    We did not receive any estimate from EMA or its members regarding
the costs to perform the J-turn test. However, EMA and its members did
not object to the cost of its suggested performance test, nor did any
commenter discuss the difference in cost of the J-turn test versus the
SWD and SIS tests. Instead, the agency received a recommendation from
dozens of commenters to adopt the J-turn test. The agency estimates
that it would cost approximately $13,400 per truck tractor and $20,100
per large bus to conduct the full series of J-turn test maneuvers
contained in this final rule.
    We also note that the J-turn maneuver is similar to the Ramp Steer
Maneuver (RSM), which was discussed at length in the NPRM. Both
maneuvers use a test course with a straight lane connected to a curved
lane. However, the RSM maneuver is an open loop type test, uses an
automated steering controller, and requires conducting an SIS maneuver
to determine the appropriate steering wheel angle for testing. The J-
turn is a path-following maneuver and the vehicle is steered by the
driver. We have

chosen a path-following maneuver over the fixed-steering RSM because of
track space concerns regarding the SIS maneuver. We believe that the
amount of track space necessary to conduct the SIS maneuver may only be
available at one or two test facilities in the United States. While one
of these facilities is readily available to NHTSA for compliance
testing purposes, we recognize that manufacturers may wish to test
their own vehicles as part of their compliance certification.
    We emphasize that the adoption of the J-turn maneuver should not in
any way diminish the roll stability performance we have observed from
ESC systems. The performance criteria associated with the J-turn test
maneuver in this final rule have been chosen to ensure a level of roll
instability mitigation performance similar to that required to satisfy
the SWD maneuver. Although the test is conducted at a lower speed, the
radius of the curve will increase lateral acceleration to a level that
would generate roll instability in vehicles without ESC systems. We
believe that all large trucks and buses equipped with current
generation ESC systems will meet the minimum performance requirements
just as we believe they would have met the minimum performance
requirements associated with the SWD maneuver. Therefore, we do not
believe that the use of a different test maneuver will change the
expected performance of ESC systems.
    We also observe that, like the sine with dwell maneuver, the J-turn
maneuver that is one of the demonstration tests in Annex 21 of UN ECE
Regulation 13. If a manufacturer chooses the J-Turn test as a
demonstration test to show compliance with Annex 21 and can achieve the
performance criteria established in this final rule, then there would
be compatibility between the performance tests of FMVSS No. 136 and UN
ECE Regulation 13.
    NTSB provided comments indicating that rollover performance
standards should be measured by static rollover stability. NHTSA does
not agree with the NTSB's suggestion. NHTSA developed test methods that
could evaluate an ESC system's performance dynamically. The goal is to
create a measure of performance that will ensure that an ESC system
could prevent a rollover. A static stability test would not measure how
an ESC system reduces lateral acceleration to reduce untripped
rollovers.
2. Test Procedure and Performance Requirements
    The J-turn test procedure developed based on EMA's suggestion is a
sequential procedure in which the test vehicle is repeatedly driven
through a 150-foot radius curve. The test is conducted on the same test
course and is generally performed in the manner suggested by EMA with
only minor changes added to test lateral responsiveness and to test the
ESC system's ability to reduce engine output. We have also modified the
minimum performance criteria to use forward speed rather than
deceleration rate. We found that using deceleration rate as a minimum
performance criteria would not address vehicle wheel lift and
subsequent rollover, especially when the vehicle has a load with a high
center-of-gravity. EMA's suggestion only measures the braking rate, but
it does not measure the ESC system's capability to lower vehicle
lateral acceleration to an acceptable threshold.
    A diagram of the curve is included in the regulatory text to
clarify any ambiguities in the description of the course. Although the
lane markings are depicted with dots on the figure, there is no
specification for how the lane is marked. It may, for example, be
marked with cones or painted lines. Although the figure depicts a
counter-clockwise layout, the test is conducted in both directions.
    The start gate is placed at the point of the test course where the
straight lane section intersects with the curved section of the lane.
An end gate is placed on the curved portion of the lane at 120 degrees
of arc angle from the start gate. It will take a test vehicle more than
4 seconds to pass through the end gate. Therefore, all of the necessary
data will be collected by that point.
    For truck tractors, the lane width is 3.7 meters (12 feet) for both
the straight section and curved section of the course. However, large
buses require additional lane width on the curved section of the course
because buses have longer wheelbases, which make it substantially more
difficult to maintain a narrower lane within the curve. The large buses
that the agency tested did not physically fit in the curved section of
the 12-foot lane because of their long wheelbases. During testing, the
rear wheels of the buses departed the lane even at very low entrance
speeds because of the geometry of the buses, not because of a lack of
stability. Therefore, for buses, the lane width on the curved section
of the course is 4.3 meters (14 feet).
    Each is subjected to multiple J-turn test runs with a test speed
starting at 32 km/h (20 mph) and increased in 1.6 km/h (1 mph)
increments until ESC service brake activation is observed. The test
driver will not apply the service brakes or the engine exhaust braking
during the maneuver. For air-braked vehicles, ESC service brake
activation occurs when the ESC system causes the pressure in the
service brake system to reach at least 34 kPa (5 psi) for a continuous
duration of at least 0.5 second. For vehicles with hydraulic brakes,
ESC service brake application occurs when the ESC system causes the
pressure in the service brake system to reach at least 172 kPa (25 psi)
for a continuous duration of at least 0.5 second. This speed is
considered the Preliminary Reference Speed. This procedure is conducted
separately using clockwise and counterclockwise steering.
    The J-turn maneuver is then repeated four times at the Preliminary
Reference Speed to confirm that this is the speed at which ESC service
brake activation occurs. To do this, four test runs are performed and
ESC service brake application is verified. If ESC service brake
application is verified, this speed is considered the Reference Speed.
If ESC service brake activation does not occur during at least two of
the four test runs, the Preliminary Reference Speed is incremented by
1.6 km/h (1 mph) and ESC service brake application is again verified.
Again, the Reference Speed is determined for both the clockwise and
counterclockwise direction.
    Once the Reference Speed is determined, the ESC system's ability to
reduce engine torque is verified. Two series of four test runs (one
series clockwise, the other series counterclockwise) are conducted at
the Reference Speed. During these maneuvers, the driver will fully
depress the accelerator pedal after entering the curve and throughout
the curve. NHTSA will verify that the engine torque output is less than
the driver-requested output. This ensures that the driver's attempt to
accelerate the vehicle does not override the ESC service brake
application and verifies the system's ability to mitigate instability
by reducing engine torque.
    Thereafter, the vehicle is subjected to multiple series of test
runs (both clockwise and counterclockwise) at an entrance speed up to a
maximum test speed, which is up to 1.3 times the Reference Speed, but
no less than 48 km/h (30 mph). At a speed between 48 km/h (30 mph) and
the maximum test speed, the vehicle is subjected to eight maneuvers,
during which ESC service brake activation is verified. The vehicle must
be able to meet the roll stability

