**DEPARTMENT OF TRANSPORTATION**

**National Highway Traffic Safety Administration**

**49 CFR Part 575**

**\[Docket No. NHTSA-2001- 9663; Notice 2\]**

**RIN 2127-AI81**

**Consumer Information Regulations;**

**Federal Motor Vehicle Safety Standards;**

**Rollover Resistance**

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

**ACTION:** Notice of Proposed Rulemaking

**SUMMARY:** The Transportation Recall Enhancement, Accountability, and
Documentation Act of 2000 requires NHTSA to develop a dynamic test on
rollovers by motor vehicles for the purposes of a consumer information
program, to carry out a program of conducting such tests, and, as these
tests are being developed, to conduct a rulemaking to determine how best
to disseminate test results to the public. In response, this notice
discusses the results of NHTSA=s evaluation of numerous driving maneuver
tests for the dynamic rollover consumer information program that
Congress mandated for the American public beginning in the 2003 model
year. This notice also proposes several alternative methods for using
the dynamic rollover test results in the agency's consumer information
for vehicle rollover resistance.

**DATES:** Comment Date: Comments must be received by \[insert date 45
days after publication\].

**ADDRESSES:** All comments should refer to Docket No. NHTSA-2001-9663;
Notice 2 and be submitted to: Docket Management, Room PL-401, 400
Seventh Street, SW, Washington, D.C. 20590. Docket hours are 10:00 a.m.
to 5:00 p.m. Monday through Friday.

For public comments and other information related to previous notices on
this subject, please refer to DOT Docket Nos. NHTSA-2000-6859 and 8298
also available on the web at [http://dms.gov/search,]{.underline} and
NHTSA Docket No. 91-68; Notice 3, NHTSA Docket, Room PL-403, 400 Seventh
Street, SW, Washington, DC 20590. The NHTSA Docket hours are from 9:30
am to 4:00 pm Monday through Friday.

**FOR FURTHER INFORMATION CONTACT:** For technical questions you may
contact Patrick Boyd, NPS-23, Office of Safety Performance Standards,
National Highway Traffic Safety Administration, 400 Seventh Street, SW,
Washington, DC 20590 and Dr. Riley Garrott, NRD-22, NHTSA Vehicle
Research and Test Center, P.O. Box 37, East Liberty, OH 43319. Mr. Boyd
can be reached by phone at (202) 366-6346 or by facsimile at (202)
493-2739. Dr. Garrott can be reached by phone at (937) 666-4511 or by
facsimile at (937) 666-3590.

**SUPPLEMENTARY INFORMATION:**

I. Executive Summary

II\. Safety Problem

# III. Background

IV\. Comments to the Previous Notice

V. National Academy of Sciences Rollover Rating Study

VI\. Choice of Maneuvers for Dynamic Rollover Resistance Tests

VII\. Proposed Rollover Resistance Rating Alternatives

VIII\. Intent to Evaluate Centrifuge Test

IX\. Handling Tests

X. Cost Benefit Statement

XI\. Rulemaking Analyses and Notices

XI\. Submission of Comments

Appendix I. Summary of Evaluation Test Results

**I. Executive Summary**

Section 12 of the "Transportation Recall, Enhancement, Accountability
and Documentation (TREAD) Act of November 2000\" directs the Secretary
to "develop a dynamic test on rollovers by motor vehicles for a consumer
information program; and carry out a program conducting such tests. As
the Secretary develops a \[rollover\] test, the Secretary shall conduct
a rulemaking to determine how best to disseminate test results to the
public." The rulemaking must be carried out by November 1, 2002.

On July 3, 2001, NHTSA published a Request for Comments notice (66 FR
35179) discussing a variety of dynamic rollover tests that we had chosen
to evaluate in our research program and what we believed were their
potential advantages and disadvantages. It also discussed other possible
approaches we considered but decided not to pursue. The driving maneuver
tests to be evaluated fit into two broad categories: closed-loop
maneuvers in which all test vehicles attempt to follow the same path;
and open-loop maneuvers in which all test vehicles are given equivalent
steering inputs. Other potential tests using a centrifuge or
computational simulation were discussed but not included in our test
plan. This notice discusses the comments we received and the results of
our test program to date.

The TREAD Act calls for a rulemaking to determine how best to
disseminate rollover test results to the public, and this Notice of
Proposed Rulemaking proposes alternatives for using the dynamic tests
results in consumer information on the rollover resistance of new
vehicles. The resulting rollover resistance ratings will be part of
NHTSA's New Car Assessment Program (NCAP). The tests will be carried out
and reported to the public by NHTSA. This program places no regulatory
requirements on vehicle manufacturers. Past NCAP ratings have been
developed using a procedure of public notice and comment, but there was
no legal requirement to do so since no regulatory requirements were
imposed on any party except NHTSA. Because the dissemination of
information will pose no regulatory burden on manufacturers, we provided
a brief statement on the potential benefits of this program and no
regulatory evaluation.

While the TREAD Act calls for a rulemaking to determine how best to
disseminate the rollover test results, the development of the dynamic
rollover test is simply the responsibility of the Secretary. Based on
NHTSA's recent research to evaluate rollover test maneuvers, the
National Academy of Sciences' study of rollover ratings, comments to the
July 3, 2000 notice, extensive consultations with experts from the
vehicle industry, consumer groups and academia, and NHTSA's previous
research in 1997-8, the agency has chosen the J-turn and the Fishhook
Maneuver as dynamic rollover tests. They are the limit maneuver tests
that NHTSA found to have the highest levels of objectivity,
repeatability and discriminatory capability. Vehicles will be tested in
two load conditions using the J-turn at up to 60 mph and the Fishhook
maneuver at up to 50 mph. Both maneuvers will be conducted with an
automated steering controller, and the reverse steer of the Fishhook
Maneuver will be timed to coincide with the maximum roll angle to create
an objective "worst case" for all vehicles regardless of differences in
resonant roll frequency. The light load condition will be the weight of
the test driver and instruments, approximating a vehicle with a driver
and one front seat passenger. The heavy load condition will add
additional 175 lb manikins in all rear seat positions.

The National Academy of Sciences recommended that dynamic maneuver tests
be used to supplement rather than replace Static Stability Factor (the
basis of our present rollover resistance ratings) in consumer
information on rollover resistance. This notice proposes two
alternatives for consumer information ratings on vehicle rollover
resistance that include both dynamic maneuver test results and Static
Stability Factor. The first alternative is to include the dynamic test
results as vehicle variables along with SSF in a statistical model of
rollover risk. This is conceptually similar to the present ratings in
which a statistical model is used to distinguish between the effects of
vehicle variables and demographic and road use variables recorded for
state crash data on a large number of single vehicle crashes. The
National Academy of Sciences demonstrated the tight confidence limits
that can be achieved using a logistic regression model for this purpose.
Such a model would be used to predict the rollover rate in single
vehicle crashes for a vehicle considering both its dynamic maneuver test
performance and its Static Stability Factor for an average driver
population (as a common basis of comparison).

Under the first alternative, the "star rating" of a vehicle would be
based on the rollover rate in single vehicle crashes predicted for it by
a statistical model. The format would be the same as for the present
rollover ratings (for example, one star for a predicted rollover rate in
single vehicle crashes greater than 40 percent and five stars for a
predicted rollover rate less than 10 percent). The present rollover
ratings are based on a linear regression model using state crash reports
of 241,000 single vehicle crashes of 100 make/model vehicles. We are
proposing to replace the current rollover risk model with one that uses
the performance of the vehicle in dynamic maneuver tests as well as its
SSF to predict rollover risk. The performance of a vehicle in dynamic
maneuver tests is simply whether it tipped-up or not in each of the four
maneuver/load combinations. The lowest entry speed of maneuvers that
caused tip-up will also be used if it improves the predictive fit of the
model. In order to compute a logistic model of rollover risk, it is
necessary to have large number of state crash reports of single vehicle
crashes to establish rollover rates of vehicles for which the dynamic
maneuver test performance and SSF are known. The agency is performing
dynamic maneuver tests on about 25 of the 100 make/model vehicles for
which we have SSF measurements and substantial state crash data. We
believe this approach will ensure that the assigned NCAP ratings for
rollover resistance correlate to the maximum extent possible with
real-world performance. However, since the agency has not finished
testing these 25 vehicles, we cannot yet say what the actual
coefficients of the model relating dynamic maneuver test performance and
SSF to predicted rollover rate will be. We are asking for comments on
the validity of this concept only in this notice.

The second alternative is to have separate ratings for Static Stability
Factor and for dynamic maneuver test performance. Dynamic maneuver tests
directly represent on-road untripped rollovers. The dynamic maneuver
test performance would be used to rate resistance to untripped rollovers
in a qualitative scale, such as A for no tip-ups, B for tip-up in one
maneuver, C for tip-ups in two maneuvers, etc. Here again the results of
ongoing dynamic testing of vehicles with established rollover rates
would guide the establishment of a qualitative scale. A statistical risk
model is not possible for untripped rollover crashes, because they
appear to be relatively rare events and they cannot be reliably
identified in state crash reports. The current Static Stability Factor
based system would be used to rate resistance to tripped rollovers.
Again we are asking for comments on the usefulness and validity of this
concept in this notice. Until our testing of the 25 vehicles is
finished, we will not know what particular NCAP rating will be assigned
to a make/model under either of these two alternatives.

**II. Safety Problem**

Rollover crashes are complex events that reflect the interaction of
driver, road, vehicle, and environmental factors. We can describe the
relationship between these factors and the risk of rollover using
information from the agency\'s crash data programs. We limit our
discussion here to light vehicles, which consist of (1) passenger cars
and (2) multipurpose passenger vehicles and trucks under 4,536 kilograms
(10,000 pounds) gross vehicle weight rating.[^1]

According to the 2000 Fatality Analysis Reporting System (FARS), 9,882
people were killed as occupants in light vehicle rollover crashes, which
represents 31 percent of the occupants killed that year in crashes. Of
those, 8,146 were killed in single-vehicle rollover crashes.
Seventy-eight percent of the people who died in single-vehicle rollover
crashes were not using a seat belt, and 65 percent were partially or
completely ejected from the vehicle

(including 53 percent who were completely ejected). FARS shows that 53
percent of light vehicle occupant fatalities in single-vehicle crashes
involved a rollover event.

Using data from the 1996-2000 National Automotive Sampling System (NASS)
Crashworthiness Data System (CDS), we estimate that 274,000 light
vehicles were towed from a police-reported rollover crash each year (on
average), and that 31,000 occupants of these vehicles were seriously
injured (defined as an Abbreviated Injury Scale (AIS) rating of at least
AIS 3).[^2] Of these 274,000 light vehicle rollover crashes, 221,000
were single-vehicle crashes. (The present rollover resistance ratings
estimate the risk of rollover if a vehicle is involved in a
single-vehicle crash.) Sixty-two percent of those people who suffered a
serious injury in single-vehicle towaway rollover crashes were not using
a seat belt, and 48 percent were partially or completely ejected
(including 41 percent who were completely ejected). Estimates from NASS
CDS indicate that 81 percent of towaway rollovers were single-vehicle
crashes, and that 84 percent (186,000) of the single-vehicle rollover
crashes occurred after the vehicle left the roadway. An audit of 1992-96
NASS CDS data showed that about 95 percent of rollovers in
single-vehicle crashes were tripped by mechanisms such as curbs, soft
soil, pot holes, guard rails, and wheel rims digging into the pavement,
rather than by tire/road interface friction as in the case of untripped
rollover events.

According to the 1996-2000 NASS General Estimates System (GES) data,
61,000 occupants annually received injuries rated as *K* or *A* on the
police KABCO injury scale in rollover crashes. (The police KABCO scale
calls *A* injuries \"incapacitating,\" but their actual severity depends
on local reporting practice. An \"incapacitating\" injury may mean that
the injury was visible to the reporting officer or that the officer
called for medical assistance. A *K* injury is fatal.) The data indicate
that 212,000 single-vehicle rollover crashes resulted in 50,000 *K* or
*A* injuries. Fifty-one percent of those with *K* or *A* injury in
single-vehicle rollover crashes were not using a seat belt, and 23
percent were partially or completely ejected from the vehicle (including
20 percent who were completely ejected). Estimates from NASS GES
indicate that 13 percent of light vehicles in police-reported
single-vehicle crashes rolled over. The estimated risk of rollover
differs by light vehicle type: 10 percent of cars and 10 percent of vans
in police-reported single-vehicle crashes rolled over, compared to 18
percent of pickup trucks and 27 percent of SUVs. The percent of all
police reported crashes for each vehicle type that resulted in rollover
was 1.7 percent for cars, 2.0 percent for vans, 3.7 percent for pickup
trucks and 5.4 percent for SUVs as estimated by NASS GES.

**III. Background**

Section 12 of the "Transportation Recall, Enhancement, Accountability
and Documentation (TREAD) Act of November 2000\" directs the Secretary
to "develop a dynamic test on rollovers by motor vehicles for a consumer
information program; and carry out a program conducting such tests. As
the Secretary develops a \[rollover\] test, the Secretary shall conduct
a rulemaking to determine how best to disseminate test results to the
public." The rulemaking must be carried out by November 1, 2002.

On July 3, 2001, NHTSA published a Request for Comments notice (66 FR
35179) discussing a variety of dynamic rollover tests that we had chosen
to evaluate in our research program and what we believed were their
potential advantages and disadvantages. It also discussed other possible
approaches we considered but decided not to pursue. The driving maneuver
tests to be evaluated fit into two broad categories: closed-loop
maneuvers in which all test vehicles attempt to follow the same path;
and open-loop maneuvers in which all test vehicles are given equivalent
steering inputs. Other potential tests using a centrifuge or
computational simulation were discussed but not included in our test
plan. This notice discusses the comments we received and the results of
our test program to date.

The TREAD Act calls for a rulemaking to determine how best to
disseminate rollover test results to the public, and this Notice of
Proposed Rulemaking proposes several alternatives for using the dynamic
tests results in consumer information on the rollover resistance of new
vehicles. The resulting rollover resistance ratings will be part of
NHTSA's New Car Assessment Program (NCAP). The tests will be carried out
and reported to the public by NHTSA. This program places no regulatory
requirements on vehicle manufacturers. Past NCAP ratings have been
developed using a procedure of public notice and comment, but there was
no legal requirement to do so since no requirements were imposed on any
party except NHTSA.

NHTSA's NCAP program has been publishing comparative consumer
information on frontal crashworthiness of new vehicles since 1979, on
side crashworthiness since 1997, and on rollover resistance since
January 2001. The present rollover resistance ratings are based on the
Static Stability Factor (SSF) which is the ratio of one half the track
width to the center of gravity (c.g.) height. (see
[www.nhtsa.dot.gov/hot/rollover/]{.underline} for ratings and
explanatory information).

SSF was chosen over vehicle maneuver tests in the present ratings system
because it represents the first order factors that determine vehicle
rollover resistance in the 95 percent of rollovers that are tripped by
impacts with curbs, soft soil, pot holes, guard rails, etc. or by wheel
rims digging into the pavement. In contrast, untripped rollovers are
those in which tire/road interface friction is the only external force
acting on a vehicle that rolls over. Driving maneuver tests directly
represent on-road untripped rollover crashes which are about 5 percent
of the total, and test performance can be improved by vehicle changes
that may not improve resistance to tripped rollovers. Other reasons for
selecting the SSF measure are: driving maneuver test results are greatly
influenced by SSF; the SSF is highly correlated with actual crash
statistics; it can be measured accurately and inexpensively and
explained to consumers; and changes in vehicle design to improve SSF are
unlikely to degrade other safety attributes.

Vehicle manufacturers generally oppose the present rollover resistance
ratings because they believe that SSF is too simple since it does not
include the effects of suspension deflections, tire traction and
electronic stability control (ESC) and because they believe that the
influence of vehicle factors on rollover risk is too slight to warrant
consumer information ratings for rollover resistance. In the conference
report dated October 23, 2000 of the FY2001 DOT Appropriation Act,
Congress permitted NHTSA to move forward with the rollover rating
proposal and directed the agency to fund a National Academy of Sciences
study on vehicle rollover ratings. The study topics are "whether the
static stability factor is a scientifically valid measurement that
presents practical, useful information to the public including a
comparison of the static stability factor test versus a test with
rollover metrics based on dynamic driving conditions that may induce
rollover events." The National Academy's report was completed and made
available in pre-publication form on February 21, 2002. Section IV
discusses the findings and recommendations of the study.

**IV. Comments to the Previous Notice**

In its July 3, 2001 Request for Comments notice (66 FR 35179), NHTSA
solicited comment on the development of a dynamic test for vehicle
rollover resistance and identified a number of tests it planned to
evaluate. The notice posed the following five sets of questions for
comments. Most commenters either supported one of the tests being
evaluated, suggested another test, or described elements the commenter
believed to be important for any test chosen for rollover resistance. In
this way, most commenters responded to the substance of question 1.
While only a few commenters responded specifically to the other
questions, parts of the general comments of other commenters are
discussed in the context of the questions.

**Question 1**: NHTSA has decided to devote its available time and
resources under the TREAD Act to develop a dynamic test for rollover
based on driving maneuver tests. Is this the best approach to satisfy
the intent of Congress in the time allotted? Are there additional
maneuvers that NHTSA should be evaluating? Which maneuver or combination
of maneuvers do you believe is the best for rollover rating? Are these
other approaches well enough developed and validated that they could be
implemented 18 months from now?

**Comments:** In answer to this question many commenters either voiced a
preference for one of the maneuvers in the test plan NHTSA announced in
its July RFC Notice or made specific suggestions for other tests.
Daimler-Chrysler (D-C), Continental-Teves, BMW, Mitsubishi and
Volkswagen (VW) supported the use of the ISO 3388 Part 2 double lane
change test (developed by VDA, the German vehicle manufacturers'
association) as the dynamic rollover test. VW suggested that the ratings
should include three components: a) SSF for general overall rating of
static stability, b) the ISO 3388 Part 2 test with minimum entry of 60
kph without 2 wheel lift, and c) a dynamic handling test that gives
credit to ESC.

Several commenters supported the variations of the fishhook test. Toyota
suggested a fishhook test with fixed timing using the LAR (lateral
acceleration at rollover \[tip-up\]) criterion as test for untripped
rollover. Toyota's recommendation also suggested using the ISO 3388 PART
2 test as a stability/controllability test, with entry speed and peak to
peak yaw rate as the measured criteria. Toyota also offered a
hypothetical star rating breakdown for LAR as a rollover rating and a
star rating chart relating entry speed and peak to peak yaw rate in the
ISO 3388 PART 2 test as a separate controllability rating. TRW stated
that rollover test maneuvers should excite worst case roll dynamics, but
that some conditions on the vehicle path should be observed to keep
handling tradeoffs in check. It expressed the opinion that a fishhook
test with steering based on roll rate best approached the stated goal
but that future developments in simulation could also be useful for
rollover resistance ratings. Honda recommended a fishhook maneuver with
a protocol for optimizing to the worst case timing for each vehicle as a
test for untripped rollover resistance combined with the basic
quasi-static centrifuge test to measure tripped rollover resistance.
Nissan had previously suggested a fishhook test and its own optimization
protocol, but in its comment to this notice, Nissan changed its position
stating that the fishhook may be too severe for consumer information and
that it has no data correlating it to real world accidents. It suggested
that NHTSA should test for handling properties instead of rollover
resistance.

NHTSA's July RFC Notice announced a research plan that excluded the
centrifuge test on the basis that it was not deemed sufficiently
"dynamic" for the requirements of the TREAD Act and for concern that a
vehicle optimized for the centrifuge test may have more oversteer than
the manufacturer would otherwise choose. Nevertheless, a number of
commenters were in support of rollover resistance tests that included
centrifuge testing. Ervin and Winkler of UMTRI suggested a number of
possible test modes using a centrifuge including a basic quasi-static
mode which adds suspension roll and shear effects to SSF, tether release
modes which add roll inertial forces somewhat analogous to J-turn and
fishhook maneuvers, and a curb trip mode with a sliding table. They also
suggested that a driving maneuver handling test for yaw stability be
performed in addition to the centrifuge test. As noted above, a
quasi-static centrifuge test for tripped rollover was part of Honda's
recommendation. CU also suggested a centrifuge (or SSF as an
alternative) as part of recommended suite of tests also including a
dynamic maneuver test with steering reversal (like the fishhook) and
handling tests for maximum lateral acceleration and yaw stability.
Advocates commented that driving maneuver tests by themselves are not
sufficient for rollover resistance tests because they only define
untripped rollover resistance, and Advocates recommend that UMTRI=s
centrifuge tests should be investigated because they can be applied to
both tripped and untripped rollover resistance.

GM recommended that the centrifuge test be substituted for Side Pull
Ratio or SSF in the Stability Margin concept it had recommended to NHTSA
in comments to previous notices on rollover resistance ratings. It also
supplied information addressing NHTSA's concern that the centrifuge test
could reward undesirable changes in suspension roll stiffness
distribution. The issue first arose in comments from Ford on a 1994
NHTSA proposal for rollover consumer information based on Tilt Table
Ratio. Ford stated that a vehicle's score in a tilt table test is
greatest if both the front and rear tires lift simultaneously when the
table is inclined at the minimum angle for two wheel lift, and that the
manufacturer could achieve the optimum score by stiffening the rear
suspension relative to the front. If the manufacturer did so, the result
would be a vehicle with less understeer as the trade-off for a better
Tilt Table Ratio. The same optimization principal would apply to
centrifuge tests. GM's comment included curves showing the point of
optimization of Side Pull Ratio (theoretically the same as the
centrifuge measurement) and its sensitivity to the proportion of total
roll stiffness provided by the front suspension for a typical SUV and a
typical car. GM compared the curves to the suspension characteristics of
these production vehicles and found that a) the suspension roll
stiffnesses of the production vehicles were close to the optimized
condition as designed with a very small sensitivity to further
suspension changes and b) the suspension changes to obtain the
negligible improvement in rollover test score involved a relative
stiffening at the front that would increase rather than decrease the
understeer. GM concluded that manufacturers would have little to gain by
suspension tuning for centrifuge test scores and that the tuning would
be at least as likely to increase understeer as to decrease it. We
believe that Ford's comment was correct in 1994, but NHTSA has recently
reviewed data showing a trend toward less understeer in SUVs of more
recent design. GM's dismissal of the issue may reflect more accurately
the design of today's new vehicles.

Toyota and GM were the only commenters to suggest how the results of
their rollover and handling tests could be expressed in ratings. GM
suggested that the following conditions be used to define "good rollover
resistance for light-duty vehicles": a) quasi-static centrifuge test
tip-up threshold of at least 0.9g; b) maximum lateral acceleration in a
circular driving maneuver of at least 0.6g; and c) a stability margin
(a-b) at least 0.2g or 1.5/wheelbase \[in meters\] squared. GM estimated
that a centrifuge measurement of 0.9g would correspond to a SSF of 1.06.
However, we would estimate that centrifuge measurement as corresponding
closer to a SSF of 1.00, based on comparisons with tilt table tests with
an allowance for the vertical load error inherent with the tilt table.

Based on its stability margin concept of good rollover resistance, GM
suggested the following "star rating" system. A vehicle passing all
three conditions for good rollover resistance would be rated with two
stars. Failing any one of the conditions would reduce its rating to one
star. Bonus stars above the two star level would be awarded for a
centrifuge test measurement 1.0 g or better, a maximum lateral
acceleration measurement of 0.7g or better, or a stability margin 0.1 or
more above the minimum (0.2g or 1.5/wheelbase \[in meters\] squared). A
vehicle satisfying all of these higher conditions would receive a five
star rating. GM also suggested that NHTSA consider a symbol other than a
star for rollover resistance ratings to differentiate them from frontal
and side crashworthiness ratings. As previously mentioned, Toyota
offered a hypothetical star rating breakdown for LAR in a Fishhook as a
rollover rating.

#### Previously, Ford had suggested a proprietary test method (Path Corrected Limit Lane Change (PCLLC)) involving a series of double lane change maneuvers controlled by a human driver and a mathematical technique for correcting the measurements of vehicle acceleration and wheel force to those expected if the vehicle perfectly adheres to a desired common path for vehicle comparisons. NHTSA agreed to evaluate this method but keep the details of the analytical technique confidential. Appendix I of this notice discusses the results of PCLLC testing using the same vehicles tested in other maneuver tests.

#### In its comment to the July notice, Ford announced that the same test measurements could be made using a newly developed advanced path following steering controller to replace the human driver and the proprietary mathematical correction technique. Ford expected both implementations of the protocol to produce the same measurements. But it changed its recommendation to the path following steering controller because the face validity (realistic appearance) of the test would be enhanced by having the advanced steering controller actually drive the vehicles through nominally identical paths rather than rely on corrections to the unavoidably variable paths taken by skilled human test drivers. Ford's comment was made after NHTSA had run the PCLLC maneuvers in a cooperative effort with Ford to evaluate that test method. However, we believe that the results of the tests of our vehicles using the PCLLC mathematical corrections would be representative of same maneuver tests accomplished with a path following steering controller. 

#### Ford's path following steering controller is not the same as the automated steering controller NHTSA used to obtain repeatable steering inputs for open-loop maneuvers. Ford's steering controller is designed to drive different vehicles in the same repeatable path although the steering inputs to guide the various vehicles along the same path may be quite different. It uses a real-time computer simulation of the vehicle steering responses and a differential GPS position signal as feedback signals for closed-loop control. 

Unlike the other maneuver tests in NHTSA's evaluation, Ford's maneuvers
are not intended to produce wheel lift or loss of control or invoke ESC
operation. Ford suggests four lane change maneuvers (like those shown in
Figure 9) varying in offset and length, each producing a maximum lateral
acceleration of 0.7g at a single test speed of 45 mph, but varying in
fundamental lateral acceleration frequency from 0.29 Hz to 0.40 Hz. The
scoring metric is the maximum dynamic weight transfer measured as a 400
ms moving average. It refers to the percent reduction in vertical load
for the two wheels on the side of the vehicle approaching tip-up. At
tip-up, the dynamic weight transfer is 100 percent, but dynamic weight
transfer in the range of 50 to 80 percent would be typical in the Ford
maneuver. A lower percent weight transfer score indicates a vehicle with
higher rollover resistance. The tests are performed with the vehicle
loaded to the gross vehicle weight rating and the rear axle load at the
rear axle weight rating.

