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 2001 Fatality Analysis Reporting System (FARS), 10,138 people were killed as occupants in light vehicle rollover crashes, which represent 32 percent of the occupants killed that year in crashes. Of those, 8,407 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 64 percent were partially or completely ejected from the vehicle (including 53 percent who were completely ejected).  FARS shows that 54 percent of light vehicle occupant fatalities in single-vehicle crashes involved a rollover event.

Using data from the 1997-2001 National Automotive Sampling System (NASS) Crashworthiness Data System (CDS), we estimate that 281,000 light vehicles were towed from a police-reported rollover crash each year (on average), and that 30,000 occupants of these vehicles were seriously injured or killed (defined as any fatality or an injury with an Abbreviated Injury Scale (AIS) rating of at least AIS 3).^[2]  Of these 281,000 light vehicle rollover crashes, 225,000 were single-vehicle crashes.  (The NCAP rollover resistance ratings estimate the risk of rollover if a vehicle is involved in a single-vehicle crash.)  Sixty-one percent of those people who suffered a serious injury in single-vehicle towaway rollover crashes were not using a seat belt, and 49 percent were partially or completely ejected (including 40 percent who were completely ejected).  Estimates from NASS CDS indicate that 80 percent of towaway rollovers were single-vehicle crashes, and that 83 percent (168,000) 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 1997-2001 NASS General Estimates System (GES) data, 62,000 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 215,000 single-vehicle rollover crashes resulted in 49,000 K or A injuries.  Fifty percent of those with K or A injury in single-vehicle rollover crashes were not using a seat belt, and 24 percent were partially or completely ejected from the vehicle (including 21 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 percentages of all police-reported crashes for each vehicle type that resulted in rollover were 1.7 percent for cars, 2.0 percent for vans, 3.8 percent for pickup trucks and 5.5 percent for SUVs as estimated by NASS GES.

III.       Background

A.        Existing NCAP Program and the TREAD Act

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 (66 FR 3388).  This notice does not establish a new consumer information program on rollover resistance ratings.  Rather, it refines our existing rollover resistance rating program in accordance with the requirements of the TREAD Act and the recommendations of the National Academy of Sciences.   

The present NCAP rollover resistance ratings are based on the Static Stability Factor (SSF) of a vehicle, which is the ratio of one half its track width to its center of gravity (c.g.) height (see www.nhtsa.dot.gov/hot/rollover/ for ratings and explanatory information).  After an evaluation of some driving maneuver tests in 1997 and 1998, we chose to use SSF instead of any driving maneuvers to characterize rollover resistance.  As we explained in our notices establishing rollover NCAP, we chose SSF as the basis of our ratings because it represents the first order factors that determine vehicle rollover resistance in the vast majority of rollovers which 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, but such crashes represent less than five percent of rollover crashes^[3].

At the time, we believed it was necessary to choose between SSF and driving maneuver tests as the basis for rollover resistance ratings.  SSF was chosen because it had a number of advantages: it 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.  We also considered the fact that an improvement in SSF represents an increase in rollover resistance in both tripped and untripped circumstances while maneuver test performance can be improved by reduced tire traction and certain implementations of electronic stability control that we believe are unlikely to improve resistance to tripped rollovers.

Congress funded NHTSA's rollover NCAP program, but directed the agency to enhance the program.  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 was to be carried out by November 1, 2002. 

On July 3, 2001, NHTSA published a Request for Comments notice (66 FR 35179) regarding our research plans to assess a number of possible dynamic rollover tests.  The notice discussed the possible advantages and disadvantages of various approaches that had been suggested by manufacturers, consumer groups, and NHTSA's prior research.  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.  The principal theme of the comments was a sharp division of opinion about whether the dynamic rollover test should be a closed loop maneuver test like the ISO 3388 double lane change that emphasizes the handling properties of vehicles or whether it should be an open loop maneuver like a J-Turn or Fishhook that are limit maneuvers in which vulnerable vehicles would actually tip up.  Ford recommended a different type of closed loop lane change maneuver in which a path-following robot or a mathematical correction method would be used to evaluate all vehicles on the same set of paths at the same lateral acceleration.  It used a measurement of partial wheel unloading without tip-up at 0.7g lateral acceleration as a performance criterion in contrast to the other closed loop maneuver tests that used maximum speed through the maneuver as the performance criterion.  Another unique comment was a recommendation from Suzuki to use a sled test developed by Exponent Inc. to simulate tripped rollovers.  

