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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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1
Introduction

Motor vehicle rollovers have been a source of concern for more than 30 years, not only because of the resulting fatalities and injuries, but also because they carry a relatively high risk of occupant death or injury as compared with other types of crashes. In 1999, 10,142 people were killed in light-vehicle1 rollovers—almost a quarter of the 41,717 traffic crash victims in the United States for that year.2 During the period 1995–1999, 7 percent of light-vehicle tow-away crashes involved rollover, but these crashes accounted for 31 percent of light-vehicle occupant fatalities (Kratzke 2001). The risk of death or injury is particularly high for single-vehicle rollovers, which represent approximately 80 percent of light-vehicle rollover crashes (Garrott and Boyd 2001). The Insurance Institute for Highway Safety (2000, 1) has noted that “single-vehicle crashes involving rollover accounted for 43 occupant deaths per million registered passenger vehicles in 1999, compared with 10 deaths per million in multiple-vehicle crashes.” In 1999, 8,345 people were killed in single-vehicle rollovers, representing 26 percent of all light-vehicle occupant fatalities3 for that year, and during the period 1995–1999, an average of 19,000 people annually suffered severe injuries in such crashes (Garrott and Boyd 2001). These data indicate that a reduction in light-vehicle rollovers—particularly those involving single vehicles—would likely lead to a decrease in the total numbers of occupant deaths and injuries resulting from motor vehicle crashes.

All automobile crashes—including rollovers—are complex events. Three main factors, and interactions among them, contribute to a crash: the driver, the driving environment (e.g., weather and road conditions, time of day), and the vehicle. Most experts acknowledge that reductions in the number of deaths and the number and severity of injuries associated with rollover likely would result from a combination of

  • Changes in driver behavior, notably an increase in seat belt use;

  • Design improvements in both roadsides and roadside structures, particularly in rural areas; and

1

Light vehicles are defined by the National Highway Traffic Safety Administration (NHTSA) as the combination of (1) passenger cars and (2) multipurpose passenger vehicles under 10,000 pounds gross vehicle weight rating.

2

Fatality data taken from the Fatality Analysis Reporting System (FARS) for 1999.

3

Rollover-related fatalities are not evenly distributed across vehicle classes. In 1999, single-vehicle rollover crashes accounted for 51 percent of occupant deaths in sport utility vehicles, compared with 36 percent of deaths in pickups and 19 percent of deaths in cars (IIHS 2000).

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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  • Vehicle modifications that would reduce the likelihood of rollover and provide additional occupant protection should rollover occur.4

Policy decisions about the need for and scope of federal government action in one or more of the above areas—and about relative priorities among the three areas—involve complex technical, social, and financial considerations. The present study was requested by Congress to inform its investigation of the rollover issue, with particular emphasis on the potential role of vehicle characteristics and related consumer information in achieving a reduction in rollover-related deaths and injuries. Public Law 106-346 (Department of Transportation and Related Agencies Appropriations Act, 2001) requires the U.S. Department of Transportation to fund a study by the National Academy of Sciences on “whether the static stability factor [SSF]5 is a scientifically valid measurement that presents practical, useful information to the public, including a comparison of the [SSF] test versus a test with rollover metrics based on dynamic driving conditions that may induce rollover events.”6

NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION’S INITIATIVES ON ROLLOVER

The National Highway Traffic Safety Administration’s (NHTSA’s) formal initiatives on rollover began in 1973 with the issuance of an advance notice of proposed rulemaking for a safety standard that would specify minimum performance requirements for rollover resistance. A related program of research was undertaken to investigate the handling and stability of different types of vehicles in severe steering maneuvers associated with untripped rollovers.7 Action on the proposed rollover resistance standard was terminated in 1978 because untripped rollover was found to be difficult to predict and to accomplish in tests, even on highly skid-resistant surfaces. Computer simulation of dynamic testing was tentatively identified as a more repeatable alternative to full-scale track testing (Federal Register 2000).