Intrinsic advantages of this test method are its insensitivity to
changes in pavement and tire friction because the tests are performed at
lateral force levels below the friction limit and its continuous (as
opposed to binary, tip-up or no tip-up) performance metric with a
comparative score for all vehicles. Intrinsic disadvantages are its
compression of vehicle differences as a result of tests restricted to a
smaller range of lateral acceleration, the need for very accurate and
repeatable vertical wheel force measurements to discriminate the
compressed vehicle differences, and the question of whether non-limit
dynamic tests can predict the comparative dynamic behavior of vehicles
in limit maneuvers. Ford believes that non-limit results can be
projected up to the limit, but it is certainly possible that anomalies
in suspension behavior may occur only at the limit.

Suzuki commented that driving maneuver tests should not be used as
NHTSA's dynamic rollover test because they measure only resistance to
untripped rollover, are unrealistic driving maneuvers and have many
practical problems. Suzuki argued that a dynamic tripped rollover test
should be used instead. In November 2001, Suzuki and its contractor
Exponent made a suggestion how a "dynamic tripped rollover test" could
be conducted. The test would use a braked sled with the vehicle placed
transversely on the sled adjacent to tripping curb. From a constant
speed of 25 mph, the sled would be braked at a relatively constant
deceleration which produces a steady lateral acceleration on the test
vehicle. Repeated runs of the sled at incrementally higher levels of
deceleration would be made until the vehicle lifts and rolls at least 20
degrees to a position restrained by safety straps. Such a test imposes a
step increase of lateral acceleration on the vehicle and measures the
result of weight transfer due to the static rigid body (SSF) properties
of the vehicle, to the c.g. movement due to quasi-static body roll, and
to the dynamic effects of roll inertia and suspension damping. This test
is very similar to the "straight tethered" centrifuge test suggested by
UMTRI in which the steady lateral acceleration imposed on the vehicle by
the centrifuge is resisted by a tether until the tether is released and
the vehicle experiences a step increase of lateral acceleration. Both
are also analogous to a J-turn test with an extremely high level of tire
adhesion.

**Question2:** How should NHTSA address the problem of long term and
short term variations in pavement friction in conducting comparative
driving maneuver tests of vehicle rollover resistance for a continuing
program of consumer information?

**Comments:** Toyota, D-C, and Ford addressed the question explicitly.
Toyota had suggested a fishhook maneuver using the scoring metric LAR
(lateral acceleration at roll). It believes that LAR is not very
sensitive to changes in pavement friction, but if the pavement friction
is too low it will become impossible for the vehicle to achieve
sufficient lateral acceleration in the maneuver to reach LAR. Toyota
also suggested a double lane change handling maneuver in which entry
speed and peak to peak yaw rate were scoring metrics that it considers
sensitive to pavement friction. It suggests strict limits on the course
parameters to qualify the handling tests as valid, giving as an example
the surface temperature limits (35C +/- 10C) used by the Japanese
government NCAP protocol for braking tests.

D-C suggested that a standard pavement friction monitoring trailer using
a standard ASTM tire be used to define the nominal surface friction of a
test track, and that at least five braking tests be conducted using the
same anti-lock equipped vehicle with standard tires to qualify the
surface before a test session. Limits for braking test measurements,
temperature and wind velocity would be established to qualify the
surface. VW made a similar recommendation of defined limits on
temperature, humidity, wind speed and surface friction (presumably using
a pavement friction monitoring trailer with a standard ASTM tire).

Ford explained that its test protocol for the double lane change
maneuvers performed either by a path-following robot or by mathematical
path-correction of driver-controlled tests calls for comparing the side
to side load transfer at a standard 0.7g lateral acceleration. Since
almost all vehicles can achieve this level of lateral acceleration on
ordinary dry pavement despite expected fluctuations in surface friction,
the test method is not sensitive to ordinary pavement friction
fluctuations.

Likewise, fluctuations in pavement friction are not an issue for the
centrifuge test suggested by UMTRI and the sled test suggested by
Exponent/Suzuki because both tests use a curb-like structure rather than
pavement friction to initiate an overturning moment.

**Question 3:** Some ESC systems presently have two functions. One is
yaw stability which uses one or more brakes to keep the vehicle headed
in the right direction in a limit maneuver, and the other is simple
brake intervention in excess of the braking required for yaw stability.
It is expected that the presence of a brake intervention function in ESC
will have a large effect on the rating of vehicles because the average
speed through a given test maneuver for vehicles having this function
will be much less than for vehicles without it (even if equipped with
ESC for yaw stability) under the usual test protocols of coasting
through maneuvers and using the entry speed as the test speed. Is the
value given to the brake intervention function of ESC as opposed to the
yaw stability function by potential rollover rating tests commensurate
with its safety value to consumers? Please provide all the data and
reasoning that support your view. Should NHTSA measure the vehicle speed
at the completion of the maneuver as well as vehicle speed at entry?

**Comments:** Toyota commented that automatic braking in excess of what
is required for yaw stability control to further lower the speed is a
good strategy to mitigate harm in an emergency, but it recognizes
NHTSA's concern that dynamic rollover tests could give the same credit
to less sophisticated systems as to yaw control. Toyota believes that
its suggestion of a separate handling test to accompany the dynamic
rollover test would reward controllability and show the advantage of yaw
control systems.

D-C commented that ESC should operate during rollover maneuver tests
with entry speed being the only criterion for the stringency of the
maneuver. The exit speed should not be considered[^3]. Continental-Teves
also commented that only the entry speed is an appropriate measure
because it best defines the obstacle avoidance situation facing the
driver.

TRW commented that ESC should be rewarded if it enhances roll dynamic
behavior, and it also stated that "Differential Braking Roll Prevention"
should be rewarded by the agency's rollover maneuver tests. It did not
define the term "Differential Braking Roll Prevention", but we
understand it to mean an automatic braking system in which selected
brakes are applied for the purpose of reducing the lateral force
generating capability of the selected tires rather than to augment yaw
stability or to simply slow down.

Ford also opposed using the average speed through a given test as a
criterion and pointed out that its recommended test does not use speed
as a comparative metric at all. It also stated that its test is unlikely
to invoke ESC but would measure the effect of active stabilizer bars and
electronically controlled shocks.

Several other manufacturers share Ford's view that the operation of ESC
is not essential to rollover resistance tests. GM suggested laboratory
tests of rollover resistance using a centrifuge in which ESC would not
operate. It stated that "the rollover resistance of the underlying
vehicle structure and suspension is a more important parameter than the
possible use of ESC to mask poor rollover resistance of the foundation
vehicle." Similarly, the recommendations from Suzuki and Exponent for a
tripped rollover test do not involve the use of ESC. Honda suggested
that if a vehicle is equipped with an on/off switch for ESC, it should
be tested with the switch in the off position.

One of the agency's reasons for posing this question was that ESC
systems with a component of ordinary four wheel braking above the
differential braking for yaw control are performing a braking action
that the driver is also likely to do in an emergency. However, the usual
test protocol for the maneuver tests being evaluated requires the driver
to coast rather than brake. Therefore, there was a question whether the
potential advantage of vehicles with automatic braking tied to ESC would
be unrealistically amplified by a test protocol that would prevent
driver braking in circumstances where actual drivers would be likely to
brake. Our concern over this theoretical problem has been reduced by our
observations during the recent maneuver test research that vehicles tip
up early in rollovers maneuvers minimizing the effect of automatic
braking.

**Question 4:** If open-loop (defined steering input) maneuvers are used
to determine whether a vehicle is susceptible to two wheel lift as a
result of severe steering actions, superficial changes that reduce tire
traction or otherwise reduce vehicle handling (but prevent wheel lift)
would be rewarded the same as more fundamental or costly improvements.
The same is true of closed loop (path following) maneuvers that use
wheel lift as the sole criterion. Should measures of vehicle handling be
reported so that consumers can be aware of possible trade-offs. What
indicators of vehicles handling would be appropriate to measure, and how
should this consumer information be reported?

**Comments**: Many commenters recommended handling tests either in
addition to rollover resistance maneuver tests or instead of rollover
resistance maneuver tests. Nissan had earlier recommended a fishhook
maneuver test for rollover resistance and had proposed a method of
timing the steering reversal to achieve maximum severity for each test
vehicle. However, in its comments to the July notice, Nissan recommended
that NHTSA measure handling rather than rollover resistance on the basis
that the fishhook test may be too severe for the purposes of consumer
information and that Nissan had no data regarding the correlation of
fishhook test performance to real-world crashes. It suggested a steady
state lateral acceleration test and a lateral transient response test.
D-C addressed the question directly by stating that its recommended ISO
3388 PART 2 test does not give incentives for negative trade-offs but
rather encourages optimized cornering capability and "limit condition
performance" by giving lower ratings for "bad handling". In its
recommendation of the ISO 3388 PART 2 test, Continental-Teves actually
described it as a handling test.

The combination of a rollover test and a separate handling test was
recommended by many commenters. Toyota suggested that a closed loop
stability and controllability test should be combined with an open loop
rollover resistance test to deal with the trade-off issue for rollover
tests. It suggested using the ISO 3388 PART 2 test as a handling test
with both entry speed and peak-to-peak yaw rate as performance criteria.
The peak-to-peak yaw rate would reflect on the yaw stability of the
vehicle. UMTRI suggested the centrifuge test for a rollover resistance
but recommended adding a driving maneuver test to characterize yaw
controllability. GM also recommended the centrifuge test, but suggested
combining its results with a driving test of steady state maximum
lateral acceleration to create a stability margin and set a lower limit
for handling. In addition to static and dynamic rollover resistance
tests, CU recommended a steady state lateral acceleration test on a skip
pad and "track-type tests to assess the vehicle's controllability,
response and grip." VW also suggested static and dynamic rollover
resistance tests , but called for a handling test that "would give
positive credit to ESP \[ESC in generic parlance\], since experience in
Germany appears to substantiate the real world benefits of ESP. It did
suggest a specific test, but tests of yaw stability would be expected to
measure an aspect of handling benefited by ESC operation.

**Question 5:** What criteria should NHTSA use to select the best
vehicle maneuver test for rollover resistance? Should the maneuver that
has the greatest chance of producing two wheel lift in susceptible
vehicles be chosen regardless of its resemblance to driving situations?
Is it more important that the maneuver resemble an emergency maneuver
that consumers can visualize? How important is objectivity and
repeatability?

**Comments:** One issue is the potential conflict between the ability of
a dynamic rollover test to produce tip-up in vulnerable vehicles
(severity) and its resemblance to a driving maneuver consumers can
imagine doing (face validity). Toyota commented that it views severity
as the more important property for a rollover resistance test and face
validity as the more important property for a handling test. Ford and
D-C took the opposite position. Ford stated that extreme maneuvers that
cause two wheel lift of some vehicles on a paved road surface are
unrelated to the vast majority of crashes. D-C said that resemblance to
emergency maneuvers is more important than determining "artificial
conditions" under which a particular vehicle is likely to roll over.

There were other comments about the general issue of criteria for
selecting a rollover test. Continental-Teves stated that "a dynamic test
for vehicle rollover rating should assess whether the vehicle system
(driver and vehicle) is capable of keeping the vehicle on the road"
which is consistent with the view that the ISO 3388 PART 2 test is more
of a handling test than a rollover test. Advocates disagreed with
NHTSA's conclusion that the TREAD Act called for a driving maneuver test
as a rollover test, and suggested that UMTRI's ideas for a centrifuge
test should be investigated. IIHS stated that "although some of the test
maneuvers may have considerably greater consumer face validity, the
ultimate decision as to which maneuvers to use should rest on which
provide the best correlation with real-world crash risk."

##### Commenter's Recommended Approaches 

D-C, Mitsubishi, VW, BMW and Continental-Teves recommended the ISO 3388
PART 2 closed-loop tight double lane change test as the best dynamic
rollover test, but also described it as a handling test.

Toyota, Honda, CU, and TRW recommended Fishhook tests optimized in
various ways to present the worst-case timing to each vehicle as the
best dynamic rollover test. Nissan had recommended the Fishhook earlier
but decided that the Fishhook test may be too severe for consumer
information, and recommended handling tests instead of a rollover test.

UMTRI, GM, Advocates, CU and Honda recommended a centrifuge test as at
least part of the rollover rating despite NHTSA's elimination of it from
the research plan announced in July 2001.

Honda, CU, and VW suggested the combination of a rollover maneuver test
and the centrifuge test or SSF for rollover ratings.

Toyota, UMTRI, Nissan, VW and Ford recommend a separate handling test
distinct from the rollover rating with particular emphasis on yaw
stability and ESC.

Suzuki and Ford recommended tests other than those discussed in the July
2001 Notice. Suzuki recommended a dynamic tripped rollover test such as
the sled test described by Exponent. Ford recommended using a new path
following steering controller instead of the PCLLC mathematical path
correction technique it previously recommended, but it continued to
recommend the maneuvers and performance metric used in the PCLLC.

NHTSA notes that although the Alliance criticized SSF for not measuring
the effect of ESC, the tests recommended by Ford and GM do not measure
the effect of ESC. Also, Honda recommended testing with ESC turned off
if an on/off switch is provided.

**V. National Academy of Sciences Study**

In the conference report dated October 23, 2000 of the FY2001 DOT
Appropriation Act, Congress directed the agency to fund a National
Academy of Sciences study on vehicle rollover ratings. The study topics
were "whether the static stability factor is a scientifically valid
measurement that presents practical, useful information to the public
including a comparison of the static stability factor test versus a test
with rollover metrics based on dynamic driving conditions that may
induce rollover events." The National Academy's report was completed and
made publicly available on February 21, 2002.

The National Academy of Sciences made a number of findings and
recommendations concerning NHTSA's present ratings of rollover
resistance that we view as guidance for our efforts under the TREAD Act
to improve the rating system.

**Finding 1:**

### Through a rigid-body model, SSF relates a vehicle's track width, T, and center of gravity height, H, to a clearly defined level of the sustained lateral acceleration that will result in the vehicle's rolling over. The rigid-body model is based on the laws of physics and captures important vehicle characteristics related to rollover. {#through-a-rigid-body-model-ssf-relates-a-vehicles-track-width-t-and-center-of-gravity-height-h-to-a-clearly-defined-level-of-the-sustained-lateral-acceleration-that-will-result-in-the-vehicles-rolling-over.-the-rigid-body-model-is-based-on-the-laws-of-physics-and-captures-important-vehicle-characteristics-related-to-rollover. .BlockQuote}

## **Finding 2:**

> Analysis of crash data reveals that, for higher-risk scenarios, SSF
> correlates significantly with a vehicle's involvement in
> single-vehicle rollovers, although driver behavior and driving
> environment also contribute. For these scenarios, the statistical
> trends in crash data and the underlying physics of rollover provide
> consistent insight: an increase in SSF reduces the likelihood of
> rollover.

**Finding 3:**

## Metrics derived from dynamic testing are needed to complement static measures, such as SSF, by providing information about vehicle handling characteristics that are important in determining whether a driver can avoid conditions leading to rollover*.* 

The first three findings help resolve some very important questions
facing NHTSA regarding the implementation of the TREAD Act to improve
the rollover rating system. Namely, is SSF a scientifically valid
measure of rollover resistance and should a dynamic rollover test
replace SSF? The National Academy confirmed that SSF is a scientifically
valid measure of rollover resistance for which the underlying physics
and real-world crash data are consistent in the conclusion that an
increase in SSF reduces the likelihood of rollover. It also found that
dynamic tests should complement static measures, such as SSF, rather
than replace them in consumer information on rollover resistance.

The National Academy's report describes a rollover crash as an event
having three phases: a phase in which the driver is in control of the
vehicle, a transition phase in which loss of control develops, and a
phase in which the vehicle is out of control. The report gives SSF
(along with the terrain) as the dominant determinants of rollover in the
final, out of control phase, of a crash leading to rollover. It is in
the previous transition phase of the crash that other vehicle properties
reflected in the ideal dynamic test can potentially influence whether
the crash enters the final phase in which only the geometric properties
of the vehicle matter.

In its presentation to NHTSA of the findings and recommendations, the
NAS study committee clarified that it envisions dynamic tests as limit
maneuvers where loss of control and actual on-road vehicle tip-up can be
expected for vulnerable vehicles. The NAS study panel also expressed a
preference for combining static and dynamic vehicle information in a
single rollover resistance rating, but it did not offer explicit
suggestions for accomplishing the combination or conveying the rating to
the consumer.

The next series of findings involve the statistical relationship between
SSF and rollover rate that NHTSA uses to interpret the rollover
resistance ratings.

### **Finding 4:**

### NHTSA's implementation of an exponential statistical model lacks the confidence levels needed to permit discrimination among vehicles within a vehicle class with regard to differences in rollover risk.  {#nhtsas-implementation-of-an-exponential-statistical-model-lacks-the-confidence-levels-needed-to-permit-discrimination-among-vehicles-within-a-vehicle-class-with-regard-to-differences-in-rollover-risk. .BlockQuote}

### **Finding 5:** {#finding-5 .BlockQuote}

### The relationship between rollover risk and SSF can be estimated accurately with available crash data and software using a logit model. For the analysis of rollover crash data, this model is more appropriate than an exponential model.  {#the-relationship-between-rollover-risk-and-ssf-can-be-estimated-accurately-with-available-crash-data-and-software-using-a-logit-model.-for-the-analysis-of-rollover-crash-data-this-model-is-more-appropriate-than-an-exponential-model. .BlockQuote}

**Finding 6:**

> The approximation of the rollover curve with five discrete
> levels---corresponding to the five rating categories---is coarse and
> does not adequately convey the information provided by the available
> crash data, particularly at lower SSF values where the rollover curve
> is relatively steep.

NHTSA calculated what it believed was an accurate trend line between the
rollover rate in single vehicle crashes and SSF using data from over
221,000 single vehicle crashes of 100 vehicle make/model/generations
representing the range of SSFs and vehicle classes (cars, vans, pickup
trucks and SUVs). It determined the average rollover rate for each of
the 100 vehicles, corrected the rates for differences in demographic and
road use variables (driver age, gender, alcohol use, road and weather
conditions, etc) and performed a linear regression between SSF and the
logarithm of the corrected average rollover rate of each vehicle. The
NAS report refers to this approach as the exponential model because it
creates an exponential regression line between SSF and rollover rate.
NHTSA chose this approach because the exponential form of the regression
line fits the rollover rate data well, and linear regression computes
the R^2^ goodness of fit statistic that is familiar to many scientific
readers who are not professional statisticians. However, the standard
statistical technique for determining the confidence limits of the
regression line (which estimate how well the line would be replicated
with another sample of crash data for the same vehicles) only considers
a data set of 518 points. The 518 data points are the rollover rates in
each of six states for those vehicles in the 100 make/model population
for which more than 25 single vehicle crashes were reported.
Consequently, the 95th percentile confidence limits computed for the
exponential line are much larger than what would be expected for a data
set of 221,000 points. This is the basis for Finding Number 4. Since
each of the 518 data points on average represents 486 crashes, it stands
to reason that the actual reproducibility of the line is much better
than that computed on the basis of only 518 points. As the NAS study
notes, the standard method of computing confidence limits for linear
regression is the wrong method for our regression line, but it offered
no other method of computing the confidence limits of our present model.

In Finding Number 5, the National Academy offered an alternative
solution to the confidence limits issue. It recommended that the logit
model be used in place of the exponential model (linear regression on
the logarithm of rollover rate). The logit model operates on the 221,000
crash data samples individually rather than as 518 averages.
Consequently, the confidence limits are extremely narrow as would be
expected for a regression line representing a huge database. However,
the change to logit model produces another problem. Each model
incorporates an implicit assumption about the form of the regression
line. We chose the exponential form because it appeared to follow the
locus of data points. The form of the line produced by logit model in
our application is closer to a straight line than to an exponential
line. Consequently, it does not follow the locus of the raw data points
as well. It appears to underestimate the rollover rate of vehicles at
the low end of the SSF range by a substantial margin (36% versus about
45% @ SSF=1.00). The NAS study acknowledged this shortcoming and gives
the example of a nonparametric-based rollover curve it calculated on a
subset of NHTSA data that represents the low end of the SSF range much
better than the logit curve. We are investigating non-parametric models
and logit models using various transformations of SSF to develop a model
combining the demonstrated tight confidence limits of the logit model
with the more accurate estimate of rollover risk of our exponential
model.

For the interpretation of vehicle measurements for consumer information
on rollover risk, NAS concentrated exclusively on using statistical
models relating measurements, such as SSF, to rollover risk in a single
vehicle crash. Finding 5 concerns the choice of model within this
methodology. Finding 6 suggests that a five interval system loses some
of the power of the data to discriminate rollover risk between vehicles.
The committee goes on to recommend that the agency look at a greater
number of intervals or even a continuous risk scale.

**Finding 7:**

> A gap exists between recommended practices for the development of
> safety information and NHTSA's current process for identifying and
> meeting consumer needs for such information. In particular:

-   The focus group studies used to develop the star rating system were
    limited in scope.

-   The agency has not undertaken empirical studies to evaluate
    consumers' use of the rollover resistance rating system in making
    vehicle safety judgments or purchase decisions.

Focus group testing is the most appropriate tool we can use within our
budget and time constraints. As mentioned in the response to
Recommendation 3, below, we plan to use interviewing in conjunction with
focus group testing to design second-tier information to be used by
consumers who want more information than the star ratings. The agency
has not undertaken empirical studies to evaluate consumer's use of the
rollover rating system because the program was just initiated for the
2001 model year. Such a study would provide useful feedback for the
development of additional consumer rollover information. However some
history of use by the public needs to be acquired before the current
system can be evaluated.

> **Recommendation 1:**

### NHTSA should vigorously pursue its ongoing research on driving maneuver tests for rollover resistance, mandated under the TREAD Act, with the objective of developing one or more dynamic tests that can be used to assess transient vehicle behavior leading to rollover.  {#nhtsa-should-vigorously-pursue-its-ongoing-research-on-driving-maneuver-tests-for-rollover-resistance-mandated-under-the-tread-act-with-the-objective-of-developing-one-or-more-dynamic-tests-that-can-be-used-to-assess-transient-vehicle-behavior-leading-to-rollover. .BlockQuote}

This notice describes the results of test program that is part of
NHTSA's pursuit of the requirements of the TREAD Act to develop dynamic
tests for rollover. We believe that the limit maneuver tests we are
developing will provide the evaluation of the transient vehicle behavior
that the NAS committee has recommended as a complement to the
information from static measures. We also trying to develop tests of
vehicle controllability to give consumers some information on the
relative difficulty of keeping the vehicle on the road away from
tripping mechanisms in the event of an emergency maneuver.

**Recommendation 2:**

> In the longer term, NHTSA should develop revised consumer information
> on rollover that incorporates the results of one or more dynamic tests
> on transient vehicle behavior to complement the information from
> static measures, such as SSF.

NHTSA will evaluate possible changes in its present consumer information
on rollover resistance, based on SSF, as we develop the protocol for
dynamic testing for rollover required by the TREAD Act. Part of our
research planned for March to November 2002 will be to investigate the
best way to present both static and dynamic information to consumers.

### **Recommendation 3:**  {#recommendation-3 .BlockQuote}

### NHTSA should investigate alternative options for communicating information to the public on SSF and its relationship to rollover. In developing revised consumer information, NHTSA should: {#nhtsa-should-investigate-alternative-options-for-communicating-information-to-the-public-on-ssf-and-its-relationship-to-rollover.-in-developing-revised-consumer-information-nhtsa-should .BlockQuote}

-   Use a logit model as a starting point for analysis of the
    relationship between rollover risk and SSF.

-   Consider a higher-resolution representation of the relationship
    between rollover risk and SSF than is provided by the current
    five-star rating system.

-   Continue to investigate presentation metrics other than stars.

-   Provide consumers with more information placing rollover risk in the
    broader context of motor vehicle safety.

NHTSA is considering changing to a new model in conjunction with the
incorporation of dynamic test results into the rollover resistance
rating program. While the NAS prefers the logit model because it has
tighter confidence bounds than the linear model we used, the logit model
underestimates the risk of rollover for low-SSF vehicles. To attempt to
overcome the drawbacks of both our original method and the logit model,
while keeping tight confidence bounds, we will investigate the use of
other statistical models to better estimate rollover risk in future
model years at the same time that we improve our model to include
dynamic test results.

The NAS committee stated that it believed that NHTSA had documented the
relationship between SSF and rollover risk in single-vehicle crashes so
well that we were short-changing the public by reducing this information
to five star-rating levels[^4]. The NAS committee recommended that we
provide the public with additional rating levels in order to allow the
public to better differentiate rollover risk between vehicles. The focus
groups we conducted before implementing the current program indicated
that consumers would prefer the five- star rating system. This star
rating method is also consistent with the other parts of NCAP (frontal
and side crash ratings). However, we will explore the use of greater
differentiation of the data as well as alternative presentation formats
in future consumer research. We will change our presentation of the
second-level detailed information as soon as possible. We already
provide the actual SSF number for each vehicle in NCAP in addition to
the star rating, for those consumers who want more detailed information
on the vehicles. This hierarchical approach was recommended in the 1996
NAS study, "Shopping for Safety." We are considering refining this level
of information by placing that SSF number in the context of all the
other vehicles tested. We can also provide the public with the point
estimate for the rollover risk associated with each value of the SSF
using the logit curve. We will conduct interviews and focus groups this
spring to determine the most effective way to communicate primary and
secondary level information to consumers. Different communication
methods may be developed for print and web site implementation.

We agree that providing more information about rollover risk in the
context of overall motor vehicle risk would be useful information to
consumers. The agency presently includes an explanation of rollover
resistance ratings, how they were derived, and safe driving tips on its
web site.