The subsequent test program (using four SUVs in various load conditions and with and without electronic stability control enabled on two of the SUVs) showed that open-loop maneuver tests using an automated steering controller could be performed with better repeatability of results than the other maneuver tests.  The J-Turn maneuver and the Fishhook maneuver (with steering reversal at maximum vehicle roll angle) were found to be the most objective tests of the susceptibility of vehicles to maneuver-induced on-road rollover.  Except for the Ford test, the closed loop tests were found not to measure rollover resistance.  Instead, the tests of maximum speed through a double lane change responded to vehicle agility.  None of the test vehicles tipped up during runs in which they maintained the prescribed path even when loaded with roof ballast to experimentally reduce their rollover resistance.  The speed scores of the test vehicles in the closed loop maneuvers were found to be unrelated to their resistance to tip-up in the open-loop maneuvers that actually caused tip-up.  The test vehicle that was clearly the poorest performer in the maneuvers that caused tip-ups achieved the best score (highest speed) in the ISO 3388 and CU short course double lane change, and one vehicle improved its score in the ISO 3388 test when roof ballast was added to reduce its rollover resistance.

Due to the non-limit test conditions and the averaging necessary for stable wheel force measurements, the wheel unloading measured in the Ford test appeared to be more quasi-static (as in driving in a circle at a steady speed or placing the vehicle on a centrifuge) than dynamic.  Sled tests were not evaluated because we believed that SSF already provided a good indicator of resistance to tripped rollover.   

B.        National Academy of Sciences Study

During the time NHTSA was evaluating dynamic maneuver tests in response the TREAD Act, the National Academy of Sciences (NAS) was conducting a study of the our SSF-based rollover resistance ratings and was directed to make recommendations regarding driving maneuver tests.  We expected the NAS recommendations to have a strong influence on TREAD-mandated changes to NCAP rollover resistance ratings.       

When NHTSA proposed the present SSF rollover resistance ratings in June 2000 (65 FR 34998), vehicle manufacturers generally opposed it because they believed that SSF as a measure of rollover resistance is too simple since it does not include the effects of suspension deflections, tire traction and electronic stability control (ESC).  In addition, the vehicle manufacturers argued that the influence of vehicle factors on rollover risk is too slight to warrant consumer information ratings for rollover resistance.  In the conference report of the FY2001 DOT Appropriations Act, Congress permitted NHTSA to move forward with its rollover rating program, but directed the agency to fund a National Academy of Sciences (NAS) 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 available at the end of February 2002.

The NAS study found that SSF is a scientifically valid measure of rollover resistance for which the underlying physics and real-world crash data are consistent with 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 dynamic tests the NAS recommended would be driving maneuvers used to assess "transient vehicle behavior leading to rollover."

The NAS study also made recommendations concerning the statistical analysis of rollover risk and the representation of ratings.  It recommended that we use logistic regression rather than linear regression for analysis of the relationship between rollover risk and SSF, and it recommended that we consider a higher-resolution representation of the relationship between rollover risk and SSF than is provided by the current five-star rating system.

We published a Notice of Proposed Rulemaking on October 7, 2002 (67 FR 62528) that proposed alternative ways of using the dynamic maneuver test results in consumer information on the rollover resistance of new vehicles.  We chose the J-Turn and Fishhook maneuver (with roll rate feedback) as the dynamic maneuver tests because they were the type of limit maneuver tests that could directly lead to rollover as recommended by the NAS.  We also proposed to use a logistic regression analysis to determine the relationship between vehicle properties and rollover risk, as recommended by the NAS.   The resulting rollover resistance ratings were proposed to be part of NHTSA's New Car Assessment Program (NCAP).   Also, we proposed two methods for presenting rollover resistance ratings for consumer information.