In the late 1980s, several groups and individuals renewed efforts to persuade NHTSA to develop a safety standard for rollover resistance. A 1986 petition by Congressman Wirth asked NHTSA to establish a standard based on

4

The characteristics and features of a motor vehicle that affect its safety can be classified into two broad categories: those helping the driver avoid a crash (crash avoidance) and those helping to protect vehicle occupants from harm during a crash (crashworthiness) (TRB 1996).

5

SSF is briefly defined later in this chapter; detailed technical discussion of its meaning and use is provided in Chapter 2.

6

The full text of the congressional request is given in Appendix A.

7

Untripped rollovers are defined by NHTSA as those for which there is no apparent tripping mechanism—such as a curb or roadside feature—other than normal surface friction. Tripped rollovers are defined as those resulting from contact with a mechanical obstacle (tripping mechanism) such as a curb or other surface irregularity. NHTSA’s classification of rollovers as either tripped or untripped is discussed in Chapters 2 and 3.

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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a minimum allowable value of SSF. This request was denied on the grounds that the proposed approach would “neither adequately encompass the causes of vehicle rollover nor satisfactorily ameliorate the problem,” although NHTSA acknowledged that “a vehicle’s stability factor has some relation to its overall involvement in rollover accidents” (Federal Register 1987). A 1988 petition by Consumers Union for a safety standard to protect vehicle occupants against “unreasonable risk of rollover” led to a 5-year vehicle and data analysis program during which NHTSA studied more than 100,000 single-vehicle rollover crashes.

In response to the requirements of the 1991 Intermodal Surface Transportation Efficiency Act, NHTSA issued an advance notice of proposed rule-making for a minimum performance standard for rollover resistance. In 1994, this rulemaking effort was terminated on the grounds that a standard based on static vehicle measurements “would not appreciably decrease crash fatalities and injuries in rollovers” (Federal Register 2000). Similar reasons were cited in NHTSA’s 1996 denial of a petition from Advocates for Auto and Highway Safety and the Insurance Institute for Highway Safety to reconsider the termination of rulemaking on a rollover standard. In addition, the agency noted that such a standard would eliminate a popular vehicle type—the compact sport utility vehicle (SUV).

The 1994 notice from NHTSA terminating work on the development of a rollover standard also proposed a new consumer information regulation requiring manufacturers to label vehicles with information on rollover stability based on either tilt table angle or critical sliding velocity.8 After 20 years spent considering various options for a rollover standard, NHTSA shifted its focus to consumer information, and has pursued this approach ever since.

In September 1994, Congress requested a study by the National Academy of Sciences on the communication of vehicle safety information to consumers, and required NHTSA to review the results of that study before issuing a final rule on vehicle rollover labeling. Following publication of the study report, Shopping for Safety (TRB 1996), and a revised rulemaking proposal from NHTSA, a final rule on a modified SUV rollover warning label was issued in 1999 (CFR 1999).9 Under the new rule, utility vehicles with a wheel-base of 110 inches or less are required to have the rollover alert label shown in Figure 1-1 on the driver side sun visor.

In parallel with its efforts during the late 1990s to develop a revised SUV rollover warning label, NHTSA initiated a project to develop a dynamic test for rollover and control stability in light vehicles. This action was taken, in part, in response to a petition from Consumers Union asking the agency

8

A discussion of static vehicle metrics, including tilt table angle and critical sliding velocity, is provided in Chapter 2.

9

Before this rulemaking, small and mid-sized SUVs were required to have a text-only warning label; guidelines were provided for label size, style, and content. For the modified warning label, the use of graphics, bright colors, and short bulleted text messages is mandatory.

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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FIGURE 1-1 Rollover alert label. (SOURCE: CFR 1999.)

to develop a test of vehicle emergency handling and to provide test results on new vehicles to the public as consumer information. In July 1999, NHTSA published the results of its research on dynamic emergency handling maneuvers that can induce on-road, untripped rollover (Garrott et al. 1999). The agency concluded that “several maneuvers appear to be able to discriminate between vehicles [that have] low static and dynamic rollover propensity measures and those that do not.”