performance criteria discussed below at any speed between 48 km/h (30
mph) and the maximum test speed.
3. System Responsiveness
    The NPRM described the need for a lateral displacement performance
metric because of the possibility of a manufacturer making the vehicle
poorly responsive to the speed and steering inputs required by the SWD
test. The risk of poor lateral displacement in response to the driver's
steering input was mitigated by a minimum responsiveness criterion.
Although the SWD test is being replaced with the J-turn test, we still
need to account for vehicle responsiveness. The nature of the J-turn
test provides two criteria for ensuring vehicle responsiveness:
Maintaining the lane within the fixed radius curve and a minimum test
speed.
    The first responsiveness criterion is the requirement that the
vehicle maintain the lane during at least six of eight runs in the roll
performance test series or at least two of four runs in any other test
series. This requirement ensures that, during J-turn test runs at
increasing speeds, the ESC system actually activates before the vehicle
becomes unstable. We are not imposing this requirement for each test
run within a series to account for driver variability and possible
driver error in conducting the maneuver. Absent driver error, we do not
expect any vehicle equipped with current-generation ESC systems to
leave the lane during any J-turn test.
    The other responsiveness criterion in this final rule is the
minimum vehicle entry speed of 48 km/h (30 mph) for the roll
performance test. This will discourage a manufacturer from designing a
system that will intervene only at very low speeds, thus artificially
decreasing the speed at which the vehicle will enter the curve during
the roll performance test.
4. Engine Torque Reduction
    As proposed in the NPRM, there must be at least a 10 percent
reduction in engine torque when measured 1.5 seconds after the
activation of the ESC system. The percent reduction is measured between
the actual engine torque output and the driver-requested torque input.
This measurement was to be taken during the slowly increasing steer
maneuver. However, now that the agency has adopted the J-turn test as
its performance test, the SIS test is no longer necessary.
    Accordingly, the agency has modified the engine torque reduction
test in the NPRM so that it can be used with the J-turn test. The
reference speed, which is the lowest test speed at which the ESC system
activated the vehicle's service brakes, is determined as part of the J-
turn test sequence. An additional two test series (one using clockwise
steering and the other using counterclockwise steering) are conducted
after the reference speed is calculated. The driver then fully
depresses the accelerator pedal after the vehicle crosses the start
gate. After ESC activation occurs, data is collected to determine the
difference between the actual engine torque output and the driver
requested torque. After analyzing research data from the J-turn
testing, we have determined that the ESC system must reduce the driver
requested engine torque by at least 10 percent for at least 0.5 second
during the time period between 1.5 seconds after the vehicle passes the
start gate and when it travels through the end gate. We are not
considering reduced engine torque before 1.5 seconds after the vehicle
crosses the start gate (and the driver fully depresses the accelerator
pedal) because our testing has shown that there is a lag between when
the operator of the vehicle requests full throttle and when the vehicle
responds by providing full throttle.
5. Roll Stability Performance Requirements
    Based on NHTSA's research, for a typical combination vehicle, an
ESC system must reduce the heavy vehicle's lateral acceleration to less
than 0.4g to prevent wheel lift and possible vehicle rollover.\58\
NHTSA considered how to measure lateral acceleration during the J-turn
maneuver. However, lateral acceleration is a function of longitudinal
velocity. Using the equation A=V\2\/R, where A is lateral acceleration,
V is longitudinal velocity, and R is the radius of the curve, when
driven in a fixed radius curve, with a 45.7-meter (150-foot) radius,
0.4g of lateral acceleration would be achieved at a forward velocity of
approximately 48 km/h (30 mph). That is, at speeds below 30 mph, a
vehicle would generate less than 0.4g of lateral acceleration and would
be unlikely to roll over. This was confirmed in the agency's testing,
where the test vehicles remained stable at speeds below 30 mph.
---------------------------------------------------------------------------