IV.       Notice of Proposed Rulemaking

The TREAD Act calls for a rulemaking to determine how best to disseminate rollover test results to the public, and our Notice of Proposed Rulemaking (NPRM) of October 7, 2002 (67 FR 62528) proposed two alternatives for using the dynamic test results in consumer information on the rollover resistance of new vehicles.  In this case the term  "rulemaking" refers more to the process than to the product.    This document does not amend the code of Federal Register, but establishes NHTSA's policy on consumer information regarding the rollover resistance program.     As mentioned above, this program places no requirements on vehicle manufacturers, only some on NHTSA. 

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 chose the J-Turn and the Fishhook maneuvers as dynamic rollover tests.  They are the limit maneuver tests that NHTSA found to have the highest levels of objectivity, repeatability and discriminatory capability.  The notice announced that vehicles would 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 would be conducted with an automated steering controller, and the reverse steer of the Fishhook maneuver would 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.  Figures 1 and 2 illustrate the open-loop steering wheel motions characterizing these maneuvers.  The light load condition would be the weight of the test driver and instruments, approximating a vehicle with a driver and one front seat passenger.  The notice announced that the heavy load condition would 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 in consumer information on rollover resistance.  NHTSA proposed two alternatives for consumer information ratings on vehicle rollover resistance that included both dynamic maneuver test results and Static Stability Factor.  The first alternative was to include the dynamic test results as vehicle variables along with SSF in a statistical model of rollover risk that would combine their predictive power.  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 recommended 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 its rollover rate in single-vehicle crashes predicted 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 proposed 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 would be simply whether it tipped up or not in each of the four maneuver/load combinations.     

In order to compute this logistic model for rollover risk, it is necessary to have the dynamic maneuver test results as well as SSF for a number of vehicles with rollover rates established by state crash reports of single-vehicle crashes.   We had the SSF measurements and established rollover rates for the 100 make/model vehicles upon which we based the static rating system but not their dynamic maneuver test results.   Thus, we asked for comment on the suitability of a rating method that combines static and dynamic vehicle properties in a single rating and on the validity of logistic regression analysis for the risk model that combines the properties in a way that is predictive of real-world crash experience.

The NPRM notice announced that we were going to perform the dynamic maneuver tests on about 25 of the 100 make/model vehicles for which we had SSF measurements and substantial state crash data.  Time and budget constraints would not permit testing all 100 vehicles.  With these dynamic maneuver test results and our existing crash and SSF information we would be able to compute the new risk model using a standard statistical package of computer programs (SAS) for logistic regression analysis.  This document notice presents the dynamic maneuver test results for 24 of the 100 vehicles, chosen to span the SSF range and to represent high production vehicles of each type (passenger car, van, pickup truck and sport utility vehicle (SUV)).  An additional SUV with a lower SSF than found among the 100 vehicles was also included.  The resulting risk model is presented in this document notice.

The second alternative we proposed was to have separate ratings for Static Stability Factor and for dynamic maneuver test performance.  Dynamic maneuver tests directly represent on-road untripped rollovers.  Under this alternative, the dynamic maneuver test performance would be used to rate resistance to untripped rollovers in a qualitative scale.  Barring unforeseen results of the dynamic maneuver tests of the 25 vehicle group, the obvious qualitative scale would be: A for no tip-ups, B for tip-up in one maneuver, C for tip-ups in two maneuvers, D for tip-ups in three maneuvers and E for tip-ups in all four maneuvers/load combinations.

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.  For this alternative, the current Static Stability Factor based system would be used to rate resistance to tripped rollovers (since we believe most of the rollovers reported in the state crash reports are tripped).  Again we asked for comments on the usefulness and validity of the concept in the NPRM notice, but we could not offer examples of actual vehicle ratings because the tests had not yet been conducted.

V.        Results of Dynamic Maneuver Tests of 25 Vehicles

This section presents an overview of the test maneuvers and the results for 25 vehicles that were used to develop the logistic regression risk model.  A more extensive account of the test program is contained in the Phase VI and VII Report that has been placed in Docket NHTSA-2001-9663. A detailed description of how we will perform the maneuver tests for NCAP ratings is contained in Appendix I.  