NHTSA’s work on dynamic testing is continuing in response to the requirements of the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act of November 2000 (Public Law 106-414). This legislation requires NHTSA to develop a dynamic test for consumer information on rollover, conduct appropriate tests, and determine how best to disseminate the resulting information to the public. These actions are to be completed by November 1, 2002. In July 2001, NHTSA issued a request for comment on its plans to evaluate several driving maneuver tests for rollover resistance in accordance with the requirements of the TREAD Act (Federal Register 2001b).

In June 2000, NHTSA issued a request for comments on a proposed rollover consumer information program based on SSF (Federal Register 2000). As noted in the request for comments, this recent initiative reflects a change in NHTSA’s focus from untripped to tripped rollovers, and a resulting reassessment of the agency’s previous view (Federal Register 1987) regarding the efficacy of SSF:

Since the vast majority of rollovers are tripped, we have now decided that primary consumer information should be based on factors relevant to tripped as well as untripped rollover, and we have reconsidered the merits of Static Stability Factor as an indicator of rollover risk for consumer information. (Federal Register 2000)

Crash reports in the National Automotive Sampling System Crashworthiness Data System distinguish between tripped and untripped rollovers,

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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although it is generally acknowledged that practical difficulties in interpreting field data can make such a distinction difficult (Woodill and Brophy 2001). According to NHTSA’s National Center for Statistics and Analysis, the vast majority of rollovers are tripped.10 An average of 7,866 untripped rollovers occurred annually during the period 1992–1996, constituting approximately 4 percent of all rollover crashes involving cars, light trucks, and vans (Federal Register 2000).

NHTSA’s STAR RATINGS FOR ROLLOVER RESISTANCE

In January 2001, NHTSA issued its final rule to provide consumers with star ratings for rollover resistance based on SSF (Federal Register 2001a). The definitions of the star ratings are given in Box 1-1. A five-star rating indicates the highest rollover resistance and a one-star rating the lowest. The ratings provide an estimate of the probability of rolling over in a single-vehicle crash, but do not predict the likelihood of that crash or the type or severity of injuries expected.

Rollover resistance ratings for several vehicles have been incorporated into NHTSA’s New Car Assessment Program (NCAP). This program is the main source of information on motor vehicle safety made available to consumers by the federal government. NCAP provides information on frontal

BOX 1-1

NHTSA’s Rollover Resistance Ratings

In a single-vehicle crash, a vehicle with a rating of

Five stars

*****

has a risk of rollover of less than 10 percent.

Four stars

****

has a risk of rollover between 10 percent and 20 percent.

Three stars

***

has a risk of rollover between 20 percent and 30 percent.

Two stars

**

has a risk of rollover between 30 percent and 40 percent.

One star

*

has a risk of rollover greater than 40 percent.

SOURCE: Federal Register 2001a.

10

As discussed in Chapters 2 and 3, this report is not concerned with making the distinction between tripped and untripped rollovers.

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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and side crash ratings, as well as rollover resistance ratings, for the most popular light vehicles (passenger cars, SUVs, light trucks, and vans). Each of the individual ratings uses the star system, with five stars indicating the best performance and one star the worst.

In general, SUVs receive between one and three stars for rollover resistance, pickup trucks between one and four stars, vans two or three stars, and passenger cars four or five stars. The rollover information on NHTSA’s website (www.nhtsa.dot.gov) includes guidance on interpreting the ratings, and notes that, “[as with] side crash ratings, it is possible to compare vehicles from different classes when looking at rollover resistance ratings.” Information is provided on what consumers can do to reduce rollover risk; the importance of wearing a seat belt to reduce the risk of death or serious injury in a rollover crash is emphasized. The value of SSF, which as noted forms the foundation for NHTSA’s ratings for rollover resistance, is listed for each rated vehicle.