We intend to develop further consumer information on rollovers. In the
short term, we are looking into providing consumers a better context for
rollover risk by better describing the size of the rollover crash
problem and its risk relative to other crash modes. In the long term,
the agency is trying to develop a method of combining available
information on the safety performance of each new vehicle model. The
approach we are exploring uses the front, side, and rollover measures
from NCAP combined with the safety benefits of rollover resistance and
vehicle weight estimated from real-world crash data. We would like to
combine the individual measures (for front, side, and rollover crashes)
to reflect their relative frequency in the real world. However, a
complete description of the safety of a new vehicle model should include
the effect of that vehicle on other road users (including occupants of
other vehicles on the road, pedestrians, and bicyclists). We are still
performing research that will help us better understand the factors
critical to vehicle aggressiveness and compatibility, and that will
provide a basis for a comprehensive combined safety rating.

**VI. Choice of Maneuvers for Rollover Resistance Tests**

Appendix I describes the candidate vehicle maneuver tests evaluated as
possible tests for dynamic rollover resistance and presents the results
of our evaluation program. The research to evaluate potential maneuver
tests for rollover is fully documented in the NHTSA technical report
*"Another Experimental Examination of Selected Maneuvers That May Induce
On-Road Untripped, Light Vehicle Rollover - Phase IV of NHTSA's Light
Vehicle Rollover Research Program".*

Table 1 summarizes the observations in Appendix I about each of the nine
Rollover Resistance maneuvers in the areas of Objectivity and
Repeatability, Performability, Discriminatory Capability, and Realistic
Appearance.

**Table 1: Summary of Rollover Resistance Maneuver Observations**

<table>
<colgroup>
<col style="width: 12%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
</colgroup>
<tbody>
<tr class="odd">
<td></td>
<td><p><strong>NHTSA</strong></p>
<p><strong>J-Turn</strong></p></td>
<td><strong>J-Turn with Pulse Braking</strong></td>
<td><p><strong>Fixed Timing</strong></p>
<p><strong>Fishhook</strong></p></td>
<td><p><strong>Roll Rate Feedback</strong></p>
<p><strong>Fishhook</strong></p></td>
<td><strong>Nissan Fishhook</strong></td>
<td><strong>Ford Path Corrected Limit Lane Change</strong></td>
<td><strong>ISO 3888 Part 2 Double Lane Change</strong></td>
<td><strong>Consumers Union Short Course Double Lane
Change</strong></td>
<td><p><strong>Open-</strong></p>
<p><strong>Loop</strong></p>
<p><strong>Pseudo</strong></p>
<p><strong>-Double</strong></p>
<p><strong>Lane</strong></p>
<p><strong>Change</strong></p></td>
</tr>
<tr class="even">
<td><strong>Objectivity and Repeatability</strong></td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Advantage</td>
</tr>
<tr class="odd">
<td><strong>Performability</strong></td>
<td>Advantage</td>
<td>Disadvantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Disadvantage</td>
</tr>
<tr class="even">
<td><strong>Discriminatory Capability</strong></td>
<td>Advantage*</td>
<td>Unacceptable</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Unacceptable</td>
<td>Unacceptable</td>
<td>Unacceptable</td>
</tr>
<tr class="odd">
<td><strong>Realistic Appearance</strong></td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Disadvantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
<td>Advantage</td>
</tr>
</tbody>
</table>

\* when limited to vehicles with low rollover resistance and/or
disadvantageous load condition

A.  **Closed-Loop Driver Controlled Rollover Resistance Maneuvers**

We continue to have substantial concerns about the use of maneuvers with
driver generated steering inputs to develop NCAP rollover resistance
ratings. Although fairly good driver-to-driver repeatability was seen
during the Phase IV testing, this partially reflects the approximately
equal skill levels of the test drivers. (This also partially reflects
the small range of the rating metric, maneuver entrance speeds, that was
seen.) A professional race driver could probably drive cleanly through
these maneuvers with higher entrance speeds. Conversely, an
inexperienced driver who has never done any test driving could probably
only manage lower speeds. We remain concerned that ratings generated
with a driver-closed steering loop maneuver might not be fair or helpful
to consumers if this year's test driver were not as good as last year's
or the test driver was having an off day when a particular make-model
was tested.

A further problem for maneuvers with driver generated steering inputs is
that of "clean" (none of the cones delimiting the maneuver's course were
bypassed or struck) versus "not clean" runs. Only for a "clean" run do
we know that the driver actually drove the prescribed maneuver. If the
vehicle during a run bypasses or hits one or more of the delimiting
cones, then there is no way to ensure that the driver was actually
trying to steer the prescribed course. To give two extreme examples, a
test driver could drive through the ISO 3888 Part 2 Double Lane Change
at a very high speed without a chance of two-wheel lift occurring by
going straight. Or, at the same speed, he could achieve two-wheel lift
by performing a fishhook maneuver. For either case, a "not clean" run
would be recorded.

It is extremely difficult to generate two-wheel lift while having a
"clean" run. While Consumers Union has stated that on a rare occasion it
managed to achieve two-wheel lift in a "clean" run, in general,
two-wheel lift will result in the vehicle not following the prescribed
course. Therefore, we must use maximum maneuver entrance speed for a
"clean" run as the rating metric instead of the more directly rollover
related metric of when two-wheel lift first occurs. The relationship
between maximum maneuver entrance speed and rollover resistance is not
known.

Although all Rollover Resistance maneuvers are influenced by both a
vehicle's handling characteristics and its resistance to tip-up, it
appears that handling dominates the Double Lane Change maneuvers but is
less important for the J-Turn and Fishhook maneuvers. The Double Lane
Change maneuvers are better for studying emergency vehicle handling than
rollover resistance. Clean runs of the CU and ISO 3388 tests are not
limit maneuvers in the sense of the J-Turn and Fishhook because they
cannot measure tip-up after the vehicle's direction control is lost.

One way to characterize maneuvers is by the number of major steering
movements they involve. The J-Turn has just one major steering movement,
the initial steer. A Fishhook has two major steering movements, the
initial steer and the countersteer. As shown by Figures 11 and 14, a
Double Lane Change has four major steering movements, the initial lane
change steer, the second lane change steer, the recovery steer, and the
stabilization steer, plus some minor steering movements. We believe that
these additional major steering movements increase the influence of
handling for Double Lane Change results compared to J-Turn and Fishhook
maneuvers.

During the Phase IV Rollover Research there were a number of "not clean"
runs of the CU Double Lane Change maneuver that resulted in two-wheel
lift. These two-wheel lifts always occurred just after the completion of
the second major steering movement, well before the third. In other
words, the two-wheel lifts occurred while the Double Lane Change and
Fishhook steering inputs were still similar and not after they had
diverged. No two-wheel lifts in Double Lane Change maneuvers were seen
after the third major steering movement. We believe that by the time of
the third major steering movement, the severity of the steering has
caused sufficient speed to be scrubbed-off to make two-wheel lifts at
this point in the maneuver very unlikely.

Double lane change maneuvers scored on the basis of highest "clean" run
speed had no value as dynamic tests of rollover resistance. For our
sample of test vehicles, there was actually an inverse relationship
between double lane change speed scores and the incidence of tip-up in
more severe maneuvers that induced tip-up. The test vehicle that
tipped-up the most often in other maneuvers and at a consistently lower
tip-up speed than other test vehicles would be rated the best vehicle
for rollover resistance by the CU Short Course or ISO 3888 Part 2 double
lane change on the basis of maximum clean run speed. These tests measure
a type of handling performance but do not measure rollover resistance.

**B. Sub-Limit Maneuvers Measuring Dynamic Weight Transfer**

Ford suggested two methods of implementing the same idea. It first
suggested the Path Corrected Limit Lane Change method in which vertical
wheel force measurements made in driver controlled runs over a number of
nominal double lane change paths are corrected mathematically for
variations due to the vehicle's departure from the ideal path. Appendix
I reported the results of a demonstration of this method in which Ford
assisted NHTSA in performing the test runs, and Ford performed the
mathematical corrections and calculated the Dynamic Weight Transfer
Metric (DWTM) for each of our test vehicles. In its subsequent comments
to the docket, Ford announced that it had developed an advanced path
following robot that could drive each test vehicle repeatably through
the ideal path directly, eliminating the need for mathematical path
correction. Ford expected both implementations to produce the same DWTM
for a given vehicle, and the following remarks address both
implementations.

Four double lane change courses are run at 45 mph. They are each
designed to produce a maximum lateral acceleration of 0.7g, but at a
different frequency of motion due to their different combinations of
length and offset. The performance metric for each test vehicle is
highest dynamic weight transfer produced by any of the four double lane
change courses.

Ford's use of the double lane change is much more relevant to rollover
resistance than the ISO 3888 or Consumers Union double lane change tests
described above. Dynamic weight transfer *is* the mechanism that leads
to tip-up. However, the Ford test is not a limit maneuver. It will not
cause vehicles to tip-up, lose control, or even invoke ESC in most
instances. From a theoretical point of view, this is the source of its
greatest advantage and greatest limitation. Running the tests at
sub-limit 0.7g lateral acceleration is a great advantage because any
reasonable concrete or asphalt pavement should supply sufficient
traction. It should eliminate concern about pavement traction variation
at a designated test location, and even permit comparable tests at
different locations. It should also eliminate the possibility of tire
debeading during test conditions. However, sub-limit tests require that
the comparison of dynamic performance between vehicles be extrapolated
from a test condition that does not cause control problems to the
extreme conditions that may actually produce rollover. Suspension
effects that may be important at tip-up would not necessarily appear at
the sub-limit test condition. While the swing-axle suspension design is
not in current use, it offers a clear example of the theoretical problem
of sub-limit tests. If a rear swing-axle vehicle enjoys a DWTM advantage
over a vehicle with a beam rear axle at a sub-limit condition, it is
easy to see how that advantage may not extrapolate to a limit condition
where weight jacking and severe positive camber angles associated with
swing-axle suspension manifest themselves.

Sub-limit maneuver testing also may not predict vehicle rollover
resistance at limit conditions. It is unclear how great a practical
limitation on rollover resistance testing is presented by the inability
of sub-limit tests to measure anomalies in suspension behavior that may
occur only in limit conditions. However, in the case of the Ford test,
the evaluation of the results for our test vehicles shows other
practical limitations that are certainly important. We included the 2WD
Chevrolet Blazer and the 4WD Ford Escape among our test vehicles because
they represented a large difference in static stability factor (0.21)
within the SUV class. In every test maneuver that produced tip-up and in
all load conditions, the Blazer had the worst performance and the Escape
had the best. Under the PCLLC method, the Mercedes ML320 with ESC
enabled performed worse than the Blazer and significantly worse than the
performance of the same ML320 with the ESC disabled. Since no other test
showed a loss of rollover resistance due to the operation of ESC, we
conclude that there was an error in the PCLLC method for this vehicle.
Aside from the ML320 with ESC, the Blazer and Escape set the performance
range among our test vehicles in the Ford test as well. However, the
standard deviation of DTWM measurement is so large in comparison to the
range of differences in DTWM between vehicles, that the large difference
in rollover resistance between the 2WD Blazer and the 4WD Escape barely
attains statistical significance. Aside from the erroneous result for
the ML320s with ESC, none of the other differences in DTWM between test
vehicles were statistically distinguishable from random measurement
variation. The measurement repeatability of the present form of the Ford
test makes it not suitable for comparisons of vehicles within a class.
The measurement variation of DWTM relative to the range of values across
vehicle population is at least 20 times that of SSF measurements.

A surprising limitation of the Ford test was that there was no
discernable dynamic weight transfer component in the measured Dynamic
Weight Transfer Metric. Except for the measurement of the ML320 with ESC
that we consider erroneous, the "dynamic" weight transfer measurements
were not different from the quasi-static weight transfer calculated from
c.g height, track width, and an allowance for steady state body roll.
This suggests that the same weight transfer would be measured if the
vehicle were simply driven in a circle at 0.7 g lateral acceleration.

The centrifuge is a theoretically ideal way to make the same
measurement. The weight transfer measurement could be made by placing
the vehicle on stationary scales on the centrifuge platform. Stationary
scales are a much more accurate way of measuring vertical load than the
method used in the Ford test. Both the PCLLC method and the
path-following robot method of Ford's test rely on measurements of axle
height and camber relative to the road to deduce vertical loads from
separate studies of tire deflection versus vertical and lateral loads
and camber angle. The centrifuge test could directly measure
quasi-static weight transfer at 0.7 g, but it could also measure the
lateral acceleration at tip-up for each vehicle which would increase the
measurement range across the population of vehicles. We expect that the
repeatability of centrifuge measurements would approach that of SSF
measurements, and Section VIII describes our plans to investigate the
potential of centrifuge testing. The "straight tether release" method of
centrifuge testing suggested by UMTRI also provides for a dynamic
component of load transfer that can be measured under laboratory
conditions. It is identical in concept to the sled tests for tripped
rollover suggested by Exponent.

Although Ford's PCLLC test produces results that are more quasi-static
than dynamic, rollover resistance ratings based on quasi-static load
transfer are useful if measured precisely, and they are likely to
correlate very well with real-world crash statistics. However, only true
limit maneuver tests measure the effects of ESC and potential anomalies
in suspension behavior on rollover resistance. Unfortunately, limit
maneuver tests are affected by pavement friction to a much greater
degree than Ford's test or centrifuge tests that do not involve pavement
friction. We do not expect pavement effects to be an insurmountable
obstacle to practical limit maneuver tests, but should that occur, we
believe that the centrifuge test has a great advantage in precision,
simplicity, and cost of operation over the PCLLC method while sharing
its advantage of pavement insensitivity.

**C. Choice of the Fishhook Test with Roll Rate Feedback and the J-turn
as an Effective Pair of Dynamic Rollover Resistance Test Maneuvers**

The fishhook and J-turn maneuvers turned out to be the only true limit
maneuvers in the test program. Unlike the other maneuvers they were
capable of causing tip-up in vehicles susceptible to on-road untripped
rollover. They were able to detect an increase in resistance to on-road
untripped rollover as a result of ESC operation, and they place the
vehicle in a circumstance where anomalies in suspension behavior will
manifest themselves. They were very objective and repeatable because
they were performed using a steering controller. We estimate that the
speed at tip-up is repeatable within 2 mph on the same surface. A test
performance criterion of tip-up or no tip-up would be absolutely
repeatable except for vehicles with a tip-up speed within 2 mph of the
maneuver cut-off speed set by safety concern for test drivers. We are
examining the repeatability of limit maneuver tests on different
pavements and in different seasonal conditions on the same pavement.

Our reasons for not choosing a Double Lane Change maneuver are
summarized in Table 1, discussed in Appendix I of this notice and
further clarified in subsections A and B above. However, to briefly
repeat, our primary concerns with the Double Lane Change maneuvers are:
a) the Ford version appears to be a very complex and expensive way of
measuring quasi-static load transfer with poor measurement precision;
also it does not measure ESC effects or anomalies in suspension behavior
at the limit; and b) the ISO 3388 and CU Short Course simply do not
measure rollover resistance under the performance criteria of maximum
entry speed of a clean run, nor are they limit tests.

Table 1 summarizes the observations that point to the Fishhook maneuver
as the best choice for a dynamic rollover resistance test maneuver. We
prefer the Roll Rate Feedback Fishhook to the Fixed Timing Fishhook
because roll rate feedback feature adapts the timing of steering to
characteristics of the vehicle being tested. This feature resolves
long-standing criticism of double lane change maneuvers for rollover
testing that the inherent timing of the course could favor the frequency
response of some vehicles over others. (The Ford test used a variety of
double lane change courses to address the same issue.) The Nissan
Fishhook also contains a procedure to adjust the steering timing to the
vehicle characteristic, but it is a more difficult test to perform than
is the automated Roll Rate Feedback Fishhook maneuver.

One of the problems with using the Roll Rate Feedback Fishhook (or any
other Fishhook) maneuver for consumer information is that Fishhook does
not give people an understanding as to how this maneuver occurs during
driving. To help people understand this test, we have decided to rename
Fishhook maneuvers (all variants) as Road Edge Recovery Maneuvers. The
Roll Rate Feedback Fishhook will be renamed the NHTSA Road Edge Recovery
Maneuver.

NHTSA analyses of crash databases have found that the most common
scenario leading to untripped rollover is road edge recovery. This
scenario begins with the vehicle dropping two wheels off the right edge
of the paved roadway onto an unpaved shoulder. The reasons for this
occurring include, among others, driver inattention, distraction and
fatigue. The driver attempts to regain the paved roadway by steering to
the left. Due to the lip between the pavement and the shoulder, a
substantial steer angle is required to start the vehicle moving to the
left. However, once the vehicle overcomes the lip and starts moving, it
quickly threatens to depart from the left side of the road. Therefore,
the driver rapidly countersteers to the right. This pattern of steering
during a road edge recovery was discovered during research done by the
Texas Transportation Institute[^5].

The similarity between the characteristic pattern of steering used by
drivers during a road edge recovery and a fishhook maneuver is apparent.
We note that fishhook maneuvers do not simulate the lip between the
pavement and the shoulder. However, we do not believe that this matters
since the effects of this lip occur at the very beginning of the
maneuver, well before the vehicle is likely to have two-wheel lift.

The NHTSA J-Turn maneuver (without pulse braking) was the easiest limit
maneuver to perform repeatably and objectively. However, it was not
chosen as a stand-alone dynamic rollover resistance test because it is
not severe enough. While our research has shown that the J-Turn can
discriminate between vehicles that have a low rollover resistance,
J-Turns generally do not induce tip-up for modern production vehicles
loaded only with a driver and instrumentation. Fishhook maneuvers induce
two-wheel lifts for more production vehicles.

The discriminatory power of the dynamic rollover test program will be
maximized by having test maneuvers with different levels of stringency
rather than just a single maneuver with tip-up speed as the only metric.
The NHTSA J-Turn is our choice for a lower severity dynamic rollover
resistance test maneuver. We have selected it because it has excellent
objectivity and repeatability, is easy to perform, and has a well worked
out test procedure. Having only a single major steering movement, it is
a logical step down from the Fishhook. This maneuver has a long history
of industry use. During NHTSA's discussions with the automotive
industry, every manufacturer stated that they routinely perform J-Turn
testing during vehicle development. Another way to increase the range of
test severity is by testing vehicles in different load conditions. Ford
suggested using the PCLLC tests with vehicles loaded to their Gross
Vehicle Weight Rating with the rear axle carrying its maximum rated
load. The tests described in this notice used a roof load as a second
load configuration. The rating system alternatives described in the next
section presume that the vehicles will be tested in two load conditions.
We have tentatively decided that the light load condition will be just
the driver and instruments and that the heavy load condition will be the
equivalent of fiftieth percentile male dummies in all seating positions.
Thus, we will test in four levels of stringency: J-turn with light and
heavy loads; and Roll Rate Feedback Fishhook with light and heavy loads.
The J-turn with light load is the least stringent, and the Fishhook with
heavy load is the most stringent. The rating example in the next section
assumes only four binary dynamic performance variables, namely did it
tip-up or not in each of the four maneuver/load combinations. The speed
at tip-up will be available as another level of stringency, but it is
not clear whether it will be needed. A greater number of dynamic
variables may not further improve the fit of the statistical model.

**VII. Proposed Rollover Resistance Rating Alternatives**

While many commenters suggested or supported specific dynamic rollover
tests, only two of them made suggestions about how to use the results of
dynamic rollover tests in ratings of rollover resistance. GM defined
minimum levels of performance for the centrifuge tip-up test, the
constant radius driving maneuver test of maximum lateral acceleration,
and the stability margin which is the difference between centrifuge test
result and the constant radius maneuver test result. A vehicle meeting
all three minimum levels of performance would be rated 2 stars. It also
defined a single higher "bonus star" level for each of the three
performance criteria, making it possible to rate up to 3 bonus stars for
total rating of 5 stars. Toyota presented an example of a range of
Lateral Acceleration for Rollover (LAR) in a fishhook maneuver (with
pulse braking if necessary) for a number of hypothetical vehicles
divided into 5 star levels of increasing LAR, noting that the actual
star levels should be determined "through NHTSA testing/data analysis."
GM's suggestion is based on the idea of being directionally correct -- a
vehicle with better rollover stability attributes should earn a higher
rating. Toyota's example is based on directional correctness as a
minimum; it is unclear whether its reference to NHTSA data analysis
refers to the analysis of test data to determine the likely extremes of
LAR or to the analysis of rollover statistics for vehicles of known LAR.

NHTSA's present rollover resistance ratings based on SSF are interpreted
in terms of a predicted rollover rate for the vehicle if it is involved
in a single vehicle crash. This goes far beyond the GM-suggested minimum
quality of directional correctness for a rating system. The NAS study
strongly supported the use of SSF to predict rollover rate as long as
the model relating SSF and rollover risk could be demonstrated to be
repeatable across data sets (shown by a tight confidence limits about
the regression line). While the logit model underestimates the rollover
risk of vehicles with very low SSF, its tight confidence limits can be
calculated by standard statistical software, and NAS concluded that the
repeatability of the model would support the discrimination of more than
5 levels of rollover resistance for light vehicles.

**Should Rollover Resistance be Rated Using Dynamic Maneuver Tests
Alone?**

The requirements of the TREAD Act refer only to a "dynamic test on
rollovers" and are silent about rollover resistance information derived
from static measures. However, the NAS study of the present rollover
rating system recommended that "NHTSA should vigorously pursue the
development of dynamic testing to [supplement]{.underline} the
information provided by SSF" \[emphasis added\]. NAS did not suggest
that any combination of dynamic tests alone was sufficient for consumer
information on rollover resistance, and its report explained that in the
final out-of-control phase of a rollover crash "SSF and the terrain over
which the vehicle is moving are the dominant determinants of whether
rollover will occur."

NHTSA agrees that the dynamic tests should supplement rather than
replace the static measures for the reasons given by NAS, but also
because ratings derived only from dynamic driving maneuver tests would
severely limit the scope of the consumer information. The terrain over
which dynamic driving maneuver tests for rollover take place is smooth
dry pavement, but the vast majority of rollovers take place on terrain
that includes soft soil, curbs and other objects that can place higher
tripping forces on the vehicle than can tire/pavement friction. There
are a number of vehicle design strategies for preventing tip-up in
maneuver tests. Those that involve lowering the center of gravity of the
vehicle, increasing its track width or reducing body sway would be
expected to increase the vehicle's general rollover resistance both
on-road and in the event of contact with a curb, soft soil or other
tripping mechanism.

There are also a number of vehicle design strategies to prevent tip-up
in maneuver tests that involve reducing the lateral tire/pavement
friction. These strategies range from simply using low traction tires to
sophisticated "rollover prevention" systems that can apply one or more
brakes in response to sensing a potential rollover situation. When a
tire is subjected to heavy braking, its capacity for lateral traction is
greatly reduced. This principle can be used to cause the vehicle to skid
rather than tip-up under control of a "rollover prevention" system (that
uses the brake intervention capability of ESC under control of a tip-up
sensing rather than yaw sensing computer program). Design strategies
that depend on the active or passive management of tire traction can be
effective in reducing the risk of a vehicle rolling over on the road
where tire traction matters. However, the on-road untripped rollover is
a special and limited case of rollover crash; most rollovers are
initiated by a tripping mechanism other than tire traction. NAS found
that dynamic maneuver tests for rollover are important because they are
sensitive to vehicle properties that are not reflected in static
measures of rollover resistance. But, a dynamic maneuver test
[alone]{.underline} can only assure the measured level of rollover
resistance in the case of on-road untripped rollover because tip-up in
the dynamic test can be prevented by tire traction management strategies
that have no effect when a tripping mechanism (other than tire traction)
initiates the rollover. Using dynamic maneuver tests to supplement the
information on rollover resistance obtained from static measurements
represents a potential improvement in consumer information, but the use
of dynamic maneuver tests alone would result in rollover resistance
ratings that may not apply to the most common type of real-world
rollover crash in which the vehicle strikes a tripping mechanism. That
would significantly reduce the correlation of rollover resistance
ratings to real-world rollover crashes.

**Rollover Resistance Ratings Based on Both Static Measures and Dynamic
Maneuver Tests**

**Alternative 1 - Combine Static and Dynamic Vehicle Measurement in a
Statistical Model of Rollover Risk**

The ideal rollover resistance rating system would give consumers
information on the risk of rollover in a single vehicle crash taking
into account both the static properties of a vehicle and its performance
in dynamic maneuver tests. The risk based system is better than a system
that is merely directionally correct. In addition to answering the
question "is the rollover risk lower for vehicle A or vehicle B?", it
can answer also the questions, "how much lower?" and "what is the
absolute risk?".

The present rollover resistance ratings are based on a statistical model
that considers about 221,000 single vehicle crashes of 100 popular
make/model vehicles for which we have SSF measurements. In addition,
each state accident report provides a number of driver demographic
variables (sex, age, sobriety), road characteristic variables (speed
limit, hill, curve, slippery surface), and weather variables (storm,
darkness). A statistical model can use the real-world crash data to
determine the effect of any variable on the proportion of single vehicle
crashes that result in rollover (rollover risk) in the presence of other
variables that may also exert an influence. In the present case, the
only vehicle variable is SSF, and the model predicts the risk of
rollover as a function of SSF in the presence of the many combinations
of confounding variables in the data sample of 221,000 crashes. The
predicted rollover risk of a vehicle in a single vehicle crash, based on
its SSF, becomes its rollover resistance rating which is expressed in
five discrete levels (less than 10%, 10% to 20%, 20% to 30%, 30% to 40%,
more than 40%) designated by one to five stars.

As mentioned previously, the NAS recommended that we use a logistic
regression model instead of the linear regression model in order to
establish tight confidence limits on the repeatability of the model, and
it found that the differences of rollover risk between vehicles
predicted by the statistical model were significant enough to support
more than five discrete levels. Also, the NAS study recommended that
NHTSA develop a risk model that combines the SSF measurement with the
results of one or more dynamic maneuver tests for a more robust consumer
information rating on rollover resistance.

The NAS study was not concerned with the distinction between tripped and
untripped rollovers because it is the magnitude and duration of the
forces that cause rollover in all circumstances. NHTSA has considered
the distinction between tripped and untripped rollovers important in
making a choice between a road maneuver test or a general rollover
resistance indicator metric like SSF for consumer information because
tripped rollovers are much more common occurrences. However, the NAS
recommendation of including both SSF and road maneuver test results in a
risk model makes the distinction between tripped and untripped rollovers
unnecessary. The recommendation does not require a choice between the
two types of rollover resistance measures because both are included.
Also, the risk model will be calculated using all available rollover
data including tripped and untripped rollovers from several states for a
number of vehicles that we will test using J-Turn and Fishhook maneuvers
and measure for SSF. The predictive power of both SSF and road maneuver
tests determined by real-world data will be reflected in the risk model.