    \58\ See 77 FR 30776-78.
---------------------------------------------------------------------------

    NHTSA track testing has shown that the minimum test speed for
effectively testing the ESC system is 48 km/h (30 mph). However, where
the ESC system activates at a speed such that 1.3 times the minimum
activation speed is greater than 48 km/h (30 mph), the vehicle may be
tested at a speed up to 1.3 times the minimum activation speed. A
multiplication factor of 1.3 will be used to ensure that ESC systems
operate over a range of speeds. A factor of 1.3 allows the vehicle's
ESC system to reach a level where maximum brake force is applied by the
system, and, as a result, ensures the ESC system reduces the
longitudinal velocity and lateral acceleration of the vehicle are below
the threshold values. At factors below 1.3, our testing has shown that
ESC systems have not yet achieved their maximum braking force. At
factors above 1.3, we have concerns about the safety of testing because
the ESC systems have achieved their maximum braking force and the
lateral acceleration of the vehicle could remain high.
    In contrast, using a performance requirement such as EMA's
suggested minimum deceleration metric provides no assurance that the
deceleration will be sufficient to prevent rollover. For example, using
EMA's suggested procedure, if a vehicle is able to enter a curve at a
relatively high rate of speed before an ESC system activates, the
performance requirement will be more stringent than if a system is
tuned to activate at lower rates of speed. Particularly, if a test is
conducted at an entrance speed of less than 48 km/h (30 mph), the
system's ability to prevent rollover is not challenged because the
vehicle is unlikely to experience lateral forces that have the
potential to cause instability, even if the vehicle was not equipped
with an ESC system.
    We considered, but rejected, using the lateral acceleration ratio,
which was the proposed performance criteria for both the SWD maneuver
and the alternative RSM, rather than the reduction in absolute lateral
acceleration. Using the J-turn maneuver, it was sufficient to ensure
that the absolute lateral acceleration was below the threshold for
wheel lift after the vehicle has begun its turn. Furthermore, unlike
the SWD and RSM maneuver where the beginning of steer can be
determined, the beginning of the J-turn maneuver occurs when the
vehicle crosses the start gate. At this point, the lateral acceleration
of the vehicle is zero or close to zero because the vehicle is
traveling in a straight line. After the vehicle crosses the start gate,
the driver has some discretion for steering the vehicle and maintaining
the lane. The low initial lateral acceleration and the driver variation
both make the lateral acceleration ratio an inappropriate performance
metric for the J-Turn test. Instead, we found that reduction in the
absolute lateral acceleration of a vehicle, which on a fixed curve is a
function of velocity, was sufficient to determine the performance

of an ESC system with respect to roll stability control.
    Thus, the minimum performance requirement to demonstrate roll
stability performance in this final rule is expressed in terms of a
vehicle's forward speed (longitudinal velocity) at two points in time.
The specific requirements are:
     The longitudinal velocity measured at 3.0 seconds after
vehicle passes through the start gate to the J-turn maneuver must not
exceed 47 km/h (29 mph).

     The longitudinal velocity measured at 4.0 seconds after
vehicle passes through the start gate to the J-turn maneuver must not
exceed 45 km/h (28 mph).

NHTSA's research indicates than an ESC system's ability to maintain an
absolute lateral acceleration below the criteria would provide an
acceptable probability that the vehicle would remain stable and that a
level of absolute lateral acceleration above the criteria would result
in a high probability of the vehicle becoming unstable.