Static Stability Factor

The SSF of a vehicle is defined as its track width, T, divided by twice its center of gravity height, H; i.e., SSF = T/2H. The at-the-curb value of H is typically just over 20 inches for a passenger car and several inches higher for an SUV. The corresponding values of SSF are approximately 1.35–1.45 for passenger cars and 1.05–1.20 for SUVs (Heydinger et al. 1999). Typically, loading the vehicle changes the center of gravity height. For many SUVs, the center of gravity height increases—and SSF decreases—when the vehicle is loaded because the loads are placed above the center of gravity of the empty vehicle. For many passenger cars, loading results in a minimal change in center of gravity height and SSF (Heydinger et al. 1999).

According to NHTSA’s analyses of 220,000 actual single-vehicle crashes, taller, narrower vehicles, such as SUVs, are more likely than lower, wider vehicles, such as passenger cars, to trip and roll over (NHTSA 2001). NHTSA’s five-star rollover resistance rating system is based on a statistical correlation between SSF and probability of rollover in a single-vehicle crash, as determined from crash data (Federal Register 2000, 2001a). The number of stars awarded to a vehicle depends on the value of SSF, as shown in Table 1-1.

TABLE 1-1 Relationship Between NHTSA’s Rollover Resistance Star Ratings and SSF Values

Star

SSF

Comments

1

1.03 or less

 

2

1.04–1.12

Typical SSF values for SUVs

3

1.13–1.24

4

1.25–1.44

Typical SSF values for passenger cars

5

1.45 or more

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Reactions to the Ratings

Automobile manufacturers and some consumer groups have expressed concern about NHTSA’s decision to base the five-star ratings for rollover resistance on SSF alone (see, for example, Alliance of Automobile Manufacturers 2000; Consumers Union 2000), and view the use of a purely static metric, with no consideration of dynamic vehicle behavior, as a serious deficiency in NHTSA’s ranking of rollover propensity. During its meetings with representatives of automobile manufacturers, the committee established that SSF is one of many metrics—both static and dynamic—that are taken into consideration in optimizing overall vehicle performance. The meaning and use of a variety of static vehicle metrics, as well as features of dynamic testing, are discussed in Chapter 2.

The Alliance of Automobile Manufacturers (2000) has further criticized NHTSA’s rollover resistance ratings on the grounds that (a) the ratings are based on flawed statistical analyses, and (b) consumers are likely to be misled and confused by the star rating system. These issues are addressed in Chapters 3 and 4, respectively.

As noted during presentations to the committee (see Appendix B), other groups, including Advocates for Highway and Auto Safety (2000) and Public Citizen (2001), have acknowledged NHTSA’s rollover resistance rating system as a step in the right direction from the consumer’s perspective, but regard it as insufficient to protect the American public from the injuries and fatalities resulting from rollover crashes. Some of these groups believe a rollover performance standard that reduced the likelihood of rollover would be more effective than consumer information in addressing the fundamental causes of the rollover problem. The committee was unable to obtain any empirical data on the reactions of consumers themselves to NHTSA’s rollover resistance ratings, other than limited information on visits to the rollover sections of the agency’s website (see Chapter 4).

CONSUMER INFORMATION ON MOTOR VEHICLE SAFETY

Meeting Consumer Needs

As noted earlier, in 1994 Congress requested that the National Academy of Sciences conduct an independent study of consumer needs for automotive safety information. The authors of the resulting report, Shopping for Safety (TRB 1996), recommended a number of short-term improvements to existing automotive safety information, as well as the longer-term development of new summary measures to provide consumers with comparative safety information on overall vehicle performance. Such summary measures were identified as potentially more helpful than currently used data in informing consumers’ vehicle

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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buying decisions. Progress on developing summary measures and their relevance in informing the public about rollover are discussed briefly in Chapter 4.