We plan to conduct dynamic rollover tests of various levels of
stringency. The J-turn maneuver with a driver and instruments (light
load configuration) is the least stringent. It would be rare for this
maneuver to cause tip-up of a modern vehicle. The same J-turn test
performed with a passenger load in every seating position (heavy load
configuration) is a more stringent test that is likely to cause tip-up
for a few vehicles. The Fishhook test with roll rate feedback is more
stringent than the J-turn test because it includes a steering reversal
designed to occur at the least favorable instant for each vehicle. It
would also be performed in both light and heavy vehicle load
configurations for a total of four levels of test stringency. Each
maneuver is repeated in a series of increasing speeds until it tips-up
or reaches the maximum test speed. The speed at tip-up offers a
discriminator within each stringency level if needed.

We believe that this suite of dynamic rollover tests will identify
vehicles vulnerable to rolling over without the presence of a tripping
mechanism, and identify a relative rank order of vehicles regarding this
vulnerability. However, the vehicle's rank order alone does not predict
the rollover risk associated with its level of vulnerability to tip-up
in dynamic rollover tests. Also, the dynamic test program is not
expected to distinguish between vehicles having an SSF of about 1.2 or
greater because they are unlikely to tip-up in any dynamic maneuver test
for rollover. This expectation is based upon NHTSA's rollover maneuver
research from 1997 to present.

Combining the dynamic rollover test results with SSF in a risk model
should overcome the limitations discussed above. Consider two vehicles
with a similar SSF. If one vehicle tips up during dynamic rollover tests
but the second does not, we would expect this advantage to manifest
itself in the rollover crash statistics of real vehicles. Likewise, a
vehicle that tips-up only in high severity maneuvers should have better
real-world performance than a vehicle of similar SSF that tips up in
lower severity maneuvers as well. Even if the real-world reduction in
rollover risk associated with better dynamic maneuver test performance
proves to not be large, it is certainly reasonable to expect it to
affect the statistical risk model when it is entered along with SSF as
one or more additional vehicle variables.

The logistic regression model recommended by NAS (referred to as the
logit model) gives an example of how the dynamic and static information
could be combined in a risk model. As presented in the NAS report, the
model operated on three driver description variables, four road
description variables, two weather variables, but only one vehicle
variable. There is no obvious reason why the same model could not
operate on additional vehicle variables. While we are particularly
interested in differences in rollover risk between vehicles with
different dynamic test performance but similar SSF, we recognize that
dynamic test results and SSF are not independent variables. But some of
the variables describing the driver, road and weather also were not
independent. The hypothetical exercise described below seems to confirm
that logistic regression can use interrelated variables without
difficulty.

The data base we have used to construct linear and logistic regression
models for the existing rating program and to assist NAS in its study of
rollover ratings contains the state crash data for 100 vehicle
make/models and their SSF measurements, but we do not have dynamic
maneuver test results for these vehicles. In order to evaluate the
logistic regression process when dynamic test results as well as SSF are
used as vehicle variables, we selected 25 vehicles from our 100 vehicle
data base and tried to estimate their probable dynamic maneuver test
results based on previous dynamic tests of similar make/models. In the
absence of real test results these hypothetical maneuver test results
allowed us to use the logistic regression software with vehicle multiple
variables. The hypothetical dynamic maneuver test results were in the
form of 4 binary (yes/no) variables representing whether the vehicle
would tip-up in the four maneuver tests of differing stringency
(J-turn/light load, J-turn/heavy load, Fishhook/light load,
Fishhook/heavy load). The possible sub-levels of performance defined by
test speed at tip-up were not used. The data base included about 88,000
single vehicle crashes of the 25 vehicle make/models with the real
driver, road, weather and SSF data, but only our estimates for dynamic
"data".

![](media/image1.wmf)First, logistic regression was performed with SSF
as the only vehicle variable. The result is presented by the dashed line
in Figure 1. It is essentially identical to the result of the "logit
model" recommended by NAS that was constructed using a 221,000 crash
data base of which the 88,000 crashes are a subset. The similarity of
the results is consistent with the finding of very tight confidence
limits for the model.

Next, the logistic regression was repeated using the hypothetical
dynamic maneuver test results in addition to SSF as vehicle variables.
The points on the graph are the predicted rollover rates for each of the
25 vehicles considering both its static and dynamic measurements under
the mean distribution of the driver, road and weather variables. The
locus of points generally follows the line predicted by SSF alone but
shows differences in predicted rollover rates as a result of
hypothetical dynamic test performance, especially at the low end of the
SSF range. We estimated in the hypothetical dynamic maneuver test
results that, with one exception, none of the vehicles with an SSF
greater than 1.17 would tip up in even our most severe dynamic maneuver
test. However, even if a vehicle does not tip-up in our maneuver tests,
its risk of rollover is not zero, and it is strongly related to SSF as
shown in the model. The model also allows for the possibility that
vehicles with the same SSF may have significant differences in dynamic
test results that influence the real rollover risk. These are the
characteristics we expect in a reasonable risk model. While this
preliminary investigation of logistic regression as a means to combine
static and dynamic measurements is encouraging, NHTSA will continue to
examine the theoretical soundness and confidence limits of the model in
keeping with the recommendations of NAS[^6].

The relative value of static versus dynamic measurements for determining
the rollover resistance of vehicles is a significant question.
Certainly, the use of both types of information to determine rollover
resistance should lead to the most accurate information, but one must
determine the relative weighting of the static and dynamic measurements.
The combination of the static and dynamic information in a statistical
model of rollover risk is an objective way to let real-world crash data
determine the weighting that best represents the outcomes of crashes.
Besides providing the best rollover risk estimates, the statistical
model also has the advantage of not requiring judgments about
appropriate data weighting from NHTSA or any of the interested parties.
Regardless of the rating method, the NCAP program will make available
the test results for SSF and for each of the dynamic maneuver tests, so
that consumers can see the basis of our rating and exercise their own
judgments about their particular concerns.

However, this method of rollover resistance rating has some drawbacks.
Dynamic maneuver test results for vehicles with large samples of single
vehicle crash data are needed to compute a robust risk model. In order
to use dynamic test results in risk-based ratings, NHTSA must first test
a number of older vehicles to correlate the combined vehicle information
of dynamic test performance and SSF to rollover rate using a large crash
database. Eventually the NCAP test results will supply the risk model
with vehicle information, but sufficient corresponding crash data will
trail the vehicle measurements by at least four years. State accident
records are reported to NHTSA yearly, but they lag by about two model
years. Even a high production vehicle requires about two years of
exposure to accumulate sufficient single vehicle crash data in the few
states with reliable reporting of both vehicle identification and
rollover crashes. Consequently, it will be a number of years before the
effects on rollover rate of traction management strategies and other
technologies that improve dynamic maneuver test results are represented
directly in the risk model. In the mean time, vehicle characteristics
that improve rollover resistance only in the special case of on-road
untripped rollover may be overvalued in the risk model in comparison to
vehicle characteristics that improve resistance to both untripped and
tripped rollover.

Critics of the SSF- based rating system may view the combination of
dynamic and static measurements in a risk model as an attempt by NHTSA
to devalue the dynamic tests. That is not the case[^7]. It is true that
SSF is a strong predictor of the risk of rollover especially in a
tripping situation and that most rollovers are tripped. Consequently, we
expect SSF to have a strong effect in a risk model even when dynamic
test variables are also included. However, the strong effect of SSF is
not likely to diminish the differences in rollover rate predicted for
difference in dynamic performance. We note that the example of Figure 1
is based only on estimates of dynamic test performance. We will not know
until we have actual dynamic test results for some of the 100 vehicles
in our 221,000 crash database whether the effect of dynamic test
performance on the rollover risk model is as great as expected.

**Alternative 2: Separate Ratings for Dynamic Rollover Test Results and
Static Vehicle Measurements**

An alternative rating system is proposed to address concerns that
combining the dynamic and static information in a risk model could give
the dynamic tests less influence than concerned parties would prefer. It
is based on the idea that the dynamic rollover maneuver tests are a
direct representation of an on-road untripped rollover. Therefore, the
dynamic test results may be reported separately as ratings of resistance
to untripped rollover. Likewise, the SSF measurements would be presented
separately as ratings of resistance to tripped rollover.

We believe that the vast majority of the rollovers in our 221,000 single
vehicle crash database are tripped rollovers. However, it is impossible
to identify those that may be untripped because state accident reports
are not concerned with that level of detail. About 95 percent of the
small number of rollover crashes investigated directly by NHTSA in great
detail (the NASS-CDS program) were tripped. Assuming a similar
distribution of tripped and untripped rollovers, our large database is a
suitable basis for a risk model of tripped rollover using SSF. The
tripped rollover risk predictions would be the same as the present risk
predictions except for the changes in statistical methodology
recommended by NAS.

Unfortunately, the NASS-CDS database receives reports of only about 10
untripped rollovers (and about 200 tripped rollovers) a year, precluding
any possibility of risk prediction on a make/model basis for untripped
rollover. Ratings of resistance to untripped rollover would have to be
based simply on the principal of directional correctness. For instance,
a vehicle that did not tip-up in any maneuver at any load condition
would be rated "A"; a vehicle that would tip-up in a maneuver test only
when loaded at every seating position would be rated "B"; and a vehicle
that would tip-up in a maneuver test even in the lightly loaded
condition would be rated "C".

This rating system also has some disadvantages. The use of two sets of
ratings about the same general type of crash would be difficult to
communicate effectively to consumers. It will also be hard to explain to
consumers why the SSF rating may be expressed in terms of risk but not
the dynamic rating. Since the only risk information in the rating system
would be associated with the static measures, those most interested in
the dynamic tests may find that more dismissive of the dynamic tests
than the combination of both types of information in a single risk
model. Since an unknown portion of our crash database does contain
untripped rollovers, the risk model based on that data without the use
of untripped rollover test data at hand may also be perceived as not the
best use of all data available to NHTSA.

Some of the parties most interested in dynamic tests have commented
repeatedly that SSF should not be used in the rollover resistance rating
of vehicles. However, consumer information based only on dynamic
maneuver tests greatly reduces the assessment of the physical forces
that cause real world rollovers. That would make the consumer
information less useful to the public.

SSF measures the steady, rigid body load transfer common to all
rollovers. The quasi-static centrifuge test adds a measurement of the
load transfer due to body roll which should also be common to all
rollovers. The Exponent sled test and the straight tethered centrifuge
test add roll momentum effects typical of tripped rollovers and possibly
J-turn tests. The dynamic maneuver tests add to these only a measurement
of the effect of ESC and other electronic "rollover prevention" systems
and a measurement of dynamic suspension behavior that may detect unusual
problems at limit conditions. However, the test conditions of dynamic
maneuver tests are limited by on-road tire traction and represent only
the special case of on-road untripped rollover. Hence, we believe the
dynamic maneuver tests should be used to supplement in some way one of
the other three types of tests with relevance to tripped rollovers
because tripped rollovers represent the vast majority of real world
rollovers.

**Consumers Preferences for Presentation of Rollover Ratings**

In response to the NAS recommendations and in order to better refine
approaches to developing and delivering consumer information on
rollover, NHTSA recently initiated additional consumer research on
rollover. This research was to further explore the perceptions,
opinions, beliefs and attitudes of drivers about vehicle rollover, and
to gather reactions to different presentations of ratings and other
rollover information.

The consumer research conducted was iterative in that it utilized
individual in-depth interviews as a first phase, and focus group testing
as a second phase. The in-depth interviews were conducted with 22
persons in Baltimore, MD in March, 2002. A total of 12 focus groups of
106 persons were conducted in Chicago, Dallas, and Richmond in April,
2002. Participants for both the interviews and focus groups had to have
purchased or planned to purchase a vehicle within the year. They also
had to rate safety as somewhat or very important in their vehicle
purchase decisions. One-third of the participants also had to rate
rollover as somewhat or very important in their purchase decisions.

The in-depth interviews were conducted with the intention of exploring
consumer beliefs and perceptions in a probing more detailed way than is
possible in focus groups. The interviews also served to provide insights
as to how the focus groups could be most effectively conducted to
acquire the desired findings. The interview results provided the basis
for modifying approaches and sample materials presented at the focus
groups. This iterative process did not, however, render opposing or
contradictory results. The findings of the interviews and focus groups
were remarkably and consistently similar. The key findings are as
follows:

## **Understanding of and Preference for Dynamic and/or Static Rating for Rollover** 

-   Virtually all participants were able to identify the difference
    between the tests for the Static Stability Factor (SSF) Rollover
    Rating and the Dynamic Test rollover rating, i.e., that the first is
    a vehicle measurement and that the latter involves maneuver tests.

-   Most participants preferred a combined rating, especially once they
    understood that 95% of real-world rollovers are accounted for by
    SSF. Those who said they should be presented separately thought they
    would provide consumers with more information; but they also thought
    that the different (5 pt vs.3 pt) rating scales presented would
    confuse people. Many thought that a dynamic test was more realistic.