G. Yaw Stability

    NHTSA has decided to defer research on the yaw maneuver suggested
by EMA, the single lane change on a wet surface test. EMA did not
provide any data showing how its performance criterion (differential
brake pressure) measures the capability of the ESC system to prevent
yaw instability. Moreover, EMA submitted data showing that at least
three of its tested vehicles failed to meet the criteria. NHTSA would
need to further research the EMA maneuver and determine adequate
performance metrics. More data is needed to create criteria that
represent appropriate stability thresholds by showing an acceptable
probability that the vehicle would remain stable if the ESC system
maintains those criteria.
    The SWD maneuver was designed to test the ESC system's ability to
prevent yaw instability by measuring how quickly the vehicle stops
turning, or rotating about its vertical axis, after the steering wheel
is returned to the straight-ahead position. The vehicle that continues
to turn or rotate about its vertical axis under these conditions is
most likely experiencing oversteer, which is what ESC is designed to
prevent. EMA's data does not show how its yaw maneuver will adequately
test the ESC system's capabilities to prevent oversteer. Likewise, the
Bendix test, a ramp with dwell maneuver, will not be examined by NHTSA
at this time for yaw stability testing. In order to create a
performance test, NHTSA would need to do further research on the Bendix
maneuver and determine adequate performance metrics.
    We are also concerned that the maneuver is conducted on a low-
friction wet Jennite surface. EMA stated that it disagrees with the
statement in the NPRM that low-friction surfaces such as wet Jennite
are too variable to make them unusable for ESC testing. EMA believes
that the use of wet Jennite in FMVSS No. 121 for air-brake testing
makes wet Jennite suitable for ESC testing. However, we remain
concerned about the potential for variability in surface friction on a
wet Jennite surface for ESC system testing.
    To date, we have found that only the SWD maneuver proposed in the
NPRM is suitable for testing yaw stability, and even that test is
limited to testing oversteer. As discussed above, we have decided not
to conduct compliance tests on vehicles using the SWD because of the
substantial time and instrumentation burden associated with the SWD
maneuver. We do not believe that this additional time and cost is
justified solely to test yaw stability when a majority of the benefits
of this final rule are derived from rollover prevention. Moreover, the
SWD maneuver would only test oversteer mitigation of yaw instability,
whereas understeer is the primary type of yaw instability that we
observed in our testing. However, we are continuing to examine possible
yaw performance maneuvers, including the SWD maneuver and the lane
change maneuver suggested by EMA to test yaw stability control
performance in the future.

H. Understeer

    As we stated in the NPRM, the agency has no performance test to
evaluate how the ESC responds when understeer is induced. The technique
used by a stability control system for mitigating wheel lift, excessive
oversteer or understeer conditions is to apply unbalanced wheel braking
so as to generate moments (torques) to reduce lateral acceleration and
to correct excessive oversteer or understeer. However, for a vehicle
experiencing excessive understeer, if too much oversteering moment is
generated, the vehicle may oversteer and spin out with obvious negative
safety consequences. In addition, excessive understeer mitigation acts
like an anti-roll stability control where it momentarily increases the
lateral acceleration the vehicle can attain. Hence, too much understeer
mitigation can create safety problems in the form of vehicle spin out
or rollover.
    During the testing to develop FMVSS No. 126 for light vehicles, the
agency concluded that understanding both what understeer mitigation can
and cannot do is complicated, and that there are certain situations
where understeer mitigation could potentially produce safety
disbenefits if not properly tuned. Therefore, the agency decided to
enforce the requirements to meet the understeer criterion included in
the ESC definition using a two-part process. First, the requirement to
meet definitional criteria ensured that all had the hardware needed to
limit vehicle understeer. Second, the agency required manufacturers to
make available engineering documentation to NHTSA upon request to show
that the system is capable of addressing vehicle understeer.
    Based on the agency's experience from the light vehicle ESC
rulemaking and the lack of a suitable test to evaluate understeer
performance, the agency did not propose a test for understeer to
evaluate ESC system performance for truck tractors and large buses. The
agency sought comment on the lack of an understeer test.
    Advocates stated in its comment that there should be a compliance
test for understeer performance. It said the ESC equipment requirement
for understeer is not enough to ensure sufficient performance to
mitigate understeer conditions.
    While we agree with the Advocates goal of having an understeer
test, we have not been able to develop a test that safely challenges an
ESC system's ability to mitigate understeer. Moreover, we believe the
definitional criteria are robust enough to ensure that an ESC system
will reduce loss-of-control crashes in both understeer and oversteer
conditions.