               The NHTSA J-Turn and Fishhook (with roll rate feedback) maneuver tests were performed for 25 vehicles representing four vehicle types including passenger cars, vans, pickup trucks and SUVs.  We chose mainly high production vehicles that spanned a wide range of SSF values, using vehicles NHTSA already owned where possible.  Except for four 2001 model year vehicles NHTSA purchased new, the vehicle suspensions were rebuilt with new springs and shock absorbers, and other parts as required for all the other vehicles included in the test program.

A.        J-Turn Maneuver

The NHTSA J-Turn maneuver represents an avoidance maneuver in which a vehicle is steered away from an obstacle using a single input. The maneuver is similar to the J-Turn used during NHTSA's 1997-98 rollover research program and is a common maneuver in test programs conducted by vehicle manufacturers and others.  Often the J-Turn is conducted with a fixed steering input (handwheel angle) for all test vehicles.  In its 1997-98 testing, NHTSA used a fixed handwheel angle of 330 degrees.  In the testing that preceded the NPRM notice, we developed an objective method of specifying equivalent handwheel angles for J-Turn tests of various vehicles, taking into account their differences in steering ratio, wheelbase and linear range understeer properties.  (See NHTSA's Phase IV report docketed with the NPRM notice as item 38 in Docket No. NHTSA 2001-9663).   Under this method, one first measures the handwheel angle that would produce a steady-state lateral acceleration of 0.3 g at 50 mph on a level paved surface for a particular vehicle.  In brief, the 0.3 g value was chosen because the steering angle variability associated with this lateral acceleration is quite low and there is no possibility that stability control intervention could confound the test results.  Since the magnitude of the handwheel position at 0.3 g is small, it must be multiplied by a scalar to have a high maneuver severity.  In the case of the J-Turn, the handwheel angle at 0.3 g was multiplied by eight.  When this scalar is multiplied by the average handwheel angle at 0.3 g (observed during NHTSA's 1997-98 rollover research program), the result is approximately 330 degrees.  Figure 1 illustrates the J-Turn maneuver in terms of the automated steering inputs commanded by the programmable steering machine.  The rate of the handwheel turning is 1000 degrees per second. 

To begin the maneuver, the vehicle was driven in a straight line at a speed slightly greater than the desired entrance speed.  The driver released the throttle, coasted to the target speed, and then triggered the commanded handwheel input. The nominal maneuver entrance speeds used in the J-Turn maneuver ranged from 35 to 60 mph, increased in 5 mph increments until a termination condition was achieved.  Termination conditions were simultaneous two inch or greater lift of a vehicle's inside tires (two-wheel lift) or completion of a test performed at the maximum maneuver entrance speed without two-wheel lift.  If two-wheel lift was observed, a downward iteration of vehicle speed was used in 1 mph increments until such lift was no longer detected.  Once the lowest speed for which two-wheel lift could be detected was isolated, two additional tests were performed at that speed to monitor two-wheel lift repeatability.

B.        Fishhook Maneuver

The second maneuver test, the fishhook maneuver, uses steering inputs that approximate the steering a driver acting in panic might use in an effort to regain lane position after dropping two wheels off the roadway onto the shoulder.  In the NPRM notice, we described it as a road edge recovery maneuver.  As pointed out by some commenters, it is performed on a smooth pavement rather than at a road edge drop-off, but its rapid steering input followed by an over-correction is representative of a general loss of control situation.  The original version of this test was developed by Toyota, and variations of it were suggested by Nissan and Honda.  NHTSA has experimented with several versions since 1997, and the present test includes roll rate feedback in order to time the counter-steer to coincide with the maximum roll angle of each vehicle in response to the first steer.

Figure 2 describes the Fishhook maneuver in terms of the automated steering inputs commanded by the programmable steering machine and illustrates the roll rate feedback.  The initial steering magnitude and countersteer magnitudes are symmetric, and are calculated by multiplying the handwheel angle that would produce a steady state lateral acceleration of 0.3 g at 50 mph on level pavement by 6.5. The average steering input is equivalent to the 270 degree handwheel angle used in earlier forms of the maneuver but, as in the case of the J-Turn , the procedure above is an objective way of compensating for differences in steering gear ratio, wheelbase and understeer properties between vehicles.  The fishhook maneuver dwell times (the time between completion of the initial steering ramp and the initiation of the countersteer) are defined by the roll motion of the vehicle being evaluated, and can vary on a test-to-test basis.  This is made possible by having the steering machine monitor roll rate (roll velocity).  If an initial steer is to the left, the steering reversal following completion of the first handwheel ramp occurs when the roll rate of the vehicle first equals or goes below 1.5 degrees per second.  If an initial steer is to the right, the steering reversal following completion of the first handwheel ramp occurs when the roll rate of the vehicle first equals or exceeds -1.5 degrees per second.  The handwheel rates of the initial steer and countersteer ramps are 720 degrees per second.