-   Some participants had trouble understanding "track width" and
    "center of gravity height" in the description of SSF.

```{=html}
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```
-   Even though most participants did not explain rollover in the same
    way it was described to them, most stated that the description of
    rollover they read (from NHTSA web-site information on rollover) was
    understandable.

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```
-   Some of the rollover terminology; "rollover resistance rating,"
    "tripped by" and especially "tripped by a [ditch,]{.underline}" were
    confusing or did not make sense to many of the participants.

## **Preferences for Presentation of Rollover Ratings and Information**

-   Participants were presented with stars, numbers, letters and
    descriptive language as alternatives for presenting rollover
    ratings. Stars were overwhelmingly preferred by both interview and
    focus group participants. They clearly disliked number ratings, and
    were ambivalent about letters and descriptors. Graphics presented to
    participants are shown in Figure 2 and in the report "Findings of 21
    In-Depth Interviews and 12 Focus Group Discussions Regarding Vehicle
    Rollover," which is available in the docket for this notice.

-   Participants accurately interpreted the star ratings, with and
    without the key that explained what each star meant and which was
    better. However, many did not fully grasp that the ratings were
    vehicle ratings and were therefore confused by or did not find
    credible the actual data sets that showed percentages from over 40%
    to under 10% for rollover risk.

-   When presented with a bar graph that showed an individual vehicle
    among all vehicles, most interview participants found the bar graph
    complicated and too vague. Some said it might be useful to decide
    between different vehicle classes. The bar graph was refined
    visually and presented as a way of checking an individual vehicle
    through the web-site for the focus groups. When shown this graph
    depicting where a certain vehicle ranked in relationship to other
    vehicles in it's class, and against all classes as well as where it
    fell in the star rating range, most participants understood it and
    thought it useful.

## **Preferences for Rating Levels for Rollover Ratings**

-   Nearly all of the participants preferred five rating levels.
    Alternatives of three and ten ratings were presented through the use
    of numbers, letters, half-stars and narrative descriptors. Most said
    they did not like the half stars, but when probed said it might make
    a difference in whether or not they would consider a vehicle.
    Interestingly, many assigned different values to half-star ratings;
    e.g. 3 ½ stars was considered more

![](media/image2.jpeg){width="6.5in" height="4.875in"}

######### Figure 2a. Graphics presented to focus groups -- 4 out of 5 stars, without key 

> ![](media/image3.jpeg){width="6.5in" height="4.875in"}

######### Figure 2b. Graphics presented to focus groups -- 4 out of 5 stars, with key 

![](media/image4.jpeg){width="6.5in" height="4.875in"}

> Figure 2c. Graphics presented to focus groups -- 5 star system with
> half stars used
>
> ![](media/image5.jpeg){width="6.5in" height="4.875in"}
>
> Figure 2d. Graphics presented to focus groups -- separate presentation
> of static and dynamic ratings
>
> ![](media/image6.jpeg){width="6.5in" height="4.875in"}
>
> Figure 2e. Graphics presented to focus groups -- 5 level system
> expressed with numbers rather than stars
>
> ![](media/image7.jpeg){width="6.5in" height="4.875in"}
>
> Figure 2f. Graphics presented to focus groups -- 10 level system
> expressed with numbers rather than half-stars
>
> ![](media/image8.jpeg){width="6.5in" height="4.875in"}
>
> Figure 2g. Graphics presented to focus groups -- rating of the vehicle
> of interest shown graphically in context of all other rated vehicles
>
> ![](media/image9.jpeg){width="6.5in" height="4.875in"}

#### Figure 2h. Graphics presented to focus groups -- 3 level system expressed with letters 

#### ![](media/image10.jpeg){width="6.5in" height="4.875in"}

Figure 2i. Graphics presented to focus groups -- 3 level system
expressed with words

important than 4 ½ stars.

-   Most participants felt three rating levels was too few. Very few
    felt that 10 rating levels was appropriate. Most thought it was too
    much information and unnecessary.

The findings of this research will help NHTSA to develop appropriate and
useful rollover ratings and consumer information in the future. NAS has
recommended that the agency provide the public with additional rating
levels in order to allow better differentiation of rollover risk between
vehicles. While clearly there are improvements to be made in how
rollover resistance and ratings are explained and made useful to the
consumer, there does not seem to be any basis in our research to date
for deviating from stars or from the five rating levels presently being
used. However, for consumers who desire more information than just
star-ratings, we will provide detailed information on each vehicle on
the web-site. Consumers will also be able to differentiate between
vehicles through use of the internet based bar-graph data that tested
positively, and through other as yet undeveloped presentations.

**VIII. Intent to Evaluate Centrifuge Test**

The test device for the centrifuge test is similar in concept to a
merry-go-round. A person seated at the edge of the merry-go-round feels
a lateral force pushing him or her away from the spinning surface that
increases with the rotational speed of the merry-go-round. The
centrifuge device test shown in Figure 3 consists of an arm attached to
a powered vertical shaft. At the end of the arm is a horizontal platform
upon which the test vehicle is parked. As the vertical shaft rotates,
the parked vehicle is subjected to a lateral acceleration that can be
precisely controlled and measured. The basic quasi-static measurement is
the lateral acceleration at which the parked vehicle experiences
two-wheel lift. The outside tires are restrained by a low curb so the
measurement is independent of surface friction, and the vehicle is
tethered for safety to prevent excessive wheel lift. This test method
was suggested by the University of Michigan Transportation Research
Institute (UMTRI) both in comments to our notice about the present
rollover resistance ratings and more recently in the context of the
TREAD Act. As discussed in Section III, the quasi-static centrifuge test
was also recommended by GM, Honda, CU and Advocates as a possible
improvement on SSF to measure general rollover resistance. The test
method is directed primarily at tripped rollover, which UMTRI noted
accounts for all but a small percentage of rollovers.

![](media/image11.wmf)

The centrifuge test has many advantages. Like SSF, it is a measurement
that that can be performed accurately, repeatably and economically (at
least in labor costs). It is arguably more accurate than SSF in
evaluating tripped rollover resistance because it includes the effect of
the outward c.g. movement as a result of suspension and tire
deflections. Its correlation to SSF would be high, and it would be
expected to correlate well with the actual rollover rates of vehicles,
because those statistics are largely driven by tripped rollovers. The
quasi-static centrifuge measurement of a vehicle's lateral acceleration
at two-wheel lift is expected to be roughly 10 percent less than the
vehicle\'s SSF with about a +/- 5 percent range to cover extremes in
roll stiffness.

Despite these advantages, we did not include the centrifuge test in the
test evaluation plan that was the subject of our July 2001 notice. We
stated the following reasons:

Improvements in centrifuge test performance can be made by suspension
changes that degrade handling. The best performance in the centrifuge
test (and in the closely related but less accurate tilt table test)
occurs when the front and rear inside tires lift from the platform at
the same time. The tuning of the relative front/rear suspension roll
stiffness to accomplish this will cause the vehicle to oversteer more
than most manufacturers would otherwise desire. We do not want to tempt
manufacturers to make this kind of trade-off. Further, we understood the
intention behind TREAD to be that NHTSA should give the American public
information on performance in a driving maneuver that would evaluate the
performance of new technologies like ESC. The centrifuge test would not
do so.

As discussed in Section III of this notice, GM provided some data
disputing our concern that improvements in centrifuge test scores could
be obtained at the expense of changing the understeer/oversteer
suspension tuning of vehicle from what the manufacturer would otherwise
choose as optimum for handling and consumer satisfaction. We request
that other manufacturers and vehicle designers review GM's information
(comment 6 to docket NHTSA-2001-9663 notice 1) and comment on the
validity of NHTSA's concern.

In view of the interest expressed by several commenters in centrifuge
testing and the potential importance GM's information, NHTSA intends to
evaluate the practicability of centrifuge testing. To our knowledge,
centrifuge tests for rollover resistance of vehicles have never been
performed. The interest of commenters is based on theoretical advantages
over SSF. NHTSA will develop a test fixture and test a number of
vehicles in the quasi-static mode using a very large centrifuge at
NASA's Goddard Space Flight Center in Greenbelt, Maryland.

**IX. Handling Tests**

**A. The Need for Handling Testing and a Handling Rating**

NHTSA expects that implementation of a rollover rating system using
dynamic tests will, over time, influence vehicle designs. Therefore, it
is of the utmost importance that we do **not** encourage designers to
maximize vehicle performance in rollover resistance tests by degrading
other safety relevant areas of vehicle performance.

Several possible ways to maximize vehicle performance in rollover
resistance tests would degrade vehicle handling. For example, better
performance in rollover resistance tests could be achieved by one or
more of:

-   Making the vehicle have less turning capability. Unfortunately, this
    would make it harder, in difficult situations, for drivers to keep
    the vehicle on the road or to avoid colliding with other vehicles,
    pedestrians, animals, and other objects.

-   Equalizing the roll stiffnesses of the front and rear suspensions.
    Unfortunately, this may make the vehicle spin-out in limit
    maneuvers.

-   Making the vehicle respond slowly to steering inputs. Again, this
    would make it harder, in some situations, for drivers to keep the
    vehicle on the road or to avoid colliding with other vehicles or
    pedestrians.

To discourage vehicle designers from maximizing rollover resistance at
the expense of handling, NHTSA believes that if our rollover ratings are
directly influenced by dynamic tests then we must also have a handling
rating based on handling tests.

In addition to discouraging vehicle designers from maximizing rollover
resistance at the expense of handling, having a handling rating based on
handling tests should also encourage the adoption of yaw stability
control. While the crash prevention benefits of yaw stability control
have not yet been proven, we anticipate that it may help prevent
crashes. Based on NHTSA's Phase IV Rollover Research, we will see some
improvement in a vehicle's rollover resistance rating due to yaw
stability control. However, a handling rating provides another
opportunity for showing the beneficial effects of yaw stability control.

**B. Guiding Principles for NHTSA Handling Testing and Handling Rating**

What is handling? In this document, what we mean by handling is the
lateral response of the vehicle to a driver's control inputs. Clearly
steering inputs are the most important control inputs for handling,
however, brake and throttle pedal inputs can also have an effect.

Traditionally, handling assessments have been made subjectively. Several
test drivers drive a vehicle for a period of time through a broad
variety of maneuvers. The maneuvers range in severity from mild to
severe to limit. After driving the vehicle, each driver independently
assigns a numerical handling rating to the vehicle. Ratings from all of
the test drivers are averaged to obtain an overall handling rating.

We do not believe that a subjective handling rating is suitable for
inclusion in the New Car Assessment Program. Government generated
handling ratings must be objectively and repeatably determined.

There are two perspectives for handling ratings. One perspective is how
safe the vehicle is to drive. The other is how well the vehicle gives an
enthusiast driver a pleasurable sense of control. Given its mission, a
NHTSA generated handling rating can only assess how safe a vehicle is to
drive, not how pleasurable it is to drive.

What aspects of handling affect safety? NHTSA has identified the
following four:

1\. Amount of turning capability. A vehicle that can turn more sharply
should be easier for drivers to keep on the road and to avoid colliding
with other vehicles, pedestrians, animals, and other objects.

2\. Graceful degradation at/near limits. When a driver approaches or
tries to exceed the maximum turning capability of a vehicle the vehicle
should plow-out (saturate traction on the front wheels first) instead of
spin-out (saturate traction on the rear wheels first).

3\. Predictability. When the driver steers, brakes, or changes the
throttle level, the vehicle should do what the driver expects the
vehicle to do. Since all vehicles have delays between steering, braking,
or throttle application and the response of the vehicle, drivers must
predict the response of the vehicle to a control input. If the vehicle
does not perform as expected, there may not be time for the driver to
react to the unexpected motion before a crash occurs.

4\. Responsiveness. When the driver steers, brakes, or changes the
throttle level, the vehicle should respond quickly to the driver's
inputs. A slowly responding vehicle would be harder for drivers to keep
on the road or to avoid colliding with other vehicles, pedestrians,
animals, and other objects.

We have discussed the aspects of handling that affect safety with
Consumers Union. In addition to the four aspects listed above, Consumers
Union uses a fifth, appropriate feedback to the steering handwheel, in
developing ratings for their magazine. While we do not dispute the
importance of appropriate feedback to the steering handwheel, this seems
to us to be such an inherently subjective assessment that we have not
included it in the above list.

We welcome comments as to the correctness of the above list of handling
aspects that affect safety. Are the aspects that are listed appropriate?
Have we left anything out?

**C. Handling Tests Being Considered by NHTSA**

NHTSA is considering developing a handling rating based upon results
from the three handling maneuvers. The handling maneuvers are:

1\. Slowly Increasing Steer maneuver. Using a programmable steering
controller, the steering handwheel is turned slowly (13.5 degrees per
second) from zero to well beyond the point at which the maximum lateral
acceleration occurs (a handwheel steering angle of 270 degrees). The
driver applies the throttle to keep the vehicle's speed as constant at
50 mph as possible during the turn.

> The Slowly Increasing Steer maneuver provides data to assess the
> amount of turning capability of a vehicle (the Maximum Attainable
> Lateral Acceleration) and whether the vehicle's handling degrades
> gracefully at the limit (did the vehicle plow or spin when the maximum
> achievable turn was attained). We performed this maneuver for every
> vehicle tested during Phases II, III, and IV of NHTSA Rollover
> Research. Based on our experience we believe that this maneuver can be
> performed with excellent objectivity and repeatability. There is a
> well worked out and widely accepted procedure for the Slowly
> Increasing Steering maneuver that is contained in the Society of
> Automotive Engineers Standard J266.

2\. Dropped Throttle in a Turn maneuver. Using a programmable steering
controller, the steering handwheel is turned quickly, and then held at,
the angle required to attain 90 percent of the vehicle's maximum
achievable lateral acceleration. The driver initially applies the
throttle to keep the vehicle's speed as constant as possible during the
turn. The throttle is then suddenly released and the resulting vehicle
motion measured.

> The Dropped Throttle in a Turn maneuver provides data to assess the
> predictability of the vehicle. Desirable behavior is for the vehicle
> to either maintain the same radius of curvature or to "tuck-in" a bit
> (slightly decrease the radius of curvature). While we have not
> performed this maneuver in the past, we expect that this maneuver can
> be performed with excellent objectivity and repeatability. There is a
> well worked out and widely accepted procedure for the Dropped Throttle
> in a Turn maneuver that is contained in the International Standards
> Organization's Standard 9816.
>
> Multiple measures of vehicle performance are determined from this
> test. One is the Dropped Throttle Yaw Rate Ratio, defined as the
> maximum yaw rate attained at any time during the three seconds after
> the throttle was released divided by the initial yaw rate. The second
> is the Dropped Throttle Path Deviation, defined as the lateral
> displacement of the vehicle's center of gravity two seconds after the
> throttle has been released from the anticipated path if the throttle
> had not been released.

3\. The Step Steer maneuver. This maneuver is performed in the same
manner as the NHTSA J-Turn except that the handwheel steering angle used
is less. Instead of turning the steering handwheel to 8.0 times the
angle needed to achieve 0.3 g lateral acceleration in the Slowly
Increasing Steer maneuver (the angle used for the NHTSA J-Turn), for
this maneuver the steering wheel is only turned to the angle needed to
achieve 4.0 meters per second squared lateral acceleration. A handwheel
steering rate of 1,000 degrees per second is used. The maneuver entrance
speed is 50 mph (80 kph) and the throttle is held constant through the
test.

> Multiple measures of vehicle performance are determined from this
> test. One is the Yaw Rate Response Time, defined as the time from when
> the steering handwheel reaches 50 percent of its final value to the
> time when the yaw rate reaches 90 percent of its steady-state value.
> The second is the Peak Yaw Rate Response Time, defined as the time
> from when the steering handwheel reaches 50 percent of its final value
> to the time when the yaw rate reaches it peak value. The third is
> Percent Overshoot, defined as the difference between the peak and
> steady state yaw rates divided by the steady state yaw rate.
>
> The Step Steer maneuver provides data to assess the predictability
> (from the Percent Overshoot measure) and the responsiveness (from the
> Yaw Rate Response Time and the Peak Yaw Rate Response Time measures)
> of the vehicle. We performed this maneuver for every vehicle tested
> during Phase IV of NHTSA Rollover Research; based on our experience we
> believe that this maneuver can be performed with excellent objectivity
> and repeatability. There is a well worked out and widely accepted
> procedure for the Step Steer maneuver that is contained in the
> International Standards Organization's Standard 7401.

Each Handling Maneuver would be performed at two loading conditions,
Nominal Load and Rear Load. The Nominal Load consists of the curb weight
vehicle plus the driver plus NHTSA's instrumentation package plus
NHTSA's titanium outriggers. The Rear Load adds to the Nominal Load
ballast positioned such that the vehicles rear Gross Axle Weight Rating
(GAWR) and Gross Vehicle Weight Rating (GVWR) are achieved
simultaneously. The ballast is comprised of bags of lead shot,
positioned as flat as possible across the rear cargo area of the test
vehicle. The ballast will be secured in a manner that insures it does
not shift during testing. We will use a ¾ inch enclosed plywood box to
contain the ballast used in the Rear Load condition. Due to the wide
range of shapes and sizes of light vehicle cargo areas, such boxes will
need to be constructed on a per-vehicle basis.

We welcome comments as to the appropriateness of the above list of
handling maneuvers. What have we left out?

NHTSA is seeking tests of handling and controllability both as way of
dealing with potential trade-offs between handling properties and
rollover tests and as a way of giving credit to technologies that
improve controllability. We request comment on the value of such tests
to resolve the concern for design compromises that could improve
centrifuge test scores.

One of our concerns is that yaw stability control is supposed to
increase a vehicle's predictability; however, our Dropped Throttle in a
Turn Maneuver test is may not be adequate for measuring the effects of
yaw stability control. What other objective and repeatable tests exist
for measuring vehicle predictability?

**D. Combining Handling Test Results to Generate a Handling Rating**

As is the case for rollover resistance ratings, an ideal handling rating
system would use data obtained from the above mentioned handling tests
to predict the risk, for a vehicle make/model assuming an "average"
driver, of a single vehicle crash. The risk based ratings are better
than ratings that are merely directionally correct because in addition
to answering the question "Is the single vehicle crash risk lower for
Vehicle A or Vehicle B?", it can also answer the questions, "How much
lower?", and "What is the absolute risk?".

The influence of drivers on whether or not a single vehicle crash occurs
is very high. The driver demographic variables that are available in the
crash data bases are believed not to be sufficient to quantify this
influence (i.e., there is no variable quantifying a driver's
aggressivity). Therefore, we believe that, unlike rollover resistance
ratings, handling ratings will not be able to predict single vehicle
crash risk. They can, at best, be directionally correct.

We envision a three level handling rating system, tentatively, from best
to worst, A, B, and C. A star rating system would not be used for
handling ratings because they are not risk based but only directionally
correct.

The handling rating calculation method proposed below contains many
constants whose values NHTSA will specify at a later date (e.g.,
a~YMinN~ and a~YRangeN~). Our intention is to determine values for these
constants based on data collected during the Phase VI testing. During
Phase VI 25 vehicles for which we have state crash data on rollover will
be tested using both rollover maneuver tests and handling tests
concluding in Fall 2002. We have tried to choose the Phase VI test
vehicles so as to cover the full range of handling that is seen in the
current fleet, from excellent to average. (We do not believe that any
current production vehicle has handling we would characterize as bad.)
Once we have the Phase VI data, we will select values for the constants
so that approximately one-third of the vehicles earn A ratings,
one-third earn B ratings, and one-third earn C ratings.

The handling rating would be determined from the measurements results of
the handling tests as follows:

1\. Calculate a Handling Score, HS, from the formula:

HS = W~1~ \* H~1~ + W~2~ \* H~2~ + W~3~ \* H~3~ + W~4~ \* H~4~

~+~ W~5~ \* H~5~ + W~6~ \* H~6~ + W~7~ \* H~7~ + W~8~ \* H~8~

~+~ W~9~ \* H~9~ + W~10~ \* H~10~ + W~11~ \* H~11~ + W~12~ \* H~12~

where W~1~ through W~12~ are weights that NHTSA will select values for
at a later date, H~1~ is the Maximum Attainable Lateral Acceleration at
Nominal Load sub-score, H~2~ is the Dropped Throttle Yaw Rate Ratio at
Nominal Load sub-score, H~3~ is the Dropped Throttle Path Deviation at
Nominal Load sub-score,H~4~ is the Yaw Rate Response Time at Nominal
Load sub-score, H~5~ is the Peak Yaw Rate Response Time at Nominal Load
sub-score, and H~6~ is the Percent Overshoot at Nominal Load sub-score,
H~7~ is the Maximum Attainable Lateral Acceleration at Rear Load
sub-score, H~8~ is the Dropped Throttle Taw Rate Ratio at Rear Load
sub-score, H~9~ is the Dropped Throttle Path Deviation at Rear Load
sub-score, H~10~ is the Yaw Rate Response Time at Rear Load sub-score,
H~11~ is the Peak Yaw Rate Response Time at Rear Load sub-score, and
H~12~ is the Percent Overshoot at Rear Load sub-score.

2\. Calculate the Maximum Attainable Lateral Acceleration at Nominal
Load sub-score, H~1~, from the formulas:

If a~YMaxN~ \< a~YMinN~ then H~1~ = 0

If a~YMaxN~ \> (a~YMinN~ + a~YRangeN~) then H~1~ = 1

Otherwise

a~BarN~ = (a~YMaxN~ -- a~YMinN~) / a~YRangeN~

H~1~ = a~BarN~ \* (2 -- a~BarN~)

where a~YMaxN~ is the measured Maximum Attainable Lateral Acceleration
at Nominal Load, and a~YMinN~ and a~YRangeN~ are constants that NHTSA
will select values for at a later date.

3\. Calculate the Dropped Throttle Yaw Rate Ratio at Nominal Load
sub-score, H~2~, from the formula:

> If R~MaxN~ \> R~RangeN~ then H~2~ = 0
>
> Otherwise
>
> H~2~ = 1 -- ((R~MaxN~ -- 1) / R~RangeN~)^2^

where R~MaxN~ is the measured Dropped Throttle Yaw Rate Ratio at Nominal
Load, and R~RangeN~ is a constant that NHTSA will select a value for at
a later date. Note that R~MaxN~ can never be less than one.

4\. Calculate the Dropped Throttle Path Deviation at Nominal Load
sub-score, H~3~, from the formula:

If Y~DevN~ \< Y~MinN~ then H~3~ = 0

If Y~DevN~ ≥ Y~MinN~ and Y~DevN~ \< 0 then

H~3~ = 1 - (Y~DevN~ / Y~MinN~)^2^

If Y~DevN~ ≥ 0 and Y~DevN~ \< Y~OkN~ then H~3~ = 1

If Y~DevN~ ≥ Y~OkN~ and Y~DevN~ \< Y~MaxN~ then

Y~BarN~ = (Y~DevN~ -- Y~OkN~) / (Y~MaxN~ -- Y~OkN~)

H~3~ = Y~BarN~ \* (2 -- Y~BarN~)

If Y~DevN~ \> Y~MaxN~ then H~3~ = 0

where Y~DevN~ is the measured Dropped Throttle Path Deviation at Nominal
Load, and Y~MaxN~, Y~MinN~, and Y~OkN~ are constants that NHTSA will
select values for at a later date.

5\. Calculate the Yaw Rate Response Time at Nominal Load sub-score,
H~4~, from the formula:

If t~rN~ \< t~rMinN~ then H~4~ = 1

If t~rN~ \> (t~rMinN~ + t~rRangeN~) then H~4~ = 0

Otherwise

H~4~ = ((t~rMinN~ + t~rRangeN~) -- t~rN~) / t~rRangeN~

> where t~rN~ is the measured Yaw Rate Response Time at Nominal Load,
> and t~rMinN~ and t~rRangeN~ are constants that NHTSA will select
> values for at a later date.

6\. Calculate the Peak Yaw Rate Response Time at Nominal Load sub-score,
H~5~, from the formula:

If t~pN~ \< t~pMinN~ then H~5~ = 1

If t~pN~ \> (t~pMinN~ + t~pRangeN~) then H~5~ = 0

Otherwise

H~5~ = ((t~pMinN~ + t~pRangeN~) -- t~pN~) / t~pRangeN~

where t~pN~ is the measured Yaw Rate Response Time at Nominal Load, and
t~pMinN~ and t~pRangeN~ are constants that NHTSA will select values for
at a later date.

7\. Calculate the Percent Overshoot at Nominal Load sub-score, H~6~,
from the formula:

> If O~r%N~ \< 0 then H~6~ = 1
>
> Otherwise
>
> H~6~ = 1 -- (O~r%N~ / O~rRangeN~)^2^
>
> where O~r%N~ is the measured Percent Overshoot at Nominal Load, and
> O~rRangeN~ is a constant that NHTSA will select a value for at a later
> date. Note that O~r%N~ can never be less than zero.

8\. Calculate the Maximum Attainable Lateral Acceleration at Rear Load
sub-score, H~7~, from the formulas:

If a~YMaxR~ \< a~YMinR~ then H~7~ = 0

If a~YMaxR~ \> (a~YMinR~ + a~YRangeR~) then H~7~ = 1

Otherwise

a~BarR~ = (a~YMaxR~ -- a~YMinR~) / a~YRangeR~

H~7~ = a~BarR~ \* (2 -- a~BarR~)

where a~YMaxR~ is the measured Maximum Attainable Lateral Acceleration
at Rear Load, and a~YMinR~ and a~YRangeR~ are constants that NHTSA will
select values for at a later date.

9\. Calculate the Dropped Throttle Yaw Rate Ratio at Rear Load
sub-score, H~8~, from the formula:

> If R~MaxR~ \> R~RangeN~ then H~8~ = 0
>
> Otherwise
>
> H~8~ = 1 -- ((R~MaxR~ -- 1)/R~RangeR~)^2^

where R~MaxR~ is the measured Dropped Throttle Yaw Rate Ratio at Rear
Load, and R~RangeR~ is a constant that NHTSA will select a value for at
a later date. Note that R~MaxR~ can never be less than one.

10\. Calculate the Dropped Throttle Path Deviation at Rear Load
sub-score, H~9~, from the formula:

If Y~DevR~ \< Y~MinR~ then H~9~ = 0

If Y~DevR~ ≥ Y~MinR~ and Y~DevR~ \< 0 then

H~9~ = 1 - (Y~DevR~ / Y~MinR~)^2^

If Y~DevR~ ≥ 0 and Y~DevR~ \< Y~OkR~ then H~9~ = 1

If Y~DevR~ ≥ Y~OkR~ and Y~DevR~ \< Y~MaxR~ then

Y~BarR~ = (Y~DevR~ -- Y~OkR~) / (Y~MaxR~ -- Y~OkR~)

H~9~ = Y~BarR~ \* (2 -- Y~BarR~)

If Y~DevR~ \> Y~MaxR~ then H~9~ = 0

where Y~DevR~ is the measured Dropped Throttle Path Deviation at Nominal
Load, and Y~MaxR~, Y~MinR~, and Y~OkR~ are constants that NHTSA will
select values for at a later date.

11\. Calculate the Yaw Rate Response Time at Rear Load sub-score, H~10~,
from the formula:

If t~rR~ \< t~rMinR~ then H~10~ = 1

If t~rR~ \> (t~rMinR~ + t~rRangeR~) then H~10~ = 0

Otherwise

H~10~ = ((t~rMinR~ + t~rRangeR~) -- t~rR~) / t~rRangeR~

> where t~rR~ is the measured Yaw Rate Response Time at Rear Load, and
> t~rMinR~ and t~rRangeR~ are constants that NHTSA will select values
> for at a later date.

12\. Calculate the Peak Yaw Rate Response Time at Rear Load sub-score,
H~11~, from the formula:

If t~pR~ \< t~pMinR~ then H~11~ = 1

If t~pR~ \> (t~pMinR~ + t~pRangeR~) then H~11~ = 0

Otherwise

H~11~ = ((t~pMinR~ + t~pRangeR~) -- t~pR~) / t~pRangeR~

where t~pR~ is the measured Yaw Rate Response Time at Rear Load, and
t~pMinR~ and t~pRangeR~ are constants that NHTSA will select values for
at a later date.

13\. Calculate the Percent Overshoot at Rear Load sub-score, H~12~, from
the formula:

> If O~r%R~ \< 0 then H~12~ = 1
>
> Otherwise
>
> H~12~ = 1 -- (O~r%R~ / O~rRangeR~)^2^
>
> where O~r%R~ is the measured Percent Overshoot at Rear Load, and
> O~rRangeR~ is a constant that NHTSA will select a value for at a later
> date. Note that O~r%R~ can never be less than zero.

14\. Calculate the provisional Handling Rating from the Handling Score,
HS, as follows:

> If HS \> HS~A~ then the provisional Handling Rating is an A
>
> If HS \< HS~C~ then the provisional Handling Rating is a C
>
> Otherwise the provisional Handling Rating is a B
>
> where HS~A~ and HS~C~ are constants that NHTSA will select values for
> at a later date.

15\. If the vehicle spins when determining the Maximum Attainable
Lateral Acceleration at Nominal Load, then reduce the provisional
Handling Rating by one letter (but never below a C).

16\. If the vehicle spins when determining the Maximum Attainable
Lateral Acceleration at Rear Load, then reduce the provisional Handling
Rating by one letter (but never below a C).

17\. The provisional Handling Rating now becomes the final Handling
Rating.

We welcome comments as to the appropriateness of the above technique for
determining handling ratings. How can it be improved? One possibility
would be to have two handling ratings, one for Nominal Load and one for
Rear Load. Would this be better? Or should we consider the ratings for
the different loadings to be an additional level of detail available to
interested persons who want more than just the one rating?

**X. Assessment of Costs and Benefits**

The costs are Federal Government costs for developing the test protocol
and rating system, conducting the tests, and disseminating the
information. The benefits are information to consumers. Consumers want
additional information. It is impossible for us to quantify the effect
on consumer behavior or on manufacturer behavior.

**XI. Rulemaking Analyses and Notices**

####### A. Executive Order 12866

Executive Order 12866, "Regulatory Planning and Review" (58 FR 51735,
October 4, 1993), provides for making determinations whether a
regulatory action is "significant" and therefore subject to Office of
Management and Budget (OMB) review and to the requirements of the
Executive Order. The Order defines a "significant regulatory action" as
one that is likely to result in a rule that may:

\(1\) Have an annual effect on the economy of \$100 million or more or
adversely affect in a material way the economy, a sector of the economy,
productivity, competition, jobs, the environment, public health or
safety, or State, local, or Tribal governments or communities;

\(2\) Create a serious inconsistency or otherwise interfere with an
action taken or planned by another agency;

\(3\) Materially alter the budgetary impact of entitlements, grants,
user fees, or loan programs or the rights and obligations of recipients
thereof; or

\(4\) Raise novel legal or policy issues arising out of legal mandates,
the President=s priorities, or the principles set forth in the Executive
Order.

NHTSA has considered the impact of this action under Executive Order
12866 and the Department of Transportation's regulatory policies and
procedures. This action has been determined to be economically not
significant. However, because it is a subject of Congressional interest,
this rulemaking document was reviewed by the Office of Management and
Budget under Executive Order 12866, "Regulatory Planning and Review."

> **B. Regulatory Flexibility Act**

The Regulatory Flexibility Act of 1980 (5 U.S.C. § 601 [et]{.underline}
[seq.]{.underline}) requires agencies to evaluate the potential effects
of their proposed and final rules on small business, small organizations
and small governmental jurisdictions. I hereby certify that the proposed
amendment would not have a significant economic impact on a substantial