XI. Test Conditions and Equipment

A. Outriggers

    Throughout the agency's research program, truck tractors and buses
were equipped with outrigger devices to prevent vehicle rollover.
During the program, the agency encountered many instances of wheel lift
and outrigger contact with the ground indicating that it was probable
that rollover could occur during testing. Over many years of research
of ESC systems, it has been proven that outriggers are essential to
ensure driver safety and to prevent vehicle and property damage during
NHTSA's compliance testing. Although NHTSA conducted some of its
testing with ESC systems disabled, thereby increasing the need for
outriggers, outriggers are still necessary as a safety

measure during testing of vehicles equipped with an ESC system in case
the system fails to activate.
    The agency proposed that outriggers be used on all truck tractors
and buses tested. We believe that outrigger influence on heavy vehicles
is minimal because of the higher vehicle weight and test load. To
reduce test variability and increase the repeatability of the test
results, the agency proposed to specify a standard outrigger design for
the outriggers that will be used for compliance testing. The agency
used this same approach in FMVSS No. 126 for compliance testing of
light vehicle ESC systems. The agency also made available the detailed
design specifications by reference to a design document located in the
agency public docket.
    For truck tractors, the document detailing the outrigger design to
be used in testing has been placed in a public docket.\59\ This
document provides detailed construction drawings, specifies materials
to be used, and provides installation guidance. For truck tractor
combinations, the outriggers are mounted on the trailer. The outriggers
are mounted mid-way between the center of the kingpin and the center of
the trailer axle (in the fore and aft direction of travel), which is
generally near the geometric center of the trailer. They will be
centered geometrically from side-to-side and bolted up under the
traditional flatbed control trailer. Total weight of the outrigger
assembly, excluding the mounting bracket and fasteners required to
mount the assembly to the flatbed trailer, is less than 2,500 pounds.
The bulk of the mass is for the mounting bracket which is located under
the trailer near the vehicle's lateral and longitudinal center of
gravity so that its inertial effects are minimized. The width of the
outrigger assembly is 269 inches and the contact wheel to ground plane
height is adjustable to allow for various degrees of body roll. A
typical installation on a flatbed type trailer involves clamping and
bolting the outrigger mounting bracket to the main rails of the
flatbed.
---------------------------------------------------------------------------

    \59\ Docket No. NHTSA-2010-0034-0010.
---------------------------------------------------------------------------

    The NPRM proposed that the outrigger design have a maximum weight
of 726 kg (1,600 lb.). However, the agency raised the weight limit of
the outriggers used for testing to accommodate the use of older and
heavier outrigger designs. This final rule raises the maximum weight of
the outriggers to 1,134 kg (2,500 lb.).
    For buses, the outrigger installations will not be as
straightforward as the outrigger installations on the control trailers,
and the NPRM solicited comments on bus outrigger designs. This is
because outriggers cannot be mounted under the flat structure, but
instead must extend through the bus. NHTSA used outriggers on the three
large buses tested during its research program and will use outriggers
for testing buses for compliance with this rule. The agency plans to
use the same outrigger arms of the standard outrigger design that it
plans to use for truck tractor testing. Therefore, the size, weight,
and other design characteristics will be similar.
    The location and manner of mounting the outriggers on buses cannot
be identical to truck tractors. Nonetheless, there are a limited number
of large bus manufacturers, which results in a limited number of unique
chassis structural designs. Also, the agency understands that large bus
structural designs do not change significantly from year-to-year. We
believe that once outrigger mounts have been constructed for several
different bus designs, those mountings can be modified and reused
during subsequent testing. The agency has, in the document described
above, provided additional engineering design drawings and further
installation guidelines for installing the standard outrigger assemble
to large buses.

B. Automated Steering Machine

    The NPRM proposed using an automated steering machine be used for
the test maneuvers on the truck tractors and large buses in an effort
to achieve highly repeatable and reproducible compliance test results.
In the SWD maneuver, the steering must follow an exact sinusoidal
pattern over a three-second time period. For the SWD maneuver, each
test vehicle is subjected to as many 22 individual test runs all
requiring activation at a specific vehicle speed, each of which will
require a different peak steering wheel angle and corresponding
steering wheel turning rate.
    However, the agency has chosen the J-turn maneuver for the
performance test. Although the SWD test requires a fixed steering wheel
angle, the J-turn test is a path-following maneuver. This means a
steering controller will not be required for the J-turn test because
the driver provides the steering wheel input in order to keep the
vehicle within the lane during the test maneuver.
    Because the driver must attempt to keep the vehicle within the lane
width, he has some discretion on the steering wheel angle and the
position of the vehicle within the lane as the vehicle crosses the
start gate. Depending on the experience and technique of the driver,
the vehicle may have a steering wheel angle that is varied by the time
the vehicle crosses the start gate. This variance is tolerable because
we do not expect that it will be difficult for a professional test
driver to maintain the vehicle lane. Nevertheless, to ensure that
variability in testing does not affect vehicle compliance, the
performance requirements need only be satisfied during two out of four
runs of a test series (or six out of eight runs of the final series).