Stimulating Improvements in Vehicle Safety

One of the most important effects of consumer information is that it stimulates manufacturers to modify their products. In the field of automotive safety, NCAP scores for crashworthiness have improved steadily since the program’s inception in 1978, with the largest improvements coming early on. A real-world reduction in the likelihood of fatality for drivers involved in head-on crashes similar to those simulated by the NCAP test is attributed, at least in part, to the NCAP program (TRB 1996). The Insurance Institute for Highway Safety’s frontal offset crash test program also has resulted in significant improvements in vehicle design; among 32 updated vehicle designs tested since 1995, 20 have obtained improved ratings (IIHS 2001).

The incorporation of a rollover resistance rating into NCAP could result in automobile manufacturers modifying vehicle designs to obtain higher ratings for their products. Five-star NCAP crash ratings are often featured by manufacturers when marketing vehicles to safety-conscious consumers. However, it is essential to avoid unintended—and detrimental—consequences in modifying a vehicle to improve a single aspect of its performance. The numerous and complex trade-offs involved in the vehicle design and development process make improving safety overall particularly challenging. Vehicle designers must ensure that vehicles remain safe in a wide range of maneuvers while also endeavoring to respond to consumer preferences regarding vehicle ride and handling. In light of the numerous factors that contribute to overall vehicle safety, techniques such as quantitative risk assessment (see Box 1-2) may be helpful in informing decisions about vehicle safety initiatives11 and placing individual risks in their proper context.

An improvement in rollover resistance could degrade other aspects of vehicle performance and compromise occupant safety in a variety of nonrollover driving scenarios. For example, several manufacturers have pointed out to NHTSA that some changes designed to improve a vehicle’s tilt table performance12 could degrade its control and handling attributes (Federal Register 2000). Similarly, the likelihood of experiencing two-wheel lift in an obstacle-avoidance maneuver13 can be reduced by fitting very “slippery” tires, which also result in a loss of directional control and severe difficulty in steering the vehicle around bends, corners, or obstacles. Nevertheless, experience with the NCAP frontal and side crash ratings and the Insurance Institute for Highway

11

The use of consumer information, as opposed to regulation, may not lead to optimal outcomes (see, for example, TRB 1996, Chapter 4).

12

An explanation of tilt table metrics is given in Chapter 2.

13

Obstacle avoidance maneuvers are discussed in Chapter 2.

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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BOX 1-2

Quantitative Risk Assessment

The large number of vehicles and their accident histories provides analysts with a robust database for conducting studies on how to improve vehicle safety. Generally, an analysis of accident statistics leads to an understanding of the factors contributing to accidents and provides important insights for improving future designs of vehicles, roads, and driver training programs. The committee used statistical analysis in assessing NHTSA’s rollover resistance rating system.

The committee briefly considered alternatives to the analysis of accident statistics for developing insights on how to improve vehicle safety—alternatives that do not depend on an extensive accident history. The analysis technique known as quantitative risk assessment (QRA) was developed to assess the likelihood of major accidents involving nuclear power plants for which there is little or no accident history. The foundations of QRA are rooted in uncertainty analysis and probability theory (see, for example, Garrick and Kaplan 1995). The technique has contributed to improved safety of nuclear facilities throughout the world, and also has proved to be a powerful analytical tool for improving safety in other fields, including marine, chemical, space, defense, and transportation systems.

Future studies addressing broad issues of vehicle safety could benefit from the use of the QRA thought process for scenarios for which there is little or no experience—for example, the impact of new vehicle types on the national vehicle mix or the influence of innovative highway systems on traffic flows. Vehicle manufacturers and government agencies involved in transportation likely would find QRA helpful in analyses of future systems and associated impacts on public safety.

Safety’s frontal offset crash rating indicates it is possible for vehicles to achieve good ratings in several categories simultaneously.

STUDY APPROACH AND ORGANIZATION OF REPORT

To respond to the congressional study request contained in Public Law 106-346, the Committee for the Study of a Motor Vehicle Rollover Rating System undertook three main tasks:

  • A comparison of the information provided by the SSF static metric with that obtained from tests of dynamic vehicle performance;

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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  • An assessment of whether SSF is a valid measure of vehicle rollover propensity, as indicated by the involvement of vehicles with a range of SSF values in actual rollover crashes; and

  • An assessment of whether vehicle test results, both static and dynamic, can be interpreted and used by consumers in making informed decisions about vehicle purchases.