number of small entities. The proposed action does not impose regulatory
requirements on any manufacturer or other party.

> **C. National Environmental Policy Act**

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

> **D. Executive Order 13132 (Federalism)**

The agency has analyzed this rulemaking in accordance with the
principles and criteria contained in Executive Order 13132 and has
determined that it does not have sufficient federal implications to
warrant consultation with State and local officials or the preparation
of a federalism summary impact statement. The proposal would not have
any substantial impact on the States, or on the current Federal-State
relationship, or on the current distribution of power and
responsibilities among the various local officials.

> **E. Unfunded Mandates Act**

The Unfunded Mandates Reform Act of 1995 requires agencies to prepare a
written assessment of the costs, benefits and other effects of proposed
or final rules that include a Federal mandate likely to result in the
expenditure by State, local or tribal governments, in the aggregate, or
by the private sector, of more than \$100 million annually (adjusted
annually for inflation with base year of 1995). Adjusting this amount by
the implicit gross domestic product price deflator for the year 2000
results in \$109 million (106.99/98.11 = 1.09). The assessment may be
included in conjunction with other assessments, as it is here.

The proposed action does not impose regulatory requirements on any
manufacturer or other party.

> **F. Civil Justice Reform**

This proposal would not have any retroactive effect. Under 49 U.S.C.
21403, whenever a Federal motor vehicle safety standard is in effect, a
State may not adopt or maintain a safety standard applicable to the same
aspect of performance which is not identical to the Federal standard,
except to the extent that the state requirement imposes a higher level
of performance and applies only to vehicles procured for the State's
use. 49 U.S.C. 21461 sets forth a procedure for judicial review of final
rules establishing, amending or revoking Federal motor vehicle safety
standards. That section does not require submission of a petition for
reconsideration or other administrative proceedings before parties may
file suit in court.

G.  **Paperwork Reduction Act**

This proposal does not contain "collections of information," as that
term is defined in 5 CFR Part 1320 *Controlling Paperwork Burdens on the
Public.*

> **H. Plain Language**

Executive Order 12866 and the President\'s memorandum of June 1, 1998,
require each agency to write all rules in plain language. This action
will not result in regulatory language.

**XII. Submission of Comments**

> *How Can I Influence NHTSA\'s Thinking on This Proposed Rule?*

In developing this proposal, we tried to address the concerns of all our
stakeholders. Your comments will help us improve this rule. We invite
you to provide views on options we propose, to suggest new approaches we
have not considered, provide new data, indicate how this proposed rule
may affect you, or provide other relevant information. We welcome your
views on all aspects of this proposed rule, but request comments on
specific issues throughout this document. We grouped these specific
requests near the end of the sections in which we discuss the relevant
issues. Your comments will be most effective if you follow the
suggestions below:

. Explain your views and reasoning as clearly as possible.

. Provide solid technical and cost data to support your views.

. If you estimate potential costs, explain how you arrived at the
estimate.

. Tell us which parts of the proposal you support, as well as those with
which you disagree.

. Provide specific examples to illustrate your concerns.

. Offer specific alternatives.

. Refer your comments to specific sections of the proposal, such as the
units or page numbers of the preamble, or the regulatory sections.

. Be sure to include the name, date, and docket number with your
comments.

*How Do I Prepare and Submit Comments?*

Your comments must be written and in English. To ensure that your
comments are correctly filed in the Docket, please include the docket
number of this document in your comments.

Your comments must not be more than 15 pages long. (49 CFR 553.21). We
established this limit to encourage you to write your primary comments
in a concise fashion. However, you may attach necessary additional
documents to your comments. There is no limit on the length of

the attachments.

Please submit two copies of your comments, including the attachments, to
Docket Management at the address given above under ADDRESSES.

Comments may also be submitted to the docket electronically by logging
onto the Dockets Management System website at
[http://dms.dot.gov.]{.underline} Click on \"Help & Information\" or
\"Help/Info\" to obtain instructions for filing the document
electronically.

*How Can I Be Sure That My Comments Were Received?*

If you wish Docket Management to notify you upon its receipt of your
comments, enclose a self‑addressed, stamped postcard in the envelope
containing your comments. Upon receiving your comments, Docket
Management will return the postcard by mail.

*How Do I Submit Confidential Business Information?*

If you wish to submit any information under a claim of confidentiality,
you should submit three copies of your complete submission, including
the information you claim to be confidential business information, to
the Chief Counsel, NHTSA, 400 Seventh Street, SW., Washington, DC 20590.
In addition, you should submit two copies, from which you have deleted
the claimed confidential business information, to Docket Management at
the address given above under ADDRESSES. When you send a comment
containing information claimed to be confidential business information,
you should include a cover letter setting forth the information
specified in our confidential business information regulation. (49 CFR
Part 512.)

*Will the Agency Consider Late Comments?*

We will consider all comments that Docket Management receives before the
close of business on the comment closing date indicated above under
DATES. To the extent possible, we will also consider comments that
Docket Management receives after that date. If Docket Management
receives a comment too late for us to consider it in developing a final
rule (assuming that one is issued), we will consider that comment as an
informal suggestion for future rulemaking action.

*How Can I Read the Comments Submitted by Other People?*

You may read the comments received by Docket Management at the address
given above under ADDRESSES. The hours of the Docket are indicated above
in the same location.

You may also see the comments on the Internet. To read the comments on
the Internet, take the following steps:

\(1\) Go to the Docket Management System (DMS) Web page of the
Department of Transportation (http://dms.dot.gov/).

\(2\) On that page, click on \"search.\"

\(3\) On the next page (http://dms.dot.gov/search/), type in the
four‑digit docket number shown at the beginning of this document.
Example: If the docket number were \"NHTSA‑1998‑1234,\" you would type
\"1234.\" After typing the docket number, click on \"search.\"

\(4\) On the next page, which contains docket summary information for
the docket you selected, click on the desired comments. You may download
the comments. However, since the comments are imaged documents, instead
of word processing documents, the downloaded comments are not word
searchable.

Please note that even after the comment closing date, we will continue
to file relevant information in the Docket as it becomes available.
Further, some people may submit late comments. Accordingly, we recommend
that you periodically check the Docket for new material.

Issued on:

Stephen R. Kratzke

Associate Administrator for

Safety Performance Standards

BILLING CODE: 4910‑59P

**Appendix I. Summary of Maneuver Evaluation Test Results**

Prior to the initiation of this research, NHTSA met with the Alliance of
Automobile Manufacturers, Daimler-Chrysler, BMW, Volkswagen, Mitsubishi,
Ford, Nissan, Toyota, Consumers Union of the United States, MTS Systems
Corporation, Heitz Automotive Inc., and other interested parties to
gather information on possible approaches for dynamic rollover tests.
NHTSA also corresponded with the University of Michigan Transportation
Research Institute. These parties made specific suggestions about
approaches to dynamic testing of vehicle rollover resistance. Based on
these suggestions plus NHTSA's experience in this area, a set of nine
rollover resistance maneuvers were selected for evaluation. These nine
maneuvers were listed in the July 2001 notice.

The research to evaluate potential maneuver tests for rollover is fully
documented in the NHTSA technical report *"Another Experimental
Examination of Selected Maneuvers That May Induce On-Road Untripped,
Light Vehicle Rollover - Phase IV of NHTSA's Light Vehicle Rollover
Research Program".* A number of test results and principal observations
about the maneuvers are discussed here under the following four general
headings:

1.  Objectivity and Repeatability, i.e., whether a maneuver could be
    performed objectively with repeatable results for the same vehicle.

2.  Discriminatory Capability, i.e., whether a maneuver demonstrated
    poorer performance for vehicles that have less resistance to
    rollover. Although of obvious importance, a maneuver's ability to
    discriminate between different levels of vehicle handling was not
    considered.

3.  Performability i.e., how difficult each maneuver is to objectively
    perform while obtaining repeatable results, how well developed are
    the test procedures for each maneuver, and whether the test
    procedure includes adequate means for adapting to differing vehicle
    characteristics.

4.  Realistic Appearance, i.e., whether a test maneuver looks like a
    maneuver consumers might imagine performing in an emergency.

The headings are useful for organizing the information, but they are not
mutually exclusive. For example, the discussion of whether the
performance of a vehicle in a particular maneuver is influenced more by
handling properties than by rollover resistance would be under the
heading of Discriminatory Capability. But the repeatability of the
performance measurement discussed under Objectivity and Repeatability
also influences the discriminatory capability of the maneuver.
Similarly, Performability is a catch-all category that includes
discussions of topics outside of the more specific headings.

Realistic Appearance helps consumers visualize the test maneuvers, but
it is less important than the other three categories of test attributes
because we are interested in anything that the vehicle is capable of
doing. What we desire are "worst case" maneuvers, not necessarily ones
that drivers try to perform. For example, drivers would not try to drive
in a fishhook pattern, but the steering movements are similar to what
occurs in an unsuccessful road edge recovery attempt. The maneuver only
looks like a fishhook path if the vehicle does not tip-up. If the
vehicle tips-up, it occurs shortly after the counter-steer when a driver
in a road edge recovery attempt would still be on the pavement.

The specific reasons for the choice of maneuvers we are proposing for
rollover resistance ratings are discussed in Section VI. The reasons are
a consequence of the observations made in this section plus other
practical considerations such as the desirability of multiple maneuvers
to create a range of test severity were taken into account.

Four sport utility vehicles were tested during the summer of 2001 to
obtain the data needed to perform this maneuver evaluation (the Phase IV
Rollover Research). Two of the vehicles tested during the Phase IV
research (the 1999 Mercedes ML320 and the 2001 Toyota 4Runner) came with
yaw stability control systems as original equipment. Both of these
vehicles were treated, for the purposes of maneuver evaluation, as two
vehicles, one with yaw stability control and one without.

Therefore, the six test vehicles were:

1\. 2001 Chevrolet Blazer without yaw stability control

> 2\. 2001 Ford Escape without yaw stability control. Note: The
> Automotive News Truck Market classifications classify this vehicle as
> a Sport Wagon instead of a Sport Utility Vehicle.

3\. 1999 Mercedes ML320 with yaw stability control disabled

4\. 1999 Mercedes ML320 with yaw stability control enabled

5\. 2001 Toyota 4Runner with yaw stability control disabled

6\. 2001 Toyota 4Runner with yaw stability control enabled

Each of the above test vehicles was tested in three configurations. Only
two of these configurations will be discussed in this notice; test data
from the Modified Handling configuration were not used for the maneuver
evaluations discussed in this notice. The test configurations of
interest were:

Nominal Vehicle. The vehicle load consisted of one occupant (the
driver), instrumentation, and outriggers in/on the vehicle.

Reduced Rollover Resistance Vehicle. In addition to the Nominal Vehicle
load, sufficient weight was placed on the roof to reduce the vehicle's
SSF by 0.05. The weight on the roof was positioned so that the
longitudinal/lateral position of the center of gravity did not change.

The Reduced Rollover Resistance Vehicle was used as a check on the
sensitivity of the test maneuvers. A 0.05 reduction in SSF equates, for
sport utility vehicles, to approximately a one star reduction in the
vehicle's rollover resistance rating. (A larger reduction in SSF is
necessary to achieve a one star rating reduction for vehicles, such as
passenger cars, that have higher SSFs.) NHTSA believes that a one star
reduction in the rollover resistance rating should make a vehicle
substantially easier to rollover. Maneuvers with good discriminatory
capability should measure substantially worse performance for this
vehicle configuration than for the Nominal Vehicle configuration.

Data collected during the Phase IV Rollover Research was used to
evaluate eight of the rollover resistance maneuvers (all except the
J-Turn with Pulse Braking). For each of these eight maneuvers, vehicles
were tested in the Nominal Vehicle configuration. For maneuvers which we
deemed appropriate, testing was also performed using the Reduced
Rollover Resistance configuration. For the J-Turn with Pulse Braking, we
decided that we had sufficient data from prior testing (Phases II and
III of the Rollover Research program) to evaluate this maneuver.

![](media/image12.png){width="4.584645669291339in"
height="3.7661417322834647in"} The results of the evaluation for each
rollover resistance maneuver follows. For each maneuver, a brief
description of the maneuver is given followed by its scores in each of
the four evaluation factors. Each evaluation factor score is followed by
a discussion as to how that particular score was decided upon.

**A. NHTSA J-Turn**

**Maneuver Description**

To perform this maneuver, the programmable steering controller input the
handwheel commands described by Figure 1.

The NHTSA J-Turn handwheel angle is eight times the handwheel angle that
produces a quasi-static 0.3 g lateral acceleration at 50 mph for each
particular test vehicle. The handwheel rate of the handwheel ramp was
1000 degrees per second.

J-Turn tests were performed with two directions of steer, to the left
and to the right. Vehicle speed was increased in 5 mph increments from
35 to 60 mph, unless at least two inches of simultaneous two-wheel lift
was observed. If such wheel lift was detected, entrance speeds were
iteratively reduced by 1 mph until it was no longer apparent.

**Objectivity and Repeatability**

The NHTSA J-Turn is the most objective and repeatable of all of the
rollover resistance maneuvers. Figure 2 shows the Handwheel Angle,
Vehicle Speed, Lateral Acceleration, and Roll Angle as functions of time
for three tests of the Toyota 4Runner with yaw stability control enabled
that were run at approximately the same speed (59.4, 58.1, and 58.6
mph). The Handwheel Angle graph shows that, by using the programmable
steering controller, the steering control input can be precisely
replicated from run-to-run (there are three traces in this graph). Test
drivers can repeatably achieve input speeds within ±2 mph of the target
speed. The vehicle speed, lateral acceleration and roll angle traces
clearly show the very high repeatability of this maneuver.

Data from these runs is typical of our experience with the maneuver,
with one exception. For runs that are either result in two-wheel lift or
are very near to the point at which it first occurs, the roll angle
repeatability becomes much worse. This is the case for all rollover
resistance maneuvers that induce tip up because the vehicle either falls
over or it does not. As a result, small fluctuations in test performance
can lead to large changes in roll angle in this situation. This results
in a variability of approximately ±2 mph in determining the lowest speed
at which two-wheel lift occurs. As such, roll angle variability at the
tip-up threshold did not lower the Objectivity and Repeatability rating
for this maneuver.

![](media/image13.wmf){width="4.972222222222222in"
height="5.388194444444444in"}

**Performability**

The NHTSA J-Turn is the easiest of all of the rollover resistance
maneuvers to perform. Objective and repeatable NHTSA J-Turn maneuvers
can easily be performed using a programmable steering controller. Having
only one major steering movement maximizes maneuver repeatability. The
test procedure is well developed. Procedures have been developed to
adapt the NHTSA J-Turn maneuver to the characteristics of the vehicle
being tested.

**Discriminatory Capability**

None of the vehicles tested had two-wheel lift during NHTSA J-Turn tests
in their Nominal Vehicle configuration. However, all of the vehicles
except the Ford Escape and the Toyota 4Runner with its yaw stability
control enabled did have two-wheel lift when tested in their Reduced
Rollover Resistance configuration. The NHTSA J-Turn is not a severe
enough maneuver to discriminate between typical, current generation,
sport utility vehicles loaded with a driver and passenger only. However,
it was very sensitive to the decrease in rollover resistance
attributable to a decrease in SSF of 0.05. Also the speed at tip-up
could discriminate between our individual test vehicles when the entire
group was loaded to produce a decrease in SSF of 0.05. We used a roof
load of about 200 lb to reduce the SSF by 0.05, but the addition of 5 to
6 passengers causes a similar reduction in SSF for typical current
generation SUVs, vans and pickup trucks.

**Realistic Appearance**

Drivers perform NHTSA J-Turns during actual driving on cloverleaf
entrance/exit ramps and other, essentially constant radius, curves that
are driven at substantial speeds. This maneuver is not given an
excellent rating in this category, however, because for light vehicles,
actual drivers are very unlikely to use the large steering magnitudes
needed to induce two-wheel lift without also applying sustained braking.

During NHTSA's discussions with the automotive industry, every
manufacturer stated that they routinely perform J-Turn testing during
vehicle development. This maneuver has a long history of industry use.

B.  **J-Turn with Pulse Braking**

**Maneuver Description**

To perform this maneuver, the programmable steering and braking
controller input the handwheel steering and braking commands as shown in
Figure 3. Figure 3 also shows a typical vehicle roll rate response
resulting from the steering input so as to explain the timing of the
brake pulse. Pulse braking was initiated at the first zero crossing
(determined by the roll rate being between +1.5 degrees per second and
-- 1.5 degrees per second) of the roll rate after the initiation of
steering (i.e., at the time when the maximum roll angle occurs).

The handwheel magnitudes used for the J-Turn with Pulse Braking maneuver
were always 330 degrees. The handwheel rate of the handwheel ramp was
1000 degrees per second.

The maximum brake pedal force used for the J-Turn with Pulse Braking
maneuver was 200 pounds. The brake pulse durations ranged from 0.25 to
0.55 seconds.

J-Turn with Pulse Braking tests were performed with two directions of
steer, to the left and to the right. Vehicle speed was increased in 2
mph increments from 36 to 60 mph, unless simultaneous two-wheel lift was
observed.

![](media/image14.jpeg){width="6.498611111111111in"
height="7.031944444444444in"}

**Figure 3: J-Turn with Pulse Braking Handwheel Steering**

**Angle and Brake Pedal Force**

**Objectivity and Repeatability**

The J-Turn with Pulse Braking is not as objective and repeatable as the
J-Turn due to the pulse braking. Research has shown that the results of
this test depend upon the precise timing and magnitude of the brake
pulse. Therefore, to perform this maneuver with reasonable objectivity
and repeatability, both tightly controlled steering and braking are
required. The programmable steering controller needed for the J-Turn has
now become a programmable steering and braking controller with a
corresponding increase in testing complexity, difficulty, and cost.

Figure 4 shows the Handwheel Angle, Brake Pedal Force, Lateral
Acceleration, Longitudinal Acceleration, Roll Angle, and Vehicle Speed,
as functions of time for two tests of a 1998 Chevrolet Tracker (this
vehicle did not have either antilock brakes or yaw stability control)
that were run at approximately the same speed (31.1 and 31.3 mph).
Unlike the rest of the data presented in this section, the J-Turn with
Pulse Braking data was collected during the summer of 2000 as part of
the Phase III-B Rollover research.

Like the NHTSA J-Turn, due to the use of the programmable steering
controller, the steering control input was precisely replicated from
run-to-run. The apparent non-repeatability in the steering input (and
lateral acceleration and roll angle) is actually after the test is over
and the driver has retaken control of the vehicle.

Similarly, the Brake Pedal Force graph shows that, by using the
programmable braking controller, the braking control input can be
precisely replicated from run-to-run. The precisely overlaid lateral
acceleration, longitudinal acceleration, roll angle, and vehicle speed
traces clearly show the very high repeatability achieved for these two
runs.

![](media/image15.wmf){width="5.138976377952756in"
height="6.725590551181102in"} We caution, however, that data from these
two runs is not typical of our experience with maneuver. In general, we
saw somewhat more variability in the brake pedal force than is shown in
Figure 4.

Also, as was discussed above for the NHTSA J-Turn, for runs that are
near the point at which two-wheel lift first occurs, roll angle
repeatability becomes much worse.

**Performability**

The addition of pulse braking substantially reduces the performability
of this maneuver relative to the NHTSA J-Turn. The addition of a
programmable braking controller, which is necessary to achieve the
precise pulse brake timing required for repeatable performance, makes
this test significantly harder and more costly to run. Issues remain as
to the brake pulse timing needed to achieve worst case rollover
performance.

Through the use of roll rate feedback, the timing of the brake pulse can
be adapted to the characteristics of the vehicle being tested. The
magnitude of the steering input can also be adapted from
vehicle-to-vehicle (although this was not done during the Phase III
research).

**Discriminatory Capability**

The J-Turn with Pulse Braking is a very bad maneuver for measuring the
rollover resistance of different vehicles. For vehicles equipped with
antilock braking systems (ABS), it does not appear to give any
additional information beyond that obtained from the NHTSA J-Turn
maneuver (unless the ABS is disabled; not a realistic situation). For
vehicles without ABS, it can be a very severe test vehicle provided the
timing of the brake pulse is just right. If this test were used for
NCAP, it would discriminate more on the basis of ABS equipment than
rollover resistance.

**Realistic Appearance**

Drivers could perform J-Turns with Pulse Braking during actual driving
on cloverleaf entrance/exit ramps and other, essentially constant
radius, curves that are driven at substantial speeds. However, we think
that the occurrence of this maneuver is unlikely. With the large
steering magnitudes needed to induce two-wheel lift, we believe it to be
far more probable that drivers will apply sustained braking (which
discourages rather than encourages two-wheel lift) instead of pulse
braking.

**C. Fixed Timing Fishhook**

**Maneuver Description.**

![](media/image16.jpeg){width="4.473622047244095in"
height="4.442913385826771in"}To perform this maneuver, the programmable
steering controller input the handwheel commands described by Figure 5.

Fixed Timing Fishhook handwheel angle is 6.5 times the handwheel angle
that produces a quasi-static 0.3 g lateral acceleration at 50 mph for
each particular test vehicle. The commanded dwell (amount of time after
the first steer for which handwheel position was maintained) for the
Fixed Timing Fishhook was 0.25 seconds. The handwheel rates of the
initial steer and countersteer ramps were 720 degrees per second.

Fixed Timing Fishhook tests were performed with both initial directions
of steer, to the left and to the right. Vehicle speed was increased in 5
mph increments from 35 to 50 mph, unless at least two inches of
simultaneous two-wheel lift was observed. If such wheel lift was
detected, entrance speeds were iteratively reduced by 1 mph until it was
no longer apparent.

**Objectivity and Repeatability**

The Fixed Timing Fishhook can be performed with excellent objectivity
and repeatability. Figure 6 shows the Handwheel Angle, Vehicle Speed,
Lateral Acceleration, and Roll Angle as functions of time for three
tests of the Chevrolet Blazer that were run at approximately the same
speed (37.8, 37.8, and 37.3 mph). Data from these runs is typical of our
experience with this maneuver.

The vehicle speed and lateral acceleration traces clearly show the very
high repeatability of this maneuver. The roll angle traces show the
non-repeatability in roll angle that occurs around the point of two
wheel lift. All three of these runs had two wheel lift approximately
three seconds into the test. The amount of two-wheel lift was
substantially less for one run than for the other two. Near the
initiation of two-wheel lift, the roll angle becomes mathematically
unstable because the vehicle either falls over or it does not. As was
discussed above for the NHTSA J-Turn, this roll angle non- repeatability
occurs for all maneuvers that generate two-wheel lift.

**Performability**

Objective and repeatable Fixed Timing Fishhook maneuvers can easily be
performed using a programmable steering controller. The test procedure
is well developed. Procedures have been developed to adapt the steering
magnitude used for the Fixed Timing Fishhook maneuver for the
characteristics of the vehicle being tested.

![](media/image17.wmf){width="5.090157480314961in"
height="4.068897637795276in"}

**Discriminatory Capability**

The Fixed Timing Fishhook is excellent maneuver for measuring the
rollover resistance of different vehicles. The Chevrolet Blazer and the
Mercedes ML320 (with the stability control both enabled and disabled)
had two-wheel lift when tested in their Nominal Vehicle configuration.
All vehicles (with the stability control, if present, both enabled and
disabled) had two-wheel lift when tested in their Reduced Rollover
Resistance configuration. (The Mercedes ML320 was not tested in its
Reduced Rollover Resistance configuration. However, we are certain that
it would have had two-wheel lift in this configuration because it had
two-wheel lift in its Nominal Vehicle configuration and raising its
center of gravity height is going to encourage, not prevent, two-wheel
lifts.) The maneuver initial speed (a severity measure for the Fixed
Timing Fishhook) at which two-wheel lifts first occurred varied about as
expected.

While the Fixed Timing Fishhook does an excellent job of discriminating
between vehicles for typical, current generation, sport utility
vehicles, it will not do as good a job for the entire vehicle fleet. It
is doubtful that any two-wheel lifts will occur during testing of
vehicles that have a Static Stability Factors of 1.2 or greater (e.g.,
most vehicles that earn three or more stars under NHTSA's current
rollover rating program). That said, no driving maneuver known to NHTSA
is expected to cause two-wheel lifts for vehicles in the 1.20 SSF range.
However, as the name of this maneuver implies, the timing of this
maneuver does not change from vehicle-to-vehicle. This will result in
some vehicles not being tested with the timing needed to achieve worst
case rollover performance.

**Realistic Appearance**

The Fishhook maneuver's steering input, no matter whether it's the Fixed
Timing, Roll Rate Feedback, or Nissan variant, approximates the steering
that a driver might perform in an effort to resume traveling in the
correct lane of a two lane road after dropping two-wheels off of the
road. None of the Fishhooks simulate the effects of the road-edge
drop-off.

**D. Roll Rate Feedback Fishhook**

**Maneuver Description**

This maneuver is performed similarly to the Fixed Timing Fishhook except
for the timing of the steering reversal. Figure 7 shows the handwheel
steering input, as a function of time, used for this maneuver. Note that
the magnitude of the steering is identical to that of the Fixed Timing
Fishhook. However, the steering dwell time (amount of time after the
first steer for which handwheel position was maintained) is no longer
kept at 0.25 seconds. Instead, this dwell time is varied so as to
maximize the severity of the maneuver.

Figure 7 also shows a typical vehicle roll rate response resulting from
the steering input so as to explain the timing of the steering reversal.
The steering reversal was initiated at the first zero crossing
(determined by the roll rate being between +1.5 degrees per second and
-- 1.5 degrees per second) of the roll rate after the initiation of
steering (i.e., at the time when the maximum roll angle occurs).

**Objectivity and Repeatability**

The Roll Rate Feedback Fishhook can be performed with excellent
objectivity and repeatability. Occasionally, when performing this
maneuver, the measured roll rate does not return to zero for a
substantial period of time (1 to 2 seconds) resulting in a greatly
delayed

![](media/image18.jpeg){width="4.301574803149606in"
height="6.375196850393701in"}

countersteer and an invalid test. However, this happens quite rarely,
and it is obvious to the test driver when this delay causes the need to
repeat the test run. Therefore, from a practical point of view, the
objectivity and repeatability of this maneuver was not different from
that of the Fixed Timing Fishhook.

Figure 8 shows the Handwheel Angle, Vehicle Speed, Lateral Acceleration,
and Roll Angle as functions of time for three tests of the Toyota
4Runner with stability control disabled that were run at approximately
the same speed (39.9, 40.3, and 39.5 mph). Data from these runs is
typical of our experience with this maneuver.

The vehicle speed and lateral acceleration traces show the high
repeatability of this maneuver. The roll angle traces show the
non-repeatability in roll angle that occurs around the point of two
wheel lift. As the traces show two of these runs had two wheel lift
approximately three seconds into the test while one did not. Near the
initiation of two-wheel lift, the roll angle becomes mathematically
unstable because the vehicle either falls over or it does not. As was
discussed above for the NHTSA J-Turn, this roll angle non- repeatability
occurs for all maneuvers that generate two-wheel lift.

**Performability**

Objective and repeatable Roll Rate Feedback Fishhook maneuvers can
easily be performed using a programmable steering controller equipped to
handle roll rate feedback. The test procedure is well developed.
Procedures have been developed to adapt both the steering magnitude and
the steering reversal timing used for the Roll Rate Feedback Fishhook
maneuver for the characteristics of the vehicle being tested.

#### ![](media/image19.wmf){width="5.0in" height="4.355511811023622in"} 

#### **Discriminatory Capability** 

The Roll Rate Feedback Fishhook is excellent maneuver for measuring the
rollover resistance of different vehicles. The Chevrolet Blazer and the
Mercedes ML320 (with the stability control both enabled and disabled)
had two-wheel lift when tested in their Nominal Vehicle configuration.
All vehicles (with the stability control, if present, both enabled and
disabled) had two-wheel lift when tested in their Reduced Rollover
Resistance configuration. (The Mercedes ML320 was not tested in its
Reduced Rollover Resistance configuration. However, we are certain that
it would have had two-wheel lift in this configuration because it had
two-wheel lift in its Nominal Vehicle configuration and raising its
center of gravity height is going to encourage, not prevent, two-wheel
lifts.) The maneuver initial speed (a severity measure for the Roll Rate
Feedback Fishhook) at which two-wheel lifts first occurred varied about
as expected.

While the Roll Rate Feedback Fishhook does an excellent job of
discriminating between vehicles for typical, current generation, sport
utility vehicles, as explained above for the Fixed Timing Fishhook, it