C. Anti-Jackknife System

    The agency proposed using an anti-jackknife system when testing
truck tractors. An anti-jackknife system prevents the trailer from
striking the tractor during testing in the event that a jackknife event
occurs during testing. This would prevent damage to the tractor that
may occur during testing. We do not believe that the use of an anti-
jackknife system will affect test results, nor have we observed any
damage to test vehicles, including vehicle finishes, caused by anti-
jackknife cables.
    The agency proposed using cables to limit the angle of articulation
between the truck tractor and trailer, and set a minimum angle of 45
degrees because setting the cables too tight could artificially help
the ESC system maintain control during testing. However, if the angle
of articulation is set too low the turning radius of the combination
vehicle decreases to a point where maneuverability of the vehicle
becomes an issue. A vehicle with too low of a turning radius would not
be able to drive through the J-turn test course. Therefore, we must to
set a minimum articulation angle for the jackknife system that ensures
safety during testing, but is not too low such that it would affect
test results. However, our testing has shown that 45 degrees is too
high of an angle for a 4x2 truck tractor, because the trailer could
still contact the truck tractor. Therefore agency is specifying 30
degrees as the minimum articulation angle in this final rule, which is
sufficient to provide safety during the testing of all truck tractors.

D. Control Trailer

    The agency proposed using a control trailer to evaluate the
performance of a truck tractor in the loaded condition. In FMVSS No.
121, the agency specifies the use of an unbraked control trailer for
compliance testing purposes. An unbraked control trailer minimizes the
effect of the trailer's brakes when testing the braking performance of
a tractor in

its loaded condition. Nevertheless, in the NPRM, we identified
potential variability in the control trailer that affected the
repeatability of SWD testing and asked for comments on how the control
trailer may be specified to prevent variability.\60\
---------------------------------------------------------------------------

    \60\ There were three specifications, not set forth in control
trailer specifications in FMVSS No. 121, that the agency identified
that might affect SWD test performance and prevent repeatable,
consistent test results using different control trailers. First, the
track width of the control trailer is not specified. Second, the
center of gravity of the control trailer is not specified. Third,
the center of gravity of the load in FMVSS No. 121 testing is only
specified to be less than 24 inches above the top of the tractor's
fifth wheel.
---------------------------------------------------------------------------

    Navistar and EMA commented on a specific truck tractor that
satisfied the proposed SWD criteria with the ESC system disabled. We
believe this is ``Vehicle J'' that was identified in the NPRM. NHTSA
conducted its own testing on ``Vehicle J'' using a different control
trailer. In contrast to EMA's test results, NHTSA's testing showed that
Vehicle J became laterally unstable with the ESC system disabled.
    Volvo, EMA, Advocates, and Bendix commented on the control trailer
specifications. Volvo asserted that further specifications need to be
made for the control trailer because trailer configuration greatly
affects compliance of the SWD test. EMA stated that the control
trailer's track width, deck height, ballast, suspension, tires and
torsional stiffness affect the SWD test results, and small variations
in the control trailer influence the SWD testing. EMA further indicated
that would not be practical to build trailers with stricter design
specifications in order to perform SWD tests and obtain consistent
results. Conversely, Advocates and Bendix recommended that the agency
add new specifications and tighten up existing requirements in order to
reduce the variability in testing. Advocates recommended specifying
track width, trailer CG height, and load CG height in the standard
because it would minimize variation in testing.
    Other than soliciting comments in the NPRM, the agency did not
investigate whether variations in the control trailer significantly
affect the results of the SWD maneuver. However, the agency has not
further modified the specifications of the control trailer. Rather, we
believe that, by using the J-turn maneuver rather than the SWD
maneuver, any potential test variability caused by different control
trailers is ameliorated. The agency's research shows that, because the
performance metric is vehicle speed rather than lateral acceleration
ratio, the effect that the control trailer has on the lateral
acceleration is negligible. The sole consideration in the performance
criteria in this final rule is speed reduction, which has not been
observed to be affected by variations in the control trailer.
    We note that Volvo, EMA, and Bendix recommended the adoption of the
J-turn test, which is one of the alternative tests discussed in the
NPRM. None of the commenters supporting adoption of the J-turn test
raised issues regarding variability in the control trailer with the J-
turn maneuver. Rather, their comments regarding control trailer
variability were limited to the SWD test maneuver.
    Further, the agency conducted J-turn testing using two different
control trailers. We did not find any relevant differences in the ESC
system performance of the truck tractors when connected to different
control trailers. We believe, based on our testing and the lack of
comments related to the control trailer in the J-turn maneuver, that
the potential for variability identified in the NPRM related to the
control trailer was limited to the SWD maneuver. We conclude that the
factors identified in the NPRM will have no effect on the performance
of vehicles using the J-turn maneuver.
    Volvo also commented that the control trailer specified in FMVSS
No. 121 will not work with four or more axle tractors such as 8x6 truck
tractor's because the trailer's fifth wheel position causes
interference between the tractor frame and trailer frame. NHTSA has
considered this comment and believes that there is merit in Volvo's
assertion. A control trailer at the length specified in the NPRM of
6550  150 mm (258  6 in) may be too short to
test vehicles with four or more axles. In this final rule, we are
changing the specified length of the control trailer to allow for
testing with a longer trailer. We are specifying that truck tractors
will be tested with a control trailer that is between 6400 mm and 7010
mm (252 in and 276 in), inclusive. However, for truck tractors with
four or more axles, at the manufacturer's option, NHTSA will test with
a control trailer with a length up to 13,208 mm (520 in). We do not
believe that using a control trailer longer than that specified in the
proposal would cause variability in testing.