To begin the maneuver, the vehicle was driven in a straight line at a speed slightly greater than the desired entrance speed.  The driver released the throttle, coasted to the target speed, and then triggered the commanded handwheel input described in Figure 2.  The nominal maneuver entrance speeds used in the fishhook maneuver ranged from 35 to 50 mph, increased in 5 mph increments until a termination condition was achieved.  Termination conditions included simultaneous two inch or greater lift of a vehicle's inside tires (two-wheel lift) or completion of a test performed at the maximum maneuver entrance speed without two-wheel lift.  If two-wheel lift was observed, a downward iteration of vehicle speed was used in 1 mph increments until such lift was no longer detected.  Once the lowest speed for which two-wheel lift could be detected was isolated, two additional tests were performed at that speed to check two-wheel lift repeatability.

C.        Loading Conditions

The vehicles were tested in each maneuver in two load conditions in order to create four levels of stringency in the suite of maneuver tests.  The light load was the test driver plus instrumentation in the front passenger seat, which represented two occupants.  A heavier load was used to create a higher level of stringency for each test.   In our NPRM, we announced that the heavy load would include 175 lb anthropomorphic forms (water dummies) in all rear seat positions.  During the test of the 25 vehicles, it became obvious that heavy load tests were being run at very unequal load conditions especially between vans and other vehicles (two water dummies in some vehicles but six water dummies in others).  While very heavy passenger loads can certainly reduce rollover resistance and potentially cause special problems, crashes at those loads are too few to greatly influence the overall rollover rate of vehicles.  Over 94% of van rollovers in our 293,000 crash database occurred with five or fewer occupants, and over 99% of rollovers of other vehicles occurred with five or fewer occupants.  The average passenger loads of vehicles in our crash database was less than two: 1.81 for vans; 1.54 for SUVs; 1.48 for cars; and 1.35 for pickup trucks.  In order to use the maneuver tests to predict real-world rollover rates, it seemed inappropriate to test the vehicles under widely differing loads that did not correspond to the real-world crash statistics.  Therefore, the tests used to develop a statistical model of rollover risk were changed to a uniform heavy load condition of three water dummies (representing a 5-occupant loading) for all vehicles capable of carrying at least five occupants.   Some vehicles were loaded with only two water dummies because they were designed for four occupants.  For pickup trucks, water dummies were loaded in the bed at approximately the same height as a passenger in the front seat.

To avoid disruption, the tests were completed under the original loading plan.  Then we conducted tests at a 5-occupant heavy load only for those vehicles in which loading differences might influence tip-up.  If the vehicle had completed the maneuver without tip-up with more than three water dummies in the rear it was not necessary to retest at a lighter load.  Likewise, if the vehicle tipped up in the light load (no water dummies) condition, it was not necessary to retest with three water dummies in the rear.  We have never observed a vehicle for which a greater passenger load improved performance in a tip-up test.

D.        Test Results

The test results in Table 1 reflect the performance either measured or imputed as described for a heavy load condition representing 5 occupants except for the Ford Explorer 2DR, the Chevrolet Tracker and Metro that were designed for only four occupants, and the Honda CRV, Honda Civic and Chevrolet Cavalier that could not be loaded to the 5 occupant level without exceeding a gross axle weight rating because of the additional weight of the outriggers. 