will not do as good a job for the entire vehicle fleet.

**Realistic Appearance**

See the Fixed Timing Fishhook maneuver Realistic Appearance discussion.

**E. Nissan Fishhook**

**Maneuver Description**

The Nissan Fishhook adds to the Fixed Timing Fishhook a procedure for
adjusting the steering reversal timings to the vehicle being tested.
This adjustment process has the same goal as the adjustment process used
for the Roll Rate Feedback Fishhook, i.e., to test each vehicle with the
steering reversal timing required for the vehicle to have its worst case
rollover performance. While the Roll Rate Feedback Fishhook maneuver
accomplishes this by using roll rate feedback resulting in only one test
run per initial maneuver speed, the Nissan Fishhook uses an iterative
procedure to determine the timing.

First, a J-Turn is performed followed by a series of Fixed Timing
Fishhooks (with different timings). Typically, two to four runs will be
made for each initial maneuver speed. The procedure used to determine
the final timing is too complex to give here but is fully described in
the NHTSA technical report "Another Experimental Examination of Selected
Maneuvers That May Induce On-Road Untripped, Light Vehicle Rollover -
Phase IV of NHTSA's Light Vehicle Rollover Research Program." However,
the final dwell times (the length of the pause between completion of the
first steer and the initiation of the countersteer, shown as time, T~1~,
in Figures 5 and 7) generated were close to those of the Roll Rate
Feedback Fishhook.

**Objectivity and Repeatability**

The Nissan Fishhook was performed with good objectivity and
repeatability. By using the programmable steering machine, handwheel
inputs were precisely executed, and able to be replicated from
run-to-run. Test drivers were able to achieve maneuver entrance speeds
an average of ± 0.9 mph from the desired target speed.

Note that the Objectivity and Repeatability rating of the Nissan
Fishhook maneuver was reduced from that assigned to the Fixed Timing
Fishhook. This was due to roll rate zero-crossing variability observed
in response to the step steer used in determining the timing of the
maneuver. The Nissan Fishhook requires accurate determination of the
third roll rate zero-crossing following input of the step steer. This is
because zero crossing variability directly affects what dwell time
duration will ultimately satisfy Nissan's requirements. If the third
roll rate zero crossing is delayed (e.g., due to an anomalous response
produced during the step steer) an inappropriate dwell time extension
will result.

Generally speaking the vehicle speed, lateral acceleration, and roll
angle data observed during Nissan Fishhook tests were highly repeatable.
However, as was discussed above for the NHTSA J-Turn, for runs that are
near the point at which two-wheel lift first occurs, roll angle
repeatability becomes much worse.

**Performability**

The Nissan Fishhook has a well worked out test procedure. It does not
have a procedure to adapt the steering magnitude for the characteristics
of the vehicle being tested although this could probably be added to the
current test procedure without difficulty. The steering reversal timings
used for the Nissan Fishhook maneuver are adjusted for the vehicle being
tested.

The primary advantage of the Nissan Fishhook over the Roll Rate Feedback
Fishhook is that by not using roll rate feedback you avoid the
occasional need for repetitions caused by anomalies in the roll rate
measurement and the extra expense of a programmable steering controller
that can handle roll rate feedback.

The primary disadvantage of the Nissan Fishhook over the Roll Rate
Feedback Fishhook is that the Nissan procedure requires three to four
times as many test runs than does the Roll Rate Feedback Fishhook. As a
result, greater tire wear occurs which has been shown to affect the
results of Fishhook testing. It also increases testing time and costs.

The Nissan Fishhook, as proposed by Nissan, uses a very high steering
wheel angle rate (1,080 degrees per second). Our programmable steering
controller has some difficulty with such a high rate. Changing to the
lower steering wheel angle rate (720 degrees per second) used for the
Fixed Timing and Roll Rate Feedback Fishhooks would probably only
minimally affect maneuver results. Reduction of the magnitude of the
countersteer to the amount used for the Fixed Timing and Roll Rate
Feedback Fishhooks should slightly increase maneuver severity. Our
experience has been that the large countersteer used by the Nissan
Fishhook slows the vehicle down more rapidly, decreasing maneuver
severity.

**Discriminatory Capability**

The Nissan Fishhook was an excellent maneuver for measuring the rollover
resistance of different vehicles. The dynamic rollover propensity of
only the Chevrolet Blazer and Ford Escape was assessed using the Nissan
Fishhook, and all tests were performed in the Nominal Load condition.
Two-wheel lift was produced during tests performed with the Chevrolet
Blazer.

The results obtained with Nissan's methodology were in good agreement
with those produced during Fixed Timing and Roll Rate Feedback Fishhook
testing. That said, the entrance speed of the Nissan Fishhook test for
which two-wheel lift occurred was approximately 6 mph higher than that
of either of the other Fishhooks.

While the Nissan Fishhook does an excellent job of discriminating
between vehicles for typical, current generation, sport utility
vehicles, as explained above for the Fixed Timing Fishhook, it will not
do as good a job for the entire vehicle fleet.

**Realistic Appearance**

See the Fixed Timing Fishhook maneuver Realistic Appearance discussion.

**F. Ford Path Corrected Limit Lane Change**

**Maneuver Description**

Ford's procedure is a path specific method composed of an array of
double lane change courses and a data-normalizing technique used to
address driver variability. It results in a metric based on dynamic
weight transfer.

Ford believes that a path specific method, wherein test vehicles
navigate a standard set of paths, is preferable to maneuvers that employ
open loop steering. Ford states that a specific path provides a basis
for comparison of the resulting metrics. By ensuring that all vehicles
experience the same magnitude of lateral acceleration, the effects of
surface variability on test results are negated. Ford suggests that 0.7g
is an appropriate target for lateral acceleration. Its suite of specific
paths exercises vehicles through a range of frequencies and amplitudes
at the proposed target lateral acceleration.

Three markers (short traffic cones) placed on the pavement delimit the
path's lane change apertures with the middle marker representing an
avoidance obstacle. Varying the position of the obstacle laterally and
longitudinally (with corresponding longitudinal repositioning of the
exit marker) produces an array of steering input amplitudes and
frequencies. A test vehicle approaches the course at 45 mph. The driver
releases the throttle at the course entrance and coasts while steering
through the course. Figure 9 portrays the suite of double lane change
paths to the left used for this maneuver. A similar suite of double lane
change paths to the right is also tested.

Ford addresses driver and test surface variability with the Path
Corrected Limit Lane Change (PCLLC) normalizing technique. The
mathematical procedure is executed during post-processing of test data
and is used "to normalize the varying results of physical tests to a
uniformly based metric.[^8]" The results indicate how the various
vehicles would perform had they followed the exact same path.

Ford states, "Post-test computer aided normalizing techniques have been
sufficiently developed that we have high confidence in their
applicability to this issue. The PCLLC technique uses physical test data
to define a vehicle-specific transfer function. These functions are then
used to normalize metric values, such as dynamic weight transfer, to a
specific vehicle path common to all vehicles evaluated. The data
suggests that use of these normalizing techniques eliminates concerns
that may arise because of test driver variability and by subjecting the
vehicles to the same path, help to eliminate track surface variability,
thus providing the only dynamic test method and metric unaffected by
these sources of variability. We \[Ford\] believe this is a technically
sound method to achieve reliable, repeatable and objectively stated
results that will improve upon SSF based star ratings.[^9]"

Ford reports that an analysis of the results of the normalizing
technique shows that, despite varying styles of driving indicated by
measurement of peak steering wheel angles and rates, the differences in
the mean values of Dynamic Weight Transfer Metric (DWTM) among four test
drivers driving the same vehicle are not statistically significant.

![](media/image20.wmf){width="5.995833333333334in"
height="4.04375in"}Figure 9: Ford Path Specific Double Lane Change
Course

#### 

Ford has allowed NHTSA to evaluate the PCLLC technique under a
confidentiality agreement. Thus, details of the procedure are not
available for this notice. NHTSA expects that Ford would make the
details of the procedure public if it proposed that Ford's test protocol
as the dynamic rollover test mandated by the TREAD Act.

Ford proposes a rollover resistance metric based on dynamic lateral
weight transfer. Ford defines dynamic weight transfer as the "percentage
of weight that is removed from a vehicle's two inside tires during a
severe lane change[^10]." The Dynamic Weight Transfer Metric (DWTM) is
the maximum percent of dynamic weight transfer averaged over a minimum
specific time. Ford recommends a minimum specific time of 400
milliseconds.

## **Objectivity and Repeatability** 

## The Path Corrected Limit Lane Change maneuver consists of a series of closed-loop (test driver generated steering inputs) double lane changes. Data collected during these double lane changes is then processed "to assure that all vehicles follow the same path and are subject to the same acceleration demands[^11]." For reasons that are discussed below in the Discriminatory Capability subsection for this maneuver, Ford Motor Company (Ford) recommends the calculation of a Dynamic Weight Transfer Metric (DWTM) at 0.7 g lateral acceleration for this maneuver. "Because different vehicle designs will react differently to forces of varying magnitude and time duration, a suite of various paths should be analyzed in determining an overall dynamic weight transfer metric (DWTM), based on values of maximum weight transfer[^12]." Note that higher values of DWTM are worse than lower values.

###  Ford has performed a substantial amount of Path Corrected Limit Lane Change maneuver testing. While we do not have access to this data, Ford has summarized this data as follows: "Ford's overall standard deviation for the DWT metric is 4.4 from multiple tests made on a variety of vehicles with a variety of drivers, over a time span of several months and using a new set of tires fitted for each test[^13]." To understand the meaning of this standard deviation, we need to know the expected range of the dynamic weight transfer metric.

The most basic way to estimate this range is to approximate the vehicle
as a rigid block in a steady state curve at 0.7g lateral acceleration.
Using this approximation, the expected range of DWTM values is from 46.7
percent (corresponding to a vehicle with a static stability factor of
1.50) to 70.0 percent (corresponding to a static stability factor of
1.00).

Real vehicles, of course, are not rigid bodies. They have compliant
suspensions and tires. This increases the DWTM values from those of
rigid vehicles. Based on NHTSA's Tilt Table data and assumptions about
the difference between tilt table and flat track testing, we estimate an
addition of about 4% to 8% DWTM to the rigid body calculations as a
result of quasi-static body roll at 0.7 g. Applying the average addition
of 6% DWTM makes the expected range of DWTM approximately 53 percent to
76 percent. Therefore, Ford's standard deviation of 4.4 for DWTM is 19
percent of the entire expected range of DWTM values.

Another way to understand the meaning of this standard deviation is to
analyze the values of DWTM that were measured by Ford and NHTSA during
joint testing of the Phase IV rollover test vehicles. Table 1 lists
these values, along with the number of observations that these values
are based on, the calculated dynamic weight transfer at 0.7 g lateral
acceleration based on a rigid body model, and the difference between
these two dynamic weight transfer values.

Consider the Chevrolet Blazer and the Ford Escape. The Blazer receives
one star; the lowest rating a for sport utility vehicle from NHTSA's
current rollover rating system (which is based on Static Stability
Factor). The Ford Escape has an SSF at the high end of the three star

range; one of the higher ratings for sport utility vehicles. Most sport
utility vehicles have Static Stability Factors between these two
vehicles.

**Table 1: Measured and Calculated Dynamic Weight Transfers**[^14]

+-------------+--------+------+----------+----------+--------+--------+
|             | 2001   | 2001 | 1999     | 1999     | 2001   | 2001   |
|             | Che    | Ford | Mercedes | Mercedes | Toyota | Toyota |
|             | vrolet | Es   | ML320    | ML320    | 4      | 4      |
|             | Blazer | cape | with ESC | with ESC | Runner | Runner |
|             |        |      | On       | Off      | with   | with   |
|             |        |      |          |          | ESC On | ESC    |
|             |        |      |          |          |        | Off    |
+-------------+--------+------+----------+----------+--------+--------+
| PCLLC       | 70.3%  | 6    | 74.8%    | 68.2%    | 66.2%  | 66.6%  |
| Measured    |        | 2.9% |          |          |        |        |
| DWTM        |        |      |          |          |        |        |
+-------------+--------+------+----------+----------+--------+--------+
| Number of   | 4      | 4    | 4        | 10       | 4      | 4      |
| O           |        |      |          |          |        |        |
| bservations |        |      |          |          |        |        |
+-------------+--------+------+----------+----------+--------+--------+
| Steady      | 67.3%  | 5    | 60.9%    | 60.9%    | 63.1%  | 63.1%  |
| State       |        | 5.6% |          |          |        |        |
|             |        |      |          |          |        |        |
| Rigid Body  |        |      |          |          |        |        |
| WT          |        |      |          |          |        |        |
| Calculated  |        |      |          |          |        |        |
| from        |        |      |          |          |        |        |
|             |        |      |          |          |        |        |
| SSF         |        |      |          |          |        |        |
+-------------+--------+------+----------+----------+--------+--------+
| Difference  | 3.0%   | 7.3% | 13.9%    | 7.3%     | 3.1%   | 3.5%   |
+-------------+--------+------+----------+----------+--------+--------+

Now compare the DWTM values of these vehicles as measured using the Path
Corrected Limit Lane Change and shown in Table 1. For the Chevrolet
Blazer the measured DWTM value is 70.3. However, based on Ford's
standard deviation and the number of samples, we have 95 percent
confidence that the DWTM for this vehicle is between 66.0 and 74.6.
Similarly, for the Ford Escape we have 95 percent confidence that the
DWTM is between 58.6 and 67.2. Note that these ranges overlap. However,
the difference between these two vehicles DWTM values is statistically
significant (although just barely having a t-value of 2.38 versus the
critical t-value of 2.37).

A measurement standard deviation for which the difference between a
sport utility vehicle with high rollover resistance and one with low
rollover resistance is only marginally statistically significant is too
large for generating vehicle ratings.

Table 1 shows another problem with the measured DWTM values. When we
estimated the expected range of DWTM as 53 percent to 76 over the entire
range of vehicles from SUVs to sport sedans, we considered only the
quasi-static load transfer due to the vehicle's rigid body geometry
(SSF) and to its steady state body roll. We neglected the dynamic weight
transfer that occurs as a result of body roll acceleration in an abrupt
maneuver. However, when the calculated steady state, rigid body weight
transfer in Table 1 is subtracted from the measured DWTM, the difference
is no more than that expected for the steady state body roll in all but
one case. It would appear that the Dynamic Weight Transfer Metric
produced by PCLLC generally measures quasi-static rather than dynamic
weight transfer. Quasi-static weight transfer is what occurs when a
vehicle is driven is a circle at a constant speed without abrupt changes
in speed or direction.

The exception is the DWTM measurement for the Mercedes ML320 with yaw
stability control enabled. While the DTWM for this vehicle with yaw
stability control disabled is no more than the expected quasi-static
load transfer, the DTWM increases by 6.6 percent when the yaw stability
control is enabled. The difference between these two values is
statistically significant and would seem to represent a dynamic weight
transfer component missing in the other PCLLC results in Table 1.
However, it is hard to understand why stability control should lower the
rollover resistance of this vehicle. Fishhook testing indicates just the
opposite; that yaw stability control increases the rollover resistance
of this vehicle. Therefore, we believe that the measured DWTM value for
the Mercedes ML320 with yaw stability control enabled is incorrect.

In conclusion, the objectivity and repeatability of the Path Corrected
Limit Lane Change has not yet attained an acceptable level for rating
the rollover resistance of vehicles. Future improvements to the
objectivity and repeatability of this maneuver can probably be made, but
there are other tests with more potential for making highly objective
and repeatable measurements of quasi-static weight transfer.

**Performability**

The procedure for performing this test is straight-forward. However,
substantial additional instrumentation, over and above that required to
perform a Fishhook maneuver, are required. The costs and additional
testing time associated with this equipment is expected to exceed the
costs and additional testing time saved by not having to use a
programmable steering controller. An additional test, on a tire testing
machine, is also required.

Ford has ideas for reducing the additional instrumentation required for
the Path Corrected Limit Lane Change procedure. However, this is a
future enhancement and cannot be evaluated at this time.

Since Ford processed the data collected during our testing, we are
unable to say how difficult the data processing is to perform. However,
with experience and the correct software it is expected to approximately
equal the effort required to process data from a Fishhook or J-Turn
test. There may be issues in making Ford's data processing software
publicly available.

Due to the use of a suite of paths for calculating DWTM values, the Path
Corrected Limit Lane Change procedure should adequately adapt to
differing vehicle characteristics.

We also have concerns about determining dynamic weight transfer as an
average value over a 400 millisecond window. The use of this broad a
window may filter out dynamic effects that may be important in actual
vehicle rollovers.

**Discriminatory Capability**

No two-wheel lifts occurred during Path Corrected Limit Lane Change
testing for any of the test vehicles. However, unlike the J-Turn and
Fishhook maneuvers, the occurrence/non-occurrence of two-wheel lift is
not used as a measure of vehicle performance for this maneuver. The DWTM
measured in PCLLC testing produces a continuous measure of rollover
resistance that, like SSF, that allows discrimination even among
vehicles that are not susceptible to on-road untripped rollover.

Ford recommends the calculation of a Dynamic Weight Transfer Metric
(DWTM) at 0.7 g lateral acceleration as a measure of vehicle performance
for this maneuver. Data collected during testing is processed to remove
driver effects by having all vehicles always follow the same specified
paths and be subject to the same acceleration demands. "Because
different vehicle designs will react differently to forces of varying
magnitude and time duration, a suite of various paths should be analyzed
in determining an overall dynamic weight transfer metric (DWTM), based
on values of maximum weight transfer[^15]." Ford's reasons for making
this recommendation are as follows:

"For a given velocity change, various vehicle related factors determine
the magnitude of dynamic weight transfer for events that can lead to
both tripped or un-tripped rollover. Obviously, the higher the
center-of-gravity, the greater the transfer for a given travel velocity
change. Similarly, the smaller the track width, the greater the
transfer. As is well known, many factors other than these two affect
dynamic weight transfer and it is because of this that SSF is a narrow
and inadequate concept. For example, if deflections occur in
suspensions, tires, or other parts that control overall body movements
such as active stabilizer bars or electronically controlled shock
absorbers, when dynamic forces are applied, the magnitude of the dynamic
weight transfer will also change. Inertial values, yaw plane motions,
vertical motions and pitch plane motions that arise because of a
vehicle's design details or features can affect force and moment
balances and can change vehicle configurations to affect the magnitude
of the dynamic weight transfer. It is a directionally correct
proposition that the greater the magnitude of the dynamic weight
transfer in a given high severity event, the less margin, reserve, or
resistance remains to a rollover occurring. Based on these principles,
Ford believes that dynamic weight transfer is a metric of value in a
dynamic test." "Our preliminary work has confirmed that this metric will
discriminate among specific vehicles within a class and between classes
of vehicles. We submit that DWTM is a more reliable metric than SSF
alone[^16]."

DWTM has the theoretical advantage over SSF of including load transfer
due to quasi-static body roll and true dynamic load transfer due to body
roll accelerations, but its measurement by the PCLLC method seems to be
lacking the dynamic load transfer component. The PCLLC test also is not
able to test for the effect of yaw stability control. In its comment to
the docket of the last notice, Ford suggested that the same 0.7g lane
change maneuvers and DTWM could be implemented directly with an advanced
path following robot rather than with the PCLLC method, but it cautioned
that the test would not evaluate the effect of yaw stability control. In
light of this comment, it is not surprising that the PCLLC test measured
no effect of yaw stability control of Toyota 4Runner, but it remains
troubling that it measured a significant loss of rollover resistance for
yaw stability control of the Mercedes ML320 contrary to its effect
measured in other rollover maneuver tests.

As discussed above, we do not believe that dynamic weight transfer
values determined using this maneuver have, so far, attained an
acceptable level of repeatability. We are also concerned about not
exercising vehicles to the limits of their performance. By not taking
vehicles to their limits, some important limit performance problems
could be overlooked.

**Realistic Appearance**

In general, double lane change maneuvers have an excellent appearance of
reality. These are the emergency obstacle avoidance maneuvers that
people think of first when they consider untripped rollover. While the
Path Corrected Limit Lane Change trajectories are idealized, rather than
actual, this distinction would likely not be noticed by consumers.

**G. ISO 3888 Part 2 Double Lane Change**

**Maneuver Description**

To perform ISO 3888 Part 2 Double Lane Change testing, the vehicle was
driven through the course shown in Figure 10. The driver released the
throttle 6.6 ft (2.0 m) from the entrance of the first lane. No throttle
input or brake application occurred during the remainder of maneuver.

![](media/image21.png){width="6.6in" height="2.6980314960629923in"}
Drivers iteratively increased maneuver entrance speed from approximately
35 mph in 1 mph increments. The iteration continued until valid tests
could no longer be performed (lane position could not be maintained
without striking cones). Each driver was required to perform three valid
runs at their maximum speed. This was to assess input and output
variability for tests performed by the same driver with the same
entrance speed.

The manner in which the 1 mph iterations were implemented was somewhat
driver-dependent. Some drivers preferred to increase speed until they
could no longer achieve a valid test. Once this threshold was reached,
the driver would reduce speed slightly and perform three valid tests.
Other drivers would perform three valid tests at one speed before
proceeding to the next iteration. Both methods produced similar results.

So as to examine driver-to-driver differences, during the Phase IV
research, this maneuver was performed for each vehicle by three drivers.
To reduce any confounding effect tire wear may have on ISO 3888 Part 2
Double Lane Change test results, a new tire set was installed on each
vehicle, for each driver.

## **Objectivity and Repeatability** 

## Since steering inputs for the ISO 3888 Part 2 Double Lane Change maneuver are generated by the test driver, vehicle performance in this maneuver depends upon the skill of the test driver, the steering strategy used by the test driver, plus random run-to-run fluctuations.

The ISO 3888 Part 2 Double Lane Change maneuver attempts to minimize
this variability through the use of an in-between lane of substantial
length and very tight entry, exit, and in-between lanes, thereby
minimizing a driver's steering options for getting through the course
without striking delineating cones.

##  Figure 11 shows the range of handwheel steering angles used by three different test drivers while performing this maneuver multiple times while Figure 12 shows the range of handwheel steering angles used by these drivers at selected times during this maneuver. As these figures show, there are both substantial driver-to-driver differences and substantial within driver run-to-run differences in the steering inputs. These differences tend to increase as the maneuver progresses.

## 

![](media/image22.wmf){width="4.583464566929134in"
height="3.972834645669291in"}

![](media/image23.wmf)

Arguably, the differences in steering inputs shown in Figure 11 and 12
do not really matter for the purposes of determining Rollover Resistance
Ratings. What really matters are driver-to-driver differences in vehicle
outputs, specifically the vehicle rating metrics.

The rating metric suggested by the Daimler-Chrysler Corporation is the
maximum entry speed into the test course at which a driver successfully
achieved a "clean" run. (A "clean" run is one during which none of the
cones delineating the course were struck.)

Table 2 shows the maximum achievable "clean" run speeds for three test
drivers for the Nominal Vehicle configuration for each of the Phase IV
rollover test vehicles. (While each vehicle was tested by three drivers,
four drivers actually participated in this testing.) Note that higher
values of this metric indicate a better performing vehicle.

**Table 2: Maximum Achievable "Clean" Run Speeds**

**For the ISO 3888 Part 2 Double Lane Change Maneuver --**

**Nominal Vehicle Configuration**

+---------+--------+--------+----------+----------+---------+---------+
| *       | **2001 | **2001 | **1999   | **1999   | **2001  | **2001  |
| *Test** | Che    | Ford   | Mercedes | Mercedes | Toyota  | Toyota  |
|         | vrolet | Es     | ML320    | ML320    | 4Runner | 4Runner |
| **D     | Bl     | cape** | with ESC | with ESC | with    | with    |
| river** | azer** |        | On**     | Off**    | ESC     | ESC     |
|         |        | **(    |          |          | On**    | Off**   |
|         | **(    | mph)** | *        | *        |         |         |
|         | mph)** |        | *(mph)** | *(mph)** | **      | **      |
|         |        |        |          |          | (mph)** | (mph)** |
+---------+--------+--------+----------+----------+---------+---------+
| GF/RS   | 39.0   | 36.9   | 38.0     | 37.2     | 37.6    | 35.9    |
+---------+--------+--------+----------+----------+---------+---------+
| LJ      | 40.0   | 36.6   | 37.0     | 36.7     | 36.7    | 35.3    |
+---------+--------+--------+----------+----------+---------+---------+
| RL      | 41.0   | 38.0   | 36.8     | 37.8     | 35.8    | 37.0    |
+---------+--------+--------+----------+----------+---------+---------+
| Range   | 2.0    | 1.4    | 1.2      | 1.1      | 1.8     | 1.7     |
+---------+--------+--------+----------+----------+---------+---------+

Table 3 shows a rank ordering of the Phase IV rollover test vehicles
based on the maximum "clean" run speeds achieved by the test drivers.
Note that 1 is the best rank and 6 the worst.

**Table 3: Vehicle Rankings Based on Maximum Achievable**

**"Clean" Run Speeds for the ISO 3888 Part 2 Double Lane**

**Change Maneuver -- Nominal Vehicle Configuration**

+---------+--------+--------+----------+----------+---------+---------+
| *       | **2001 | **2001 | **1999   | **1999   | **2001  | **2001  |
| *Test** | Che    | Ford   | Mercedes | Mercedes | Toyota  | Toyota  |
|         | vrolet | Es     | ML320    | ML320    | 4Runner | 4Runner |
| **D     | Bl     | cape** | with ESC | with ESC | with    | with    |
| river** | azer** |        | On**     | Off**    | ESC     | ESC     |
|         |        |        |          |          | On**    | Off**   |
+---------+--------+--------+----------+----------+---------+---------+
| GF/RS   | 1      | 5      | 2        | 4        | 3       | 6       |
+---------+--------+--------+----------+----------+---------+---------+
| LJ      | 1      | 5      | 2        | 3        | 3       | 6       |
+---------+--------+--------+----------+----------+---------+---------+
| RL      | 1      | 2      | 5        | 3        | 6       | 4       |
+---------+--------+--------+----------+----------+---------+---------+

As Table 2 shows, for the drivers used, the range of maximum achievable
"clean" run entry speeds varied from 1.2 mph for the 1999 Mercedes ML320
with yaw stability control enabled to 2.0 mph for the 2001 Chevrolet
Blazer. The average range was 1.5 mph. While these may seem like small
ranges, the entire best-to-worst range in Table 2 is only 5.7 mph. Since
we tested a fairly broad range of sport utility vehicles during the
Phase IV research, the maximum achievable "clean" run speeds for most
sport utility vehicles are expected to be in this 5.7 mph range.
Therefore, driver-to driver variability averages 27 percent of the range
of the rating metric and can be as much as 35 percent.

The problem caused by driver-to-driver variability combined with the
small range of metric values is clearly shown by Table 3. While the
Chevrolet Blazer attained the best ranking from all three test drivers,
the ranking for the Mercedes ML320 with yaw stability control enabled
varied from second best to second worst.

Driver skills and abilities vary with time. Although we did not do such
testing, if we retested the Phase IV rollover test vehicles with the
same test drivers performing the ISO 3888 Part 2 Double Lane Change
maneuver we anticipate that our results would not exactly match those
shown in Tables 2 and 3. Since we have such a small range for the rating
metric day-to-day (or even hour-to-hour) changes in test driver
performance would probably change the maximum achievable "clean" run
entry speeds by a substantial percentage of the overall range.

Due to the problems associated with driver-to-driver variability and
run-to-run for the same driver variability, the objectivity and
repeatability of this maneuver is poor.

**Performability**

The procedure for performing this test is straight-forward. However, as
discussed above, this maneuver has objectivity and repeatability issues.
Resolving these issues adds difficulty and complexity to performing
these tests.

For example, one possibility for improving objectivity and repeatability
is to use multiple drivers to perform the testing (three drivers were
used during the Phase IV testing). While this should help, there are
still potential problems. One exceptionally skilled test driver could
generate very good performance metrics for a mediocre vehicle. If this
exceptionally skilled driver did not test some other vehicle, that
vehicle's performance metrics might, incorrectly, be lower than they
should be. Therefore, in addition to using multiple drivers, procedures
would need to be developed to ensure that every vehicle is tested by
drivers of approximately equal skill.

The ISO 3888 Part 2 Double Lane Change test procedure includes
adjustments to lane width and lane change gate length for differing
vehicle sizes. These should adequately adapt this maneuver for differing
vehicle characteristics.

**Discriminatory Capability**

No two-wheel lifts occurred during any "clean" run of ISO 3888 Part 2