E. Sensors

    The vehicle speed is measured with a non-contact GPS-based speed
sensor. Accurate speed data is required to ensure that the SWD maneuver
is executed at the required 72.4  1.6 km/h (45.0  1.0 mph) test speed. Sensor outputs are available to allow the
driver to monitor vehicle speed.

F. Ambient Conditions

    The ambient temperature range specified in other FMVSSs for outdoor
brake performance testing is 0 [deg]C to 38 [deg]C (32[emsp14][deg]F to
100[emsp14][deg]F). However, when the agency proposed a range of 0
[deg]C to 40 [deg]C (32[emsp14][deg]F to 104[emsp14][deg]F) for FMVSS
No. 126, the issue of tire performance at near freezing temperatures
was raised. The agency understood that near freezing temperatures could
impact the variability of compliance test results. As a result, the
agency increased the lower bound of the temperature range to 7 [deg]C
(45[emsp14][deg]F) to minimize test variability at lower ambient
temperatures. For the same reasons, the NPRM proposed an ambient
temperature range of 7 [deg]C to 40 [deg]C (45[emsp14][deg]F to
104[emsp14][deg]F) for testing.
    In their comments, Meritor WABCO, EMA, and Bendix recommended
changes to the minimum ambient temperature allowed for testing. The
three commenters requested that the minimum temperature for performance
tests to be reduced. Meritor WABCO recommended a minimum temperature of
2 [deg]C (35[emsp14][deg]F). Both EMA and Bendix recommended a minimum
temperature of 0 [deg]C (32[emsp14][deg]F). EMA asserted that the
minimum temperature of 7 [deg]C (45[emsp14][deg]F) proposed in the NPRM
reduces the amount of time available to test vehicles during the year.
We agree that a minimum test temperature of 7 [deg]C (45[emsp14][deg]F)
restricts the agency's ability to test for compliance in certain areas
of the United States, including NHTSA's Vehicle Research and Test
Center in Ohio. Thus, we are lowering the minimum testing temperature
to 2 [deg]C (35[emsp14][deg]F). We believe this change will have a
negligible effect on the outcome of performance testing.
    EMA further recommended that the upper limit be decreased from 40
[deg]C (104[emsp14][deg]F) to 38 [deg]C (100[emsp14][deg]F) to match
the FMVSS No. 121 ambient temperature specifications. We are not
adopting this suggestion to match the temperature specifications in
FMVSS No. 121. EMA gave no reason other than consistency with FMVSS No.
121 for adopting this change. Allowing for a larger temperature range
for testing ESC systems does not have any effect on the agency's
ability to conduct consecutive FMVSS No. 121 and FMVSS No. 136 tests
because the FMVSS No. 121 testing is conducted at an ambient
temperature of not greater than 38 [deg]C (100[emsp14][deg]F). Thus,
compliance testing will be conducted at any temperature between 2
[deg]C (35[emsp14][deg]F) and 40 [deg]C (104[emsp14][deg]F). The agency
proposed a maximum wind speed for conducting the compliance testing of
no greater

than 5 m/s (11 mph). This is the same value specified for testing
multi-purpose passenger vehicles (MPVs), buses, and trucks under FMVSS
No. 126. This is also the same value used for compliance testing for
FMVSS No. 135, Light Vehicle Brake Systems.
    As for other ambient conditions, Bendix recommended that the
maximum wind speed be raised from 11 mph (5 m/s) to 22 mph (10 m/s).
Bendix did not specify any rationale for wanting the increase in the
allowable wind speed. The agency sees no reason to increase the wind
speed at this time.

G. Road Test Surface

    The NPRM proposed that the SWD maneuver be executed on a high
friction surface with a peak friction coefficient (PFC) of 0.9, which
is typical of a dry asphalt surface or a dry concrete surface. As in
other standards where the PFC is specified, we proposed that the PFC be
measured using an ASTM E1136 standard reference test tire in accordance
with ASTM Method E1337-90, at a speed of 64.4 km/h (40 mph), without
water delivery. We proposed incorporating these ASTM provisions into
the standard.
    Although we have changed the performance test maneuver, we have not
changed the specifications for the road test surface. The J-turn
maneuver is conducted on a high friction surface with a PFC of 0.9.
Thus, we are incorporating the relevant ASTM provisions into this
standard.
    Bendix recommended adding a restriction that there be no ice or
snow buildup on the test track surface. NHTSA has not adopted this
suggested change. We believe that the surface PFC specification of 0.9
already ensures that the test track will be free of snow and ice.