Note that Table 1 includes some results collected during tests performed with alternative steering angles.  Although the steering angles used during these tests were still based on the handwheel angle that would produce a steady-state lateral acceleration of 0.3 g at 50 mph on a level paved surface, the scalars used to calculate the steering angles were smaller.  These tests

VII.     Comments to the NPRM Notice and Agency Response

               We received 39 comments to the NPRM notice from vehicle manufacturers, equipment suppliers, test labs, public interest groups, the National Transportation Safety Board, the Insurance Institute for Highway Safety, attorneys, and members of the public.  Mainly, the comments addressed whether the static and dynamic measurements should be used for separate ratings of rollover resistance or for a combined rating based on a risk model.  The nature of the dynamic maneuver tests, testing of 15-passenger vans, and several practical testing issues such as the extraneous effects of tire wear, surface condition and ambient temperature were also addressed.  The notice also introduced the related subject of handling ratings that was not part of the TREAD Act requirements.  We received a number of valuable comments on handling tests, and we are still soliciting information.  However, the subject of this notice is confined to the TREAD Act requirements for dynamic rollover ratings.

A.        Combined or Separate Rollover Resistance Ratings 

The main question posed in the NPRM notice was whether the rollover resistance ratings should reflect the combined statistical power of SSF and dynamic tests for predicting rollover risk or whether ratings of rollover risk using SSF alone should continue, supplemented with a qualitative comparison of dynamic test performance.  The notice gave alternative A as a risk model determined by logistic regression analysis of state crash reports of single-vehicle crashes for about 25 vehicles with known SSF and dynamic test results.  That process led to the risk model described in Section VI, however the mathematical calculation of the model could not be performed until the completion of a lengthy dynamic test program.   Alternative B in the notice was a continuation of rollover risk prediction using SSF-only plus qualitative separate dynamic scores of A, B, C, D, or E signifying the number of maneuvers in which the vehicle tripped up without a risk interpretation.

Commenters representing TRW Automotive, National Automobile Dealers Association (NADA), General Motors (GM), Alliance of Automobile Manufacturers (Alliance), Association of International Automobile Manufacturers (AIAM), Insurance Institute for Highway Safety (IIHS), Bosch, Consumers Union, Advocates for Highway and Auto Safety (Advocates), Toyota, Continental-Teves and Public Citizen remarked directly on the question of combined versus separate use of SSF and dynamic maneuver tests in rollover resistance ratings.  Except for Continental-Teves and Bosch, the commenters were in favor of ratings that combined the SSF and dynamic maneuver tests in a single rating.  Consumers Union specifically supported the logit risk model operating on a moderate risk scenario (in which rollover rates vary in the approximate range of 0.075 to 0.55 across the range of vehicles) as a way of combining the SSF and dynamic maneuver tests.  It commented that using the risk model it described was consistent with the recommendations of the NAS study.  We believe the risk model we have developed is consistent with recommendation of NAS and Consumers Union.  It is the logit model with the risk scenario (of demographic and road condition variables) that represents the average crash conditions of 293,000 actual single-vehicle crashes.  It produces predicted rollover rates in the range of 0.09 to 0.50 for vehicles ranging from tip-up to no tip-up in maneuvers and from 1.0 to 1.55 in SSF. 

The other commenters in favor of combined ratings were primarily concerned that separate ratings would be too confusing to serve as consumer information.  They believed a combined rating was the only viable option, but they did not comment specifically on the means used by NHTSA to develop the combined risk model.  IIHS and the Alliance (along with Carr Engineering) suggested that another comment period following the notice containing the actual model (as opposed to the example given in the NPRM notice) would be necessary.  GM suggested that the risk model be developed through a collaborative effort along the lines of the Motor Vehicle Safety Research Advisory Committee, and the Alliance suggested a working-level dialog between NHTSA and the auto industry to develop the risk model.  TRW supported a single rating that would be computed on the basis of the SSF only model with a predetermined number of stars added or subtracted for dynamic maneuver performance (determined without a statistical relationship to risk).   Advocates expressed wariness that the combined rating could be misleading to consumers unless it corresponded to real-world rollover rates.  Public Citizen preferred the combined rating developed from a risk model.  It was concerned that consumers would focus more attention on the dynamic maneuvers in separate ratings although the tests represent an event (on-road untripped rollover) that occurs in less than 5 percent of actual rollover crashes.