The vehicle dynamics of rollover is described in Chapter 2, which also includes discussion of static measures and dynamic vehicle testing. The mandate for this study did not require the committee to comment on NHTSA’s dynamic vehicle testing activities under the TREAD Act or to recommend one or more dynamic vehicle tests as a basis for consumer information on rollover. Within the context of its charge, however, the committee has provided some comments on the use of dynamic testing in investigating rollover crashes and the associated challenges. Although not specifically asked to do so by Congress, the committee has also included in this chapter comments on the relevance of electronic stability control systems to rollover, in response to a request from NHTSA.

Chapter 3 presents the committee’s review of the statistical analyses used by NHTSA as the foundation for its five-star rating system for rollover resistance. Statistical analyses of rollover crash data have been conducted by Exponent Failure Analysis Associates, Inc. (Exponent) at the request of the Alliance of Automobile Manufacturers (Donelson et al. 2000; Donelson and Ray 2001). The committee has considered these studies within the broad context of its assessment, but a detailed commentary on Exponent’s methodology and results was judged to be beyond the scope of the present study.

The committee’s assessment of NHTSA’s consumer information on rollover is presented in Chapter 4. In the absence of empirical data on consumers’ use of this information, the committee assessed the practicality and usefulness of the rollover resistance ratings by extrapolating from research on a range of consumer products, using its judgment, and evaluating the process used by NHTSA to develop the ratings. The committee’s assessment also draws on the findings and recommendations of the Committee for the Study of Consumer Automotive Safety Information, which prepared the report Shopping for Safety (TRB 1996).

Finally, the committee’s major findings and recommendations for a future approach are presented in Chapter 5.

REFERENCES

Abbreviations

CFR Code of Federal Regulations

IIHS Insurance Institute for Highway Safety

NHTSA National Highway Traffic Safety Administration

TRB Transportation Research Board

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Advocates for Highway and Auto Safety. 2000. Comments on Docket No. NHTSA-2000-6859.

Alliance of Automobile Manufacturers. 2000. Comments on Docket No. NHTSA-2000-6859.

CFR. 1999. Consumer Information Regulations, §575.105 Vehicle Rollover. Title 49, Part 575.

Consumers Union. 2000. Comments on Docket No. NHTSA-2000-6859.

Donelson, A.C., F. Forouhar, and R.M. Ray. 2000. The Relative Importance of Factors Related to the Risk of Rollover Among Passenger Vehicles. Report prepared by Exponent Failure Analysis Associates, Inc., Menlo Park, Calif., and included as Appendix 4 to comment from the Alliance of Automobile Manufacturers on Docket No. NHTSA-2000-6859.

Donelson, A.C., and R.M. Ray. 2001. Motor Vehicle Rollover Ratings: Toward a Resolution of Statistical Issues. Report prepared for the Alliance of Automobile Manufacturers by Exponent Failure Analysis Associates, Inc., Menlo Park, Calif.

Federal Register. 1987. Federal Motor Vehicle Safety Standards; Denial of Petition for Rulemaking; Vehicle Rollover Resistance. Vol. 52, No. 249, Dec. 29, pp. 49,033–49,038.

Federal Register. 2000. Consumer Information Regulations; Federal Motor Vehicle Safety Standards; Rollover Prevention; Request for Comments. Vol. 65, No. 106, June 1, pp. 34,998–35,024.

Federal Register. 2001a. Consumer Information Regulations; Federal Motor Vehicle Safety Standards; Rollover Resistance; Final Rule. Vol. 66, No. 9, Jan. 12, pp. 3,388–3,437.

Federal Register. 2001b. Consumer Information Regulations; Federal Motor Vehicle Safety Standards; Rollover Prevention; Request for Comments. Vol. 66, No. 128, July 3, pp. 35,179–35,193.

