- The Washington Times - Thursday, January 13, 2005

Each year, vehicle rollovers account for about 10,000 fatalities in the United States. This is a sobering figure.

In January 2001, the National Highway Traffic Safety Administration came forward with ratings that attempted to estimate how likely a vehicle was to roll over in a “single-vehicle” crash, as if it were sliding sideways and “tripped” by a curb, ditch, soft earth or deep snow.

At first, these ratings were based solely on measurements of a vehicle’s height and width to try to determine how top-heavy it was and the chances it would roll. This mathematical calculation is called a “static stability factor.” The higher the static stability factor, the more stable the vehicle. The lower the factor, the more top-heavy the vehicle is and the more likely it is to roll over in these “single-vehicle” crashes or “tripped” rollovers.

Some safety groups and automakers complained that this measurement was not realistic and didn’t take into account the handling differences of various vehicles. But NHTSA said the stability factor correlated well with real-world events. Nevertheless, Congress ordered the federal agency to adopt a system that would use a handling test in which the vehicle is driven.

In the new handling tests, vehicles are driven through as many as 10 “fishhook” maneuvers, a jarring series of turns similar to a driver trying to avoid a high-speed collision by steering sharply in one direction and then in another while trying to regain control. Tests are conducted at speeds ranging from 35 to 50 mph and are completed at 50 mph or as soon as a vehicle tips up on two wheels.

Rollover ratings for vehicles from the 2003 model years and earlier are based on the static stability factor only. Starting with 2004 models, rollover ratings combine the results of both the static stability factor and the handling test. However, the static stability factor more heavily influences the ratings because, NHTSA said, how “top-heavy” a vehicle is, which is measured by the static stability factor, is the major factor in “tripped” rollovers. The only result from the dynamic test that affects the star rating is whether the vehicle tips up onto two wheels.

These two results are combined into a five-star rating. Five stars means a vehicle has less than a 10 percent risk of a rollover; one star indicates a greater than 40 percent chance. Because cars are much less likely to tip than pickup trucks and sport utility vehicles, NHTSA does not perform the handling test with cars.

In addition to the star rating that ranks a vehicle’