Continental-Teves and Bosch prefer separate ratings for SSF and dynamic maneuver tests.  Continental-Teves stated that "the relative effects of SSF and dynamic performance are not well understood, and may not be the same for every vehicle or every driver."  Bosch stated that "static and dynamic ratings should be separate, as they are both equally important with regards to indicating stability and safety of the vehicle."  Bosch further explained that " a combined rating may not adequately show the influence of such systems [Electronic Stability Control and Rollover Mitigation] which in turn would not encourage manufacturers to add systems to vehicles that increase overall vehicle safety in potential rollover as well as many other situations."

B.        Crash Avoidance Technologies

Some of the stated expectations of the commenters about rollover resistance ratings are unrealistic.   The rollover resistance ratings predict the likelihood of a single-vehicle crash becoming a rollover.  They do not predict the likelihood of the vehicle becoming involved in a single-vehicle crash.  Similarly, the frontal and side NCAP crashworthiness ratings do not predict the likelihood of the vehicle striking an object head-on or being struck from the side.  The Alliance comment anticipates the dilemma.   While conceding that SSF is strongly correlated with a tripped rollover once the vehicle is already off-road, it states that " the likelihood of being involved in a single-vehicle crash in the first place - particularly one involving off-road excursion - is influenced much more by demographic and environmental influences than is the scenario examined for SSF purposes."  The scenario used in the combined risk model is the same scenario used in the SSF model, namely the average demographic and environmental variables reported by the states for the entire 293,000 single-vehicle crash data base we have collected.  We think this is the best scenario to characterize single-vehicle crashes.

The Alliance is concerned that our model "may fail to account for potentially beneficial technologies for avoiding single-vehicle and rollover crashes, such as electronic stability control and variable ride high suspension systems."   Its concern is unnecessary for variable ride-height suspension systems, which will be tested in the highway rather than off-road height for both SSF and dynamic maneuver tests, and the technology will certainly improve the rating of vehicles so equipped. 

However, the Alliance is right that the model does not predict the risk of a single-vehicle crash.  NHTSA has been very clear in public notices, consumer information and web site presentations that neither the SSF risk model nor the proposed combined SSF and dynamic maneuver risk model predict the risk of having a single-vehicle crash.  From the standpoint of rollover resistance, single-vehicle crashes are a measure of exposure.  The prediction is of the risk of a rollover resulting from the exposure of the vehicle to a single-vehicle crash.  The risk of rollover in the event of a single-vehicle crash is strongly influenced by vehicle properties, but the vehicle properties of modern vehicles have far less influence in comparison to demographic and environmental factors regarding the risk of a single-vehicle crash in the first place.  However, electronic yaw stability control may provide a real-world reduction in single-vehicle crashes.

We have been optimistic about the potential of electronic yaw stability control to reduce single-vehicle crashes.   NHTSA's consumer information identifies its availability as standard or optional equipment on individual vehicles and explains how it operates to help a driver maintain control in extreme circumstances. One of the reasons we are exploring the possibility of NCAP handling ratings is to describe the effect of yaw stability control on handling predictability.   However, the technology has not been in widespread use long enough to produce much crash evidence for the evaluation of its real-world effectiveness in preventing single-vehicle crashes.  Our previous attempts at evaluating its effectiveness were thwarted by insufficient data. 

Part of the motivation for the NAS study of NHTSA's SSF-based rollover resistance ratings was the Alliance's concern that yaw stability control was not being considered.  In its public oral presentation to the NAS study committee in May 2001, NHTSA said it did not expect yaw stability control to have a large effect on the risk of rollover given a single-vehicle crash.  In its view, the large majority of rollovers were the result of various types of tripping, and SSF represented the most important vehicle attributes in those circumstances.  NHTSA believes that the greatest potential effect of yaw stability control was in reducing single-vehicle crashes in the first place.  Therefore, we suggested to the committee that rather than trying to predict rollovers per single-vehicle crash with dynamic maneuver tests, we should keep SSF for that purpose and adjust the comparative risk for vehicles with yaw stability control by the effect of yaw stability control to reduce exposure to single-vehicle crashes.  However, establishing the effectiveness of yaw stability control would require data not available for at least two or three more years.  Neither the NAS committee nor the Alliance, which was active in providing the committee information, expressed interest in this suggestion.  But the present comments indicate that finding a way to include the crash avoidance potential of yaw stability control is a principal concern of the Alliance and several suppliers of these systems.