Garrick, B.J., and S. Kaplan. 1995. Summary Report: A Risk-Based Approach to the Evaluation and Application of Disarmament Strategies and Technologies. North Atlantic Treaty Organization Advanced Workshop, Visegrád, Hungary, May 19–23.

Garrott, R.W., and P. Boyd. 2001. A Progress Report on Development of a Dynamic Rollover Rating Test. Presented at the Society of Automotive Engineers Government/Industry Meeting, May 14–16.

Garrott, R.W., J.G. Howe, and G. Forkenbrock. 1999. An Experimental Investigation of Selected Maneuvers That May Induce On-Road Untripped, Light Vehicle Rollover—Phase II of NHTSA’s 1997–1998 Vehicle Rollover Research Program. NHTSA, Vehicle Research and Test Center, East Liberty, Ohio.

Heydinger, G.J., R.A. Bixel, W.R. Garrott, M. Pyne, J.G. Howe, and D.A. Guenther. 1999.

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Measured Vehicle Inertial Parameters—NHTSA’s Data Through November 1998, SAE 1999-01-1336. Presented at International Congress and Exposition, Detroit, Mich., March.

IIHS. 2000. Fatality Facts: Passenger Vehicles. Arlington, Va.

IIHS. 2001. Crashworthiness Keeps Getting Better. Status Report, Vol. 36, No. 3, Arlington, Va.

Kratzke, S. 2001. Light Vehicle Rollover: Background on NHTSA’s Activities in This Area. Presented to the Committee for the Study of a Motor Vehicle Rollover Rating System, Washington, D.C., April 11.

NHTSA. 2001. Frequently Asked Questions About Rollover Resistance Ratings. www.nhtsa.dot.gov/hot/rollover/Index.html.

Public Citizen. 2001. Avoidable Deaths and Avoidable Shame: The Dire Need for Vehicle Rollover Prevention and Crashworthiness Standards. Presented by Joan Claybrook to the Committee for the Study of a Motor Vehicle Rollover Rating System, Washington, D.C., May 29.

TRB. 1996. Special Report 248: Shopping for Safety: Providing Consumer Automotive Safety Information. National Research Council, Washington, D.C.

Woodill, R., and J. Brophy. 2001. Collection of NASS CDS Data Relating to Rollover. Presented to the Committee for the Study of a Motor Vehicle Rollover Rating System, Washington, D.C., May 29.

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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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Suggested Citation:"Chapter 1: Introduction." Transportation Research Board. 2002. An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance: Special Report 265. Washington, DC: The National Academies Press. doi: 10.17226/10308.
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TRB Special Report 265 - An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance finds that the static stability factor is a useful indicator of a vehicle's propensity to roll over, but that U.S. government ratings for new cars, light trucks, and sport utility vehicles do not adequately reflect differences in rollover resistance shown by available crash data. According to the report, the five-star system should be revised to allow better discrimination among vehicles and incorporate results from road tests that measure vehicle control and handling characteristics.

Following the National Highway Traffic Safety Administration's (NHTSA's) issuance of vehicle ratings to inform consumers about rollover risk, Congress requested a TRB study to evaluate the appropriateness of the rating system. Motor vehicle rollovers involving passenger cars, vans, pickup trucks, and sport utility vehicles result in approximately 10,000 deaths and 27,000 serious injuries each year in the United States. NHTSA developed a five-star rating system to inform consumers about the rollover resistance of passenger cars and light-duty passenger vehicle trucks.

After thoroughly evaluating NHTSA's development of the rating system, the committee that conducted this study concurred with the agency's reliance on a static measure of vehicle stability but pointed out some inadequacies of the statistical model used to relate this static measure to rollover risk. Alternative statistical approaches would provide a better approximation of risk. The rating system itself was found wanting. The procedures used to develop and test the ratings with consumers through focus groups did not provide credible evidence that consumers understood the message about the actual risk associated with a given vehicle. By being limited to only five levels, the system also discarded valuable information. The data developed by NHTSA could be refined to enable consumers to discriminate better among vehicle models with regard to their rollover experience.

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