Motor Vehicle Rollover Rating System Study


As requested by Congress, this project will assess the adequacy of a proposed federal requirement for consumer safety information about motor vehicle rollover resistance, which would be based upon a static measure of vehicle stability.


For almost 30 years, the National Highway Traffic Safety Administration (NHTSA) has been attempting to address the very serious highway safety problem of motor vehicle rollover crashes. Roughly 9,500 people are killed each year in crashes involving a vehicle that rolls over. Safety groups have petitioned NHTSA to develop a minimum standard for vehicle rollover resistance, and various mandates from Congress over the years have either required the agency to move forward or prohibited the agency from taking action. The agency has investigated the development of dynamic stability tests that might lead to minimum standards, conducted extensive research analyzing rollover crashes, and initiated research into crash avoidance to reduce the frequency of crashes leading to rollovers.

As stated in various notices published in the Federal Register, NHTSA has found it difficult to develop a minimum stability standard that would not simply disqualify whole classes of passenger vehicles (light trucks and sport utility vehicles) that consumers demand. (Demand has grown to the point that roughly half of all new passenger vehicle sales in the United States are for light trucks or sport utility vehicles.) On June 4, 1994, NHTSA indicated through a notice in the Federal Register that it had abandoned efforts to develop a standard (for reasons summarized below) and would instead focus on developing consumer safety information about vehicle stability. On June 1, 2000, the agency issued a notice inviting public comment on its proposal to use a simple measure of stability, referred to as the static stability factor (SSF). The SSF combines track width and center of gravity, two key components of vehicle stability. The SSF is a measure that equals one half of the track width divided by the height of the center of gravity above the road. The SSF was selected from a group of other possible static measures the agency considered, including a tilt table test and a test that involves pulling a vehicle sideways over a small tripping mechanism.

Various safety groups have criticized the agency's proposal, in part because it is a static, rather than dynamic, test. Consumers Union had petitioned the agency in 1996 to develop a dynamic test. NHTSA rejected this proposal for two main reasons. First, the agency has not been able to develop a simple, repeatable test of dynamic performance. According to the agency, typical sharp turning maneuvers on test tracks that induce wheel lift have shown high variability across drivers and even across tests with the same driver. (Tests severe enough to induce rollover regularly -- involving speeds in excess of 45 mph with abrupt turns -- are also hazardous to test drivers.) Second, the agency argues that maneuver-induced rollover is a rare event, even for vehicles with a high center of gravity and narrow track width. Extensive data analyses conducted by the agency indicate that in over 95 percent of rollover crashes, the vehicle was "tripped," or induced to roll over, by events such as hitting some object, plowing into soft dirt on the roadside, or sliding down an embankment.

The automobile industry has been critical of the agency's proposal to use the SSF because it does not take into account vehicle features, such as suspension design or recommended tires and air pressure, that are important components of stability. Variations in such design features are purported to affect the SSF rating within models produced in the same year. The SSF would also not take into account new vehicle features, such as electronic stability control technologies, which are being introduced on new models (first introduced on GM's 1997 Cadillacs).

Shortly after NHTSA issued its notice to rely on the SSF for consumer information, the Senate's Transportation Appropriations subcommittee expressed strong reservations about the proposal. In the report accompanying the subcommittee's DOT appropriations for FY 2001, dated 6/13/00, the Senate criticized the proposal because it "does not take into consideration other driving conditions that induce rollover events, vehicle features to prevent rollover, or the application of technologies to protect occupants during this type of crash." The Senate proposed to prohibit NHTSA from moving forward with its requirement for consumer information pending the results of a study to be performed by the National Academy of Sciences. In the final study request that emerged from the House-Senate Conference on the DOT appropriations, the agency was allowed to proceed with its "proposal to provide rollover rating information to the public." NHTSA proposes to provide SSF ratings to the public on the upcoming (2001) models. In that same report, however, NHTSA was required to "review and respond within 30 days to the [NAS] study findings and recommendations and propose any appropriate revisions to the consumer information program based on that review."

Proposed Technical Activities

Because NHTSA's various efforts to address rollover have been documented through several rulemakings, there is an extensive amount of information available through the associated dockets. Several studies sponsored or conducted by NHTSA are in these dockets, as are extensive comments on the same by interested parties. The NHTSA website has posted electronic documents entered into the docket on NHTSA formal notices since 1996; it has over 280 citations. There are also extensive articles on the subject published through the Society of Automotive Engineers (SAE) technical paper series and in technical journals.

Statement of Task

This study will address the question posed by Congress of "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 first component of the Congressional question requires an assessment of whether the static stability factor (SSF) is a valid measure of vehicle rollover propensity. To make this assessment, the committee will, among other appropriate considerations, compare the SSF for particular models with available information on the actual involvement of such vehicles in rollover crashes and consider information about other possible static measures. The second component of the question requires a comparison between the proposed static test and tests of dynamic performance. To make this assessment, the committee will consider the efforts of NHTSA and others to develop a repeatable, valid dynamic test, and will compare such dynamic tests to the proposed static test. For both components of the Congressional request, an important criteria of utility is whether the test results can be interpreted and used by consumers in making informed decisions about vehicle purchase. Although not specifically asked to do so by Congress, the study will also consider whether safety information about vehicle features, such as electronic stability controls, could be provided to consumers in a practical and meaningful way.

The committee is not tasked with developing a better measure of vehicle stability that has all the desirable features one might wish to improve the quality of safety information provided to consumers. It is being asked to evaluate whether the NHTSA proposal "is a scientifically valid measurement that presents practical and useful information to the public..."

Preliminary Work Plan

The committee will meet three times during the first half of 2001. Two open meetings are envisioned to provide public comment. The first meeting will span multiple days and will include briefings from the sponsor on the history of its rulemakings and notices that led to the current consumer information rating system proposal. Expert panels will be formed (representing different perspectives) to comment upon this proposal. At the second open meeting, alternatives to the NHTSA proposal will be considered including other static tests and possible dynamic tests. Presentations on consumer information and risk communication will also be made during the second meeting. It is envisioned that the third meeting would be closed in its entirety to allow the committee to deliberate on its findings and recommendations and to review its draft report. Subsequent revisions to the report will take place through conference calls. After the report (prepublication manuscript) is delivered, a month of time would be provided for dissemination.


The product, a consensus study report subject to institutional review under National Research Council Report Review Committee guidelines, will be delivered to the Department of Transportation and Congress. The availability of the report will be announced to the media and public through the National Research Council's Office of News and Public Information.

Any reports resulting from this effort will be prepared in sufficient quantity to ensure their distribution to Congress, committee members, NHTSA, and to other relevant parties in accordance with Academy policy. Reports may be made available to the public without restrictions.

Federal Advisory Committee Act (FACA)

The Academy has developed interim policies and procedures to implement Section 15 of the Federal Advisory Committee Act, 5 U.S.C. App. 15. Section 15 includes certain requirements regarding public access and conflicts of interest that are applicable to agreements under which the Academy, using a committee, provides advice or recommendations to a Federal agency. In accordance with Section 15 of FACA, the Academy shall submit to the government sponsor(s) following delivery of each applicable report a certification that the policies and procedures of the Academy that implement Section 15 of FACA have been substantially complied with in the performance of the contract/grant/cooperative agreement with respect to the applicable report.

Public Information About the Project

In order to afford the public greater knowledge of Academy activities and an opportunity to provide comments on those activities, the Academy may post on its website ( the following information as appropriate under its procedures: (1) notices of meetings open to the public; (2) brief descriptions of projects; (3) committee appointments, if any (including biographies of committee members); (4) report information; and (5) any other pertinent information.