IIHS's comment also shows an expectation of more than what is possible for a rollover resistance rating.  It discusses a comparison of the 1997 Jeep Grand Cherokee and 1997 Toyota 4Runner made in one its reports.  In that report, the Toyota had four times the number of fatal rollovers per 100,000 registered vehicles as the Jeep, but they had very similar SSFs.  They also had very similar rollover rates in terms of rollovers per single-vehicle crash that were consistent with their SSFs.  IIHS expects a good dynamic rating to show a large difference between the Grand Cherokee and the 4Runner.  That will not be possible because differences in dynamic maneuver test performance predict only small differences in rollover rate, and, in fact, there is not a large difference in rollover rate between these vehicles in terms of rollovers per single-vehicle crash in our six state crash data base.  The difference is in the definition of rollover rate.  A rollover rate in terms of fatal rollovers per 100,000 vehicles depends on the rate of single-vehicle crashes per 100,000 vehicles and on the occurrence of a fatality in the rollover as well as on the rate of rollover per single-vehicle crash.  The first two of these factors depend primarily on demographic and environmental influences and can mask actual differences or similarities between vehicles as in this case.  Neither vehicle had yaw stability control, which would have created a plausible vehicle-related difference in single-vehicle crash rate.  The difference in fatality rate could involve crashworthiness features, or particularly in the case of rollover, it could merely reflect the seat belt wearing habits of a risk taking demographic that also experienced a higher rate of single-vehicle crashes.  The rate of rollovers per single-vehicle crash is much less sensitive to demographic influences than is the rate of fatal rollovers per 100,000 vehicles. 

Carr Engineering and Suzuki commented that the agency was not following the recommendations of the NAS study by performing J-Turn and Fishhook maneuver tests.  They believe that the NAS recommended handling tests to assess loss of control potential rather than limit maneuvers to assess the resistance of the vehicle to actual on-road tip-up.  We agree that the language of the NAS study report is somewhat ambiguous.  That is why we included in our NPRM notice the clarification the NAS study panel gave us during the presentation of the report to NHTSA in response to our direct questions about J-Turn and Fishhook tests versus handling tests.  The NAS study committee clarified that it envisioned dynamic maneuver 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 stated it was not in a position to recommend a specific test because that would require study of discriminatory capability, repeatability and other properties, but J-Turns and Fishhooks were of the type of tests it had in mind.  Two outside experts in vehicle dynamics and testing reviewed our test plan before the Phase VI test of the 25 vehicles.  One had been a member of the NAS study committee.  Once again, we were assured that our tests were consistent with the NAS recommendations.

We believe that both our test selection and our analysis method of developing a rollover risk model to combine SSF and dynamic test results are entirely consistent with the recommendations of the NAS study and therefore appropriate to satisfy the requirements of the TREAD Act.  We agree that it is important to inform consumers of the effectiveness of yaw stability control in reducing single-vehicle crashes, and we will determine its effectiveness from crash report data as sufficient data becomes available.

C.        The J-Turn and Fishhook Maneuvers

There were a number of comments regarding the J-Turn and Fishhook test protocols from the Alliance, GM, Toyota, Honda, Nissan, Renfroe Engineering, Carr Engineering, Mechanical Systems Analysis Inc, and Automotive Testing Inc.  In addition, Ford made a detailed presentation elaborating on some of the subjects introduced in the Alliance comment.  The Ford presentation material was placed in Docket NHTSA-2001-9663.     

A number of the commenters objected to the J-Turn maneuver because they thought it was not representative of real driving, involved too fast a steering movement, or was redundant.  Since its results were not used in the risk model, we agree that it is redundant.  As a result, we are no longer planning to use it in the NCAP testing program. 

Except for Suzuki, Carr Engineering and Ford, those who commented on the maneuver tests supported the Fishhook maneuver.  Carr Engineering and Advocates objected to calling the Fishhook maneuver a road edge recovery test as we had done in the NPRM notice.  While the Fishhook maneuver includes steering commands like a crash involving road edge recovery, it is performed on a smooth uniform surface instead of one with vertical drop-offs and friction coefficients differences that exist