H. Vehicle Test Weight

    The agency proposed that truck tractors be tested with the combined
weight of the truck tractor and control trailer be equal to 80 percent
of the tractor's GVWR. To achieve this load condition, we proposed that
the tractor be loaded with the fuel tanks filled to at least 75 percent
capacity, test driver, test instrumentation, and ballasted control
trailer with outriggers. The center of gravity of all ballast on the
control trailer was proposed to be located directly above the kingpin.
When possible, load distribution on non-steer axles will be in
proportion to the tractor's respective axle GAWRs. Load distribution
will be adjusted by altering fifth wheel position, if adjustable. In
the case where the tractor's fifth wheel cannot be adjusted so as to
avoid exceeding a GAWR, ballast will be reduced so that axle load
equals specified GAWR, maintaining load proportioning as close as
possible to specified proportioning.
    In its comments, EMA recommended changing the loading requirements
from 80 percent of the truck tractor's GVWR to 100 percent of the truck
tractor's GVWR requirements. EMA wanted this loading condition because
it is used in FMVSS No. 121 testing, and it would eliminate the burden
of changing the vehicle's load when going from FMVSS No. 121 testing to
FMVSS No. 136 testing.
    In light of the change to the J-turn maneuver, we have determined
that the vehicle should be tested at its GVWR rather than 80 percent of
the truck tractor's GVWR. The agency proposed SWD testing at 80 percent
of GVWR because it was determined that such a weight would enable the
agency to evaluate both roll and yaw stability with a single maneuver.
The J-turn maneuver is designed to evaluate only roll stability, and
testing the vehicle at its GVWR is the most severe configuration for
the maneuver. Thus, the agency can use the same loading condition that
it uses for FMVSS No. 121 testing.
    EMA also suggested removing the proposed test condition that the
fuel tank be 75 percent full. EMA reasoned that high fuel volume is
dangerous for testing. Also, EMA observed that a 75% fuel filling
condition is not included in FMVSS No. 121.
    Regarding the fuel tank filling, NHTSA specifies the 75 percent
fuel level in FMVSS No. 126 for testing light vehicles. The goal of the
fuel level specification in FMVSS No. 126 was to ensure consistent
vehicle test weights for the performance tests. With the adoption of
the J-turn maneuver, NHTSA did not find any evidence of varying fuel
levels affecting the results of the ESC performance tests. Therefore,
NHTSA agrees with EMA and will remove the specification of a minimum
fuel tank level.
    The agency proposed that liftable axles be in the down position for
testing. This was because we proposed to conduct our performance test
in a loaded condition. Although the NPRM proposed to load the truck
tractor to 80 percent of its GVWR, we believed that a truck tractor
would operate with liftable axles in the down position. In the final
rule, we are testing vehicles at GVWR. Consequently, we will test
vehicles equipped with liftable axles in the down position. This is
consistent with the test conditions for testing fully loaded air braked
vehicles under FMVSS No. 121.
    For testing buses, the agency proposed loading the vehicle to a
simulated multi-passenger configuration. For this configuration the bus
would be loaded with the fuel tanks filled to at least 75 percent
capacity, test driver, test instrumentation, outriggers and simulated
occupants in each of the vehicle's designated seating positions. The
simulated occupant loads would be obtained by securing 68 kilograms
(150 pounds) of ballast in each of the test vehicle's designated
seating positions without exceeding the vehicle's GVWR and GAWR. The 68
kilogram (150 pound) occupant load was chosen because that is the
occupant weight specified for use by the agency for evaluating a
vehicle's load carrying capability under FMVSS Nos. 110 and 120. During
loading, if any rating is exceeded the ballast load would be reduced
until the respective rating or ratings are no longer exceeded.
    In the final rule, we have removed the specification that the
ballast consists of water dummies. We do not believe that it is
necessary to specify the type of ballast in the test procedure. We note
that, for truck tractors, the type of ballast that is loaded on the
control trailer is not specified. We do not believe, especially in
light of the change to the J-turn test, that the type of ballast used
(whether it is water, sand, or some other ballast) would have an effect
on the ESC system's ability to lower the vehicle's forward speed.
    Unlike in the NPRM, this final rule specifies that buses are tested
at its GVWR. This is the most severe loading condition for testing
buses using the J-turn test maneuver. The NPRM specified that buses
would be tested with a simulated full passenger load, without any cargo
other than test equipment. We have increased the testing load, which
makes the load condition consistent with the loading NHTSA uses to test
FMVSS No. 121 compliance. We have added specification to the loading
procedure to allow for the vehicle to be loaded to GVWR. First,
simulated passengers are loaded. Next, ballast is added to the lowest
baggage compartment. If the bus does not have a baggage compartment or
additional ballast is needed because the baggage compartment is loaded
to capacity, ballast is added to the floor of the passenger compartment
to load the bus to its GVWR. During loading, if any axle rating is
exceeded, the ballast is reduced in the reverse order it is loaded
until the GVWR or GAWR of any axle is no longer exceeded.

I. Tires