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Spring 2002 Vol. 2 No. 1

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Illustration of rollover test with dummy İFord Motor Co. and Wieck Media Services Inc. Rollover and Ratings

How Useful and Accurate Is the Current Five-Star System?

Purchasing a new car, pickup truck, or the increasingly popular sport utility vehicle involves a multitude of considerations, from the ticket price and passenger capacity of the vehicle to its gas mileage and safety on the road. But one factor that may pass under a consumer's radar when comparison shopping is a vehicle's risk for rolling over.

Each year more than 10,000 people are killed and another 27,000 are seriously injured in rollover crashes. These accidents account for nearly one-third of the nation's annual deaths in cars and light trucks, which include SUVs, minivans, and pickup trucks.

Responding to a surge in public concern, the federal government undertook the development of a rating system to determine a vehicle's risk of rollover. Last year, the National Highway Traffic Safety Administration created and implemented a five-star rating system as part of its New Car Assessment Program that would provide consumers with the rollover information they need to make sound purchasing decisions. For example, the lowest-rated vehicles, which receive a one-star rating, are considered at least four times more likely to roll over than the five-star-rated vehicles, if involved in a single-vehicle crash. The star ratings are based on a top-heaviness measurement known as the static stability factor, calculated mathematically using a vehicle's track width and center-of-gravity height.

But automobile manufacturers and some consumer groups questioned NHTSA's decision to use top-heaviness as the sole measurement to rate rollover risk, without also taking into account other performance factors such as how vehicles handle when they are in motion.

According to a new Research Council report, the static stability factor is a useful indicator of a vehicle's propensity to roll over, but the consumer information NHTSA developed is likely to be of limited practical use to the public. Two vehicles given the same star rating may have significantly different rollover tendencies. The report suggests a rating system with more categories or a numerical score that would better communicate rollover risk and make the system more effective in helping buyers make informed choices.

Also, dynamic tests based on actual driving performance to evaluate how the vehicle handles leading up to the crash should be included in the assessment of rollover risk. Some vehicles are equipped with electronic stability control systems that may help a driver avoid situations that could result in a rollover. In its report, the committee suggested that the NHTSA develop a system that uses the static stability factor and information from road tests to provide a more complete picture of rollover behavior. NHTSA is currently developing dynamic tests for rollover in response to legislation.
  -- Jennifer Burris

An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance (TRB Special Report 265). Committee for the Study of a Motor Vehicle Rollover Rating System, Transportation Research Board (2002, approx. 120 pp.; available online from National Academy Press).

The committee's chair is David N. Wormley, dean of the College of Engineering, Pennsylvania State University, University Park. The study was funded by the U.S. Department of Transportation.

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Copyright 2002 by the National Academy of Sciences