Double Lane Change testing for any of the test vehicles. (A "clean" run
is one during which none of the cones delineating the course were
struck.) While some two-wheel lifts did occur during runs that were not
"clean", these should not be considered for the determination of our
rollover resistance ratings. The reason is that when a run is not
"clean", there is no way to determine whether the vehicle comes close to
following the test course. For example, a driver could perform a
fishhook maneuver or simply drive straight through. Either case would
simply be recorded as not a "clean" run.

Unlike the J-Turn and Fishhook maneuvers, the occurrence/non-occurrence
of two-wheel lift cannot be used as a measure of vehicle performance for
this maneuver because two-wheel lifts during a clean run appear very
unlikely for any NCAP vehicle. The rating metric suggested by the
Daimler-Chrysler Corporation (Daimler) is the maximum entry speed into
the test course at which a driver successfully achieved a "clean" run.

Table 4 shows the maximum achievable "clean" run speeds attained by any
of the test drivers for both the Nominal Vehicle and Reduced Rollover
Resistance configuration for each of the Phase IV rollover test
vehicles. Note that higher values of this metric indicate a better
performing vehicle.

The Reduced Rollover Resistance configuration vehicles have had weights
placed on the roof so as to raise the center of gravity height. Their
Static Stability Factors have been reduced by 0.05. A 0.05 reduction in
SSF equates, for sport utility vehicles, to approximately a one star
reduction in the vehicle's rollover resistance rating. As was previously
stated, NHTSA believes that a one star reduction in the rollover
resistance rating should make a vehicle substantially easier to
rollover. Maneuvers with good discriminatory capability should measure
substantially worse performance for Reduced Rollover Resistance the
configuration than for the Nominal Vehicle configuration.

**Table 4: Maximum Achievable "Clean" Run Speeds By Any**

**Driver for the ISO 3888 Part 2 Double Lane Change Maneuver --**

**Nominal Vehicle and Reduced Rollover Resistance Configurations**

+----------+--------+--------+----------+---------+--------+--------+
| **Test** | **2001 | **2001 | **1999   | **1999  | **2001 | **2001 |
|          | Che    | Ford   | Mercedes | M       | Toyota | Toyota |
| **       | vrolet | Es     | ML320    | ercedes | 4      | 4      |
| Driver** | Bl     | cape** | with ESC | ML320   | Runner | Runner |
|          | azer** |        | On**     | with    | with   | with   |
|          |        | **(    |          | ESC     | ESC    | ESC    |
|          | **(    | mph)** | *        | Off**   | On**   | Off**  |
|          | mph)** |        | *(mph)** |         |        |        |
|          |        |        |          | **      | **(    | **(    |
|          |        |        |          | (mph)** | mph)** | mph)** |
+----------+--------+--------+----------+---------+--------+--------+
| Nominal  | 41.0   | 38.0   | 38.0     | 38.9    | 37.6   | 37.0   |
| Vehicle  |        |        |          |         |        |        |
| Confi    |        |        |          |         |        |        |
| guration |        |        |          |         |        |        |
+----------+--------+--------+----------+---------+--------+--------+
| Reduced  | 39.0   | 37.3   | 37.4     | 37.1    | 39.3   | 38.0   |
| Rollover |        |        |          |         |        |        |
| Re       |        |        |          |         |        |        |
| sistance |        |        |          |         |        |        |
| Confi    |        |        |          |         |        |        |
| guration |        |        |          |         |        |        |
+----------+--------+--------+----------+---------+--------+--------+
| Di       | 2.0    | 0.7    | 0.6      | 1.8     | -1.7   | -1.0   |
| fference |        |        |          |         |        |        |
+----------+--------+--------+----------+---------+--------+--------+

This expected substantial change in rollover resistance ratings is not
seen for the ISO3888 Part 2 Double Lane Change maneuver. For three of
the vehicles the maximum achievable "clean" run speeds attained by any
of the test drivers in the Reduced Rollover Resistance configuration
vehicles did decrease slightly compared to the Nominal Configuration
vehicles while for the 2001 Toyota 4Runner they increased slightly. The
average change was only 0.4 mph, far less than the average
driver-to-driver variability of 1.5 mph.

The expected substantial change in rollover resistance measurement was
not observed for the ISO3888 Part 2 Double Lane Change maneuver
apparently because the sensitivity of the test to handling properties is
predominant compared to its sensitivity to rollover resistance. Placing
weight on a vehicle's roof raises its center of gravity height which
reduces its rollover resistance. However, doing this also increases a
vehicle's mass and roll moment of inertia, resulting in changes to a
vehicle's handling that are not well understood. Since handling and
rollover resistance are inextricably intertwined in the rating produced
by this maneuver, the rating generated can improve even though the
rollover resistance of a vehicle is getting worse.

Results from both J-Turn and Fishhook testing are, of course, also
influenced by the handling characteristics of the vehicle. However,
handling has less of a chance to dominate these maneuvers because they
involve fewer major steering movements (one for a J-Turn, two for a
Fishhook, and three for a Double Lane Change).

The above reasoning also explains an apparent anomaly in Table 3. In
this table, the Chevrolet Blazer has the best ranking of any of the
vehicles. However, based on its one star rating and performance in the
NHTSA J-Turn and Fishhooks, we believe it to have the lowest rollover
resistance of any of the Phase IV rollover test vehicles. The apparent
contradiction is resolved once we realize that the ISO3888 Part 2 Double
Lane Change maneuver measures mostly the handling rather than rollover
resistance of vehicles.

**Realistic Appearance**

In general, double lane change maneuvers have an excellent appearance of
reality. These are the emergency obstacle avoidance maneuvers that
people think of first when they consider untripped rollover.

**H. Consumers Union Short Course Double Lane Change**

**Maneuver Description**

To perform Consumers Union Short Course Double Lane Change testing, the
vehicle was driven through the course shown in Figure 13. As the vehicle
approached the course entrance, the driver released the throttle so as
to achieve a desired target speed as the vehicle passed over a timing
strip 35 feet from the entrance of the first lane. Otherwise, the
procedure for this maneuver was identical to that used for the ISO 3888
Part 2 Double Lane Change testing.

![](media/image21.png){width="6.625196850393701in"
height="1.9716535433070865in"}

**Objectivity and Repeatability**

Since steering inputs for the Consumers Union Short Course Double Lane
Change maneuver are generated by the test driver, vehicle performance in
this maneuver depends upon the skill of the test driver, the steering
strategy used by the test driver, plus random run-to-run fluctuations.

##  Figure 14 shows the range of handwheel steering angles used by three different test drivers while performing this maneuver multiple times while Figure 15 shows the range of handwheel steering angles used by these drivers at selected times during this maneuver. As these figures show, there are both substantial driver-to-driver differences and substantial within driver run-to-run differences in the steering inputs. These differences tend to increase as the maneuver progresses.

Arguably, the differences in steering inputs shown in Figures 14 and 15
do not really matter for the purposes of determining Rollover Resistance
Ratings. What really matters are driver-to-driver differences in vehicle
outputs, specifically the vehicle rating metrics.

The rating metric used by NHTSA is the maximum entry speed into the test
course at which a driver successfully achieved a "clean" run. (A "clean"
run is one during which none of the cones delineating the course were
struck.) Note that this is not the rating metric used by Consumers Union
for this maneuver; Consumers Union performs subjective rating of the
emergency handling capability of vehicles with vehicles that have large
amounts of two-wheel lift in this maneuver receiving an "unacceptable"
safety rating.

Table 5 shows the maximum achievable "clean" run speeds for three test
drivers for the Nominal Vehicle configuration for the Phase IV rollover
test vehicles. Note that higher values of this metric indicate a better
performing vehicle.

Table 6 shows a rank ordering of the Phase IV rollover test vehicles
based on the maximum "clean" run speeds achieved by the three test
drivers. Note that 1 is the best rank and 6 the worst.

![](media/image24.wmf){width="4.638976377952756in"
height="5.419291338582677in"}

![](media/image25.wmf){width="5.33740157480315in"
height="5.891732283464567in"}

**Table 5: Maximum Achievable "Clean" Run Speeds**

**For the Consumers Union Short Course**

**Double Lane Change Maneuver --**

**Nominal Vehicle Configuration**

+---------+--------+--------+----------+----------+---------+---------+
| *       | **2001 | **2001 | **1999   | **1999   | **2001  | **2001  |
| *Test** | Che    | Ford   | Mercedes | Mercedes | Toyota  | Toyota  |
|         | vrolet | Es     | ML320    | ML320    | 4Runner | 4Runner |
| **D     | Bl     | cape** | with ESC | with ESC | with    | with    |
| river** | azer** |        | On**     | Off**    | ESC     | ESC     |
|         |        | **(    |          |          | On**    | Off**   |
|         | **(    | mph)** | *        | *        |         |         |
|         | mph)** |        | *(mph)** | *(mph)** | **      | **      |
|         |        |        |          |          | (mph)** | (mph)** |
+---------+--------+--------+----------+----------+---------+---------+
| GF      | 39.3   | 37.0   | 38.8     | 36.7     | 36.5    | 37.7    |
+---------+--------+--------+----------+----------+---------+---------+
| LJ      | 38.1   | 37.1   | 37.1     | 36.6     | 37.4    | 35.7    |
+---------+--------+--------+----------+----------+---------+---------+
| RL      | 40.7   | 40.5   | 39.2     | 38.3     | 37.8    | 37.8    |
+---------+--------+--------+----------+----------+---------+---------+
| Range   | 2.6    | 3.5    | 1.7      | 1.7      | 1.3     | 2.1     |
+---------+--------+--------+----------+----------+---------+---------+

**Table 6: Vehicle Rankings Based on Maximum**

**Achievable "Clean" Run Speeds For the Consumers**

**Union Short Course Double Lane Change Maneuver --**

**Nominal Vehicle Configuration**

+---------+--------+--------+----------+----------+---------+---------+
| *       | **2001 | **2001 | **1999   | **1999   | **2001  | **2001  |
| *Test** | Che    | Ford   | Mercedes | Mercedes | Toyota  | Toyota  |
|         | vrolet | Es     | ML320    | ML320    | 4Runner | 4Runner |
| **D     | Bl     | cape** | with ESC | with ESC | with    | with    |
| river** | azer** |        | On**     | Off**    | ESC     | ESC     |
|         |        |        |          |          | On**    | Off**   |
+---------+--------+--------+----------+----------+---------+---------+
| GF      | 1      | 4      | 2        | 5        | 6       | 3       |
+---------+--------+--------+----------+----------+---------+---------+
| LJ      | 1      | 3      | 3        | 5        | 2       | 6       |
+---------+--------+--------+----------+----------+---------+---------+
| RL      | 1      | 2      | 3        | 4        | 5       | 5       |
+---------+--------+--------+----------+----------+---------+---------+

As Table 5 shows, for three test drivers used, the range of maximum
achievable "clean" run entry speeds varied from 1.3 mph for the 2001
Toyota 4Runner with yaw stability control enabled to 3.5 mph for the
2001 Ford Escape. The average range was 2.2 mph. While these may seem
like small ranges, the entire best-to-worst range in Table 5 is only 5.0
mph. Since we tested a fairly broad range of sport utility vehicles
during the Phase IV research, the maximum achievable "clean" run speeds
for most sport utility vehicles are expected to be in this 5.0 mph
range. Therefore, driver-to driver variability averages 44 percent of
the range of the rating metric and can be as much as 70 percent.

The problem caused by driver-to-driver variability combined with the
small range of metric values is clearly shown by Table 6. While the
Chevrolet Blazer attained the best ranking from all three test drivers,
the ranking for the Toyota 4Runner with yaw stability control enabled
varied from second best to worst.

Driver skills and abilities vary with time. Although we did not do such
testing, if we retested the Phase IV rollover test vehicles with the
same test drivers performing the Consumers Union Short Course Double
Lane Change maneuver we anticipate that our results would not exactly
match those shown in Tables 4 and 5. Since we have such a small range
for the rating metric day-to-day (or even hour-to-hour) changes in test
driver performance would probably change the maximum achievable "clean"
run entry speeds by a substantial percentage of the overall range.

Due to the problems associated with driver-to-driver variability and
run-to-run for the same driver variability, the objectivity and
repeatability of this maneuver are poor. However, it is important to
recognize that NHTSA's objective for this maneuver, the determination of
rollover resistance ratings, is not the same as Consumers Union's
objective, the evaluation of a vehicle's emergency handling
capabilities. Handling evaluation has always been a subjective process.
This appears to be a better maneuver for what Consumers Union wants to
accomplish than for what the Government wants to accomplish.

**Performability**

The procedure for performing this test is straight-forward. However, as
discussed above, this maneuver has objectivity and repeatability issues.
Resolving these issues adds difficulty and complexity to performing
these tests.

For example, one possibility for improving objectivity and repeatability
is to use multiple drivers to perform the testing (three drivers were
used during the NHTSA testing). While this should help, there are still
potential problems. One exceptionally skilled test driver could generate
very good performance metrics for a mediocre vehicle. If this
exceptionally skilled driver did not test some other vehicle that
vehicle's performance metrics might, incorrectly, be lower than they
should be. Therefore, in addition to using multiple drivers, procedures
would need to be developed to ensure that every vehicle is tested by
drivers of approximately equal skill.

The Consumers Union Short Course Double Lane Change test procedure does
not change from vehicle-to-vehicle. This reflects Consumers Union's
reason for developing this maneuver; as a test of emergency handling. On
an actual road, if an obstacle suddenly intrudes into a vehicle's lane
requiring emergency maneuvering to avoid, the parameters of the
intrusion (distance ahead of oncoming vehicle at which the intrusion
begins, amount of intrusion) do not depend on the characteristics of the
oncoming vehicle. In other words, if a child runs out in front of you,
they do not run out sooner because your vehicle is bigger or wider.

However, NHTSA has a different purpose. We are trying to rate a vehicle
resistance to rollover. As such, we would like to test with worst case
lane geometry. This may well change with vehicle size or other
characteristics. Therefore, for NHTSA's purpose, we believe that a test
maneuver should adapt for differing vehicle characteristics.

**Discriminatory Capability**

No two-wheel lifts occurred during any "clean" run of Consumers Union
Short Course Double Lane Change testing for any of the test vehicles. (A
"clean" run is one during which none of the cones delineating the course
were struck.) While some two-wheel lifts did occur during runs that were
not "clean", these should not be considered for the determination of our
rollover resistance ratings. The reason is that when a run is not
"clean", there is no way to determine whether the vehicle comes close to
following the test course. For example, a driver could perform a
fishhook maneuver or simply drive straight through. Either case would
simply be recorded as not a "clean" run.

Unlike the J-Turn and Fishhook maneuvers, the occurrence/non-occurrence
of two-wheel lift cannot be used as a measure of vehicle performance for
this maneuver because two-wheel lifts during clean run appear unlikely
for NCAP vehicles. The rating metric use by NHTSA is the maximum entry
speed into the test course at which a driver successfully achieved a
"clean" run.

We did not perform testing of the Reduced Rollover Resistance
configurations of the Phase IV test vehicles with this maneuver; so, we
cannot make the comparisons shown in Table 4 for this maneuver. However,
the discussion following Table 4 likely applies to this maneuver as well
as to the ISO 3888 Part 2 Double Lane Change. Again, this maneuver tests
both the handling and rollover resistance of vehicles. In fact, since
Consumers Union developed this maneuver to examine the emergency
handling of vehicles, and because this maneuver is not as tightly
constrained as is the ISO 3888 Part 2 Double Lane Change, we believe
that this maneuver focuses more on handling than does the ISO maneuver.
Since handling and rollover resistance are inextricably intertwined in
the rating produced by this maneuver with handling dominating, the
rating generated can easily improve even though the rollover resistance
of a vehicle is getting worse.

The above reasoning explains the apparent anomaly in Table 6. In this
table, the Chevrolet Blazer has the best ranking of any of the vehicles.
However, based on its one star rating and performance in the NHTSA
J-Turn and Fishhooks, we believe it to have the lowest rollover
resistance of any of the Phase IV rollover test vehicles. The apparent
contradiction is resolved once we realize that the Consumers Union
Double Lane Change maneuver measures both the handling and rollover
resistance of vehicles with handling dominating.

Due to the fact that this maneuver is not focused solely on a vehicle's
rollover resistance but instead measures some combination of their
handling and rollover resistance properties, its discriminatory
capability for rollover resistance (not emergency handling) is poor.

**Realistic Appearance**

See the ISO 3888 Part 2 Double Lane Change maneuver Realistic Appearance
discussion.

**I. Open-Loop Pseudo-Double Lane Change**

**Maneuver Description**

Driver-based, path-following double lane changes have historically been
associated with considerable handwheel variability. This was in evidence
during the ISO 3888 Part 2 and Consumers Union Short Course testing
performed during the Phase IV research. Although the ISO 3888 Part 2
Double Lane Change course layout attempts to minimize this variability
by relating lane width to vehicle width, handwheel variability observed
during this maneuver continues to exceed that typically observed during
steering machine-based maneuvers.

Aside from the handwheel variability issues, double lane changes have a
certain appeal. It is foreseeable that the inputs of either double lane
change used in Phase IV could emulate a driver's reaction to a variety
of crash avoidance scenarios. Furthermore, examination of what effects
the third steering input (second reversal) has on dynamic rollover
propensity is of interest. To facilitate examination of third steer
effects without the confounding effect of handwheel variability,
open-loop handwheel inputs executed with the steering machine that
approximated a double lane change were performed.

Two open-loop pseudo-double lane changes were performed during the Phase
IV research: ISO 3888 Part 2 and Consumers Union Short Course
simulations. For each maneuver, handwheel inputs were chosen to
approximate those observed during closed-loop, path-following tests
performed at VRTC by three test drivers. Specifically, steering recorded
during the three tests begun with the highest, yet most similar,
entrance speeds was considered for each driver, per maneuver. Using
these data, handwheel input composites were developed. Open-loop double
lane changes were performed in the Nominal load condition, with the
Toyota 4Runner and Chevrolet Blazer only. The Ford Escape and Mercedes
ML320 were not evaluated with these maneuvers.

Upon completion of the path-following double lane changes, the three
highest, most consistent valid maneuver entrance speeds attained by each
driver were determined. A valid test was one in which no vehicle-to-cone
contact was detected. This produced a total of nine valid runs for each
vehicle (recall the 4Runner with enabled stability control was
considered to be separate vehicle from the 4Runner with disabled
stability control).

Double lane change simulation began by plotting of the handwheel angles
for all drivers of a particular vehicle. The plots were overlaid and
centered about the middle peak of the maneuver in the time domain. After
each of the nine tests was centered, the data were averaged to form a
preliminary composite.

Once the preliminary composite was created, averages for each of the
three primary handwheel peaks were calculated. These averages were based
on peak value data (independent of time) from each of the nine
driver-based tests. Each average was then divided by the appropriate
preliminary composite value to produce a ratio. The three ratios were
averaged to produce a final, overall ratio. This final ratio was
multiplied by preliminary composite data to yield a final handwheel
input composite[^17]^.^

Piecewise approximation was used to construct ramp-based handwheel
profiles representative of the final handwheel composites. The
approximation was programmed into the steering machine, and the maneuver
performed.

Figure 16 presents the suite of piecewise approximations used to define
the Consumers Union Short Course simulations for the Toyota 4Runner
(enabled and disabled stability control) and Chevrolet Blazer.

![](media/image26.wmf){width="5.310236220472441in"
height="5.272047244094488in"}

Generally speaking, closed-loop Consumers Union Short Course tests
performed with the 4Runner (disabled stability control) and Blazer
contained four significant steering inputs (i.e., third reversals). The
drivers used the fourth steering inputs to preserve lateral stability
and insure exit lane position. These inputs were included in Consumers
Union Short Course approximations for the 4Runner with disabled
stability control and for the Blazer, but were not required for
approximation of 4Runner steering observed during tests performed with
enabled stability control.

Due to the length of the second lane in the ISO 3888 Part 2 course, each
driver made steering adjustments after the second handwheel peak to
maintain lane position. As a result, each ISO 3888 Part 2 simulation
contained five significant handwheel peaks. Figure 17 presents the
open-loop steering inputs used to simulate the ISO 3888 Part 2 Double
Lane Change maneuver for each vehicle.

During testing, runs of the Open-Loop Pseudo-Double Lane Change were
performed beginning with a maneuver entry speed of 35 mph. Vehicle speed
was iteratively increased in 5 mph increments to 50 mph or until
two-wheel lift occurred. Additionally, tests were performed at the
average maximum entrance speed attained by test drivers at VRTC during
closed-loop tests without the steering machine. No downward speed
iterations were used to isolate the lowest entrance speed capable of
producing two-wheel lift.

##### ![](media/image27.wmf){width="5.004330708661417in" height="4.836220472440945in"} 

##### Objectivity and Repeatability 

The Open-Loop Pseudo-Double Lane Change can be performed with excellent
objectivity and repeatability. Figure 18 shows the Handwheel Angle,
Vehicle Speed, Lateral Acceleration, and Roll Angle as functions of time
for two tests of the Chevrolet Blazer that were run at approximately the
same speed (40.3 and 40.7 mph). Data from these runs is typical of our
experience with this maneuver.

![](media/image28.wmf){width="4.829133858267716in"
height="3.722047244094488in"}

Since this maneuver uses the programmable steering controller, the
steering control input is once again precisely replicated from
run-to-run. However, the lateral acceleration becomes slightly less
repeatable when the vehicle is in the recovery portion (i.e., while
trying to straighten out after performing the return lane change).

As was discussed above for the NHTSA J-Turn, for runs near the point at
which two-wheel lift first occurs, roll angle repeatability becomes much
worse.

######## Performability 

Objective and repeatable Open-Loop Pseudo-Double Lane Change maneuvers
can easily be performed using a programmable steering controller.

While running this maneuver is straight-forward, we have substantial
concerns about the maneuver itself. Unfortunately, due to lack of
development time, we doubt that the steering inputs used during the
Phase IV Rollover Research correspond to worst case conditions. Work is
needed as to how to adapt this maneuver for different vehicles sizes or
characteristics. Probably at least one year of effort would be required
to develop and refine this maneuver.

**Discriminatory Capability**

Testing for the Open-Loop Pseudo-Double Lane Change maneuver was only
performed using two vehicles, the 2001 Chevrolet Blazer and the 2001
Toyota 4Runner (both with the yaw stability control enabled and
disabled). Two different steering inputs were used for this Open-Loop
Pseudo-Double Lane Change testing, one that simulated the ISO 3888 Part
2 Double Lane Change and one that simulated the Consumers Union Short
Course Double Lane Change.

For the simulated ISO 3888 Part 2 Double Lane Change, the Chevrolet
Blazer had two-wheel lift while the Toyota 4Runner with yaw stability
control enabled and disabled did not. However, the maneuver entry speed
at which the Chevrolet Blazer had two-wheel lift was substantially (5
mph) higher than the maximum speed at which Toyota 4Runner testing was
stopped. When yaw stability control was disabled, the speed at which
Toyota 4Runner testing was stopped was determined by when spin-out
occurred. When yaw stability control was enabled, the speed at which
Toyota 4Runner testing was stopped was determined by test driver
concerns about possible loss of control. So two-wheel lift was seen for
the Chevrolet Blazer but not the Toyota 4Runner because the Blazer was
able to perform this maneuver at higher speeds than was the 4Runner. As
was the case for the actual ISO 3888 Part 2 Double Lane Change, handling
and rollover resistance appear to be inextricably intertwined in the
ratings produced by this maneuver.

For the simulated Consumers Union Short Course Double Lane Change, the
Chevrolet Blazer and the Toyota 4Runner with yaw stability control
disabled had two-wheel lift while the Toyota 4Runner with yaw stability
control enabled did not. The maneuver entry speed at which the Chevrolet
Blazer had two-wheel lift was higher than the maximum speed at which
Toyota 4Runner two-wheel lift occurred. However, based on its one star
rating and performance in the NHTSA J-Turn and Fishhooks, we believe the
Chevrolet Blazer to have the lowest rollover resistance of any of the
Phase IV rollover test vehicles. The explanation for this apparent
anomaly is that, as was the case for the actual Consumers Union Short
Course Double Lane Change, handling and rollover resistance appear to be
inextricably intertwined in the ratings produced by this maneuver.

Because this maneuver is not focused solely on a vehicle's rollover
resistance but instead measures some combination of handling and
rollover resistance properties, its discriminatory capability for
rollover resistance is poor.

**Realistic Appearance**

The Realistic Appearance discussion from the Ford Path Corrected Limit
Lane Change again applies.

[^1]: For brevity, we use the term Alight trucks@ in this document to
    refer to vans, minivans, sport utility vehicles (SUVs), and pickup
    trucks under 4,536 kilograms (10,000 pounds) gross vehicle weight
    rating. NHTSA has also used the term ALTVs@ to refer to the same
    vehicles.

[^2]: A broken hip is an example of an AIS 3 injury.

[^3]: NHTSA notes that if the stringency of a rollover maneuver test was
    determined by averaging the entry and exit speeds, a test in which
    the vehicle performed automatic braking would be considered less
    stringent than one in which the vehicle entered at the same speed
    and coasted through at a higher speed.

[^4]: Finding 3-5, "The current practice of approximating the rollover
    curve with five discrete levels does not convey the richness of the
    information provided by available crash data." "An Assessment of the
    National Highway Traffic Safety Administration's Rating System for
    Rollover Resistance," TRB NRC, prepublication copy February 21,
    2002, page 3-27.

[^5]: Ivey, D. L., Sicking, D. L., "Influence of Pavement Edge and
    Shoulder Characteristics on Vehicle Handling and Stability,"
    Transportation Research Record 1084.

[^6]: We noted that the predicted rollover risk of vehicles at the low
    end of the SSF range in Figure 1 was considerably larger for the
    model including dynamic maneuver results than for the logistic model
    using SSF only. This is due in part to an apparent limitation in the
    form of the risk prediction curve with a single independent variable
    inherent to the basic logistic regression procedure that prevents
    the line from having sufficient curvature to follow the trend in
    rollover risk versus SSF in the data set presented to the model. The
    exponential risk curve upon which our current SSF rollover
    resistance ratings are based agrees more closely with the logistic
    model operating on both the SSF and the hypothetical dynamic
    maneuver tests. Our current rating system also agrees more closely
    with the actual rollover rates of vehicles than does the basic
    logistic regression procedure operating on SSF alone. We expect to
    overcome the limitation in the form of the risk prediction curve of
    the logistic regression model operating on SSF alone by using
    transformations of SSF (log(SSF) for example) as the vehicle
    variable. Once we have achieved a model with the goodness of fit of
    our current exponential model and the narrow confidence limits of
    the logistic model recommended by NAS, we can add the dynamic
    maneuver test results with the certainty that we are refining the
    risk prediction rather than compensating for the deficiencies of the
    base model. In the example of Figure 1, we would not expect much
    change in the points representing the risk predictions of the 25
    vehicle with both SSF and dynamic maneuver test results. The use of
    multiple variables tends to free the model of the restrictions in
    form that are otherwise manifested in a single variable model by the
    need to represent an exponential risk relationship by single
    continuous line with a large change in curvature in our data range.
    However, we would expect the line representing an improved logistic
    model with SSF only to conform more closely to the actual vehicle
    rollover rates, and we would expect the spread between the SSF line
    and the vehicle points to represent only the effect of the dynamic
    performance of the vehicle.

[^7]: The example of Figure 1 shows substantial differences in risk
    prediction by standard logistic regression when hypothetical dynamic
    test results are added to a model using only SSF to describe the
    vehicle**.** This example demonstrates the potential value of adding
    dynamic test results to the logit model because the predictions that
    include the hypothetical dynamic test results more closely match the
    actual rollover rates.

[^8]: Copied from Page 4 of Ford Motor Company's submission of August
    16, 2001 in response to NHTSA notice Consumer Information
    Regulations; Rollover Resistance, Docket No. NHTSA-2001-9663 (66
    Fed. Reg. 35179-35193, July 3, 2001). Referred to subsequently as
    Ford's 2001 Rollover Comments

[^9]: Copied from Page 5 of Ford's 2001 Rollover Comments

[^10]: Copied from Page 1 of a Ford Motor Company memorandum titled
    "Dynamic Weight Transfer Results from Path-Corrected Limit Lane
    Change Joint Testing with NHTSA." Referred to subsequently as Ford's
    PCLLC Report.

[^11]: Copied from Page 3 of Ford's 2001 Rollover Comments

[^12]: Copied from Page 1 of Appendix III of Ford's 2001 Rollover
    Comments

[^13]: Copied from Page 2 of Ford's PCLLC Report

[^14]: Values taken from Page 2 of Ford's PCLLC Report

[^15]: Copied from Page 1 of Appendix III of Ford's 2001 Rollover
    Comments

[^16]: Copied from Pages 5 and 6 of Ford's 2001 Rollover Comments

[^17]: Determination of the final composite was necessary because the
    peak handwheel input of a particular test did not necessarily occur
    at the same time as the others. The preliminary composite was used
    to establish trends (e.g., timing, rates, etc.) in the handwheel
    position data. The final composite increased handwheel magnitudes,
    so as to insure maneuver severity was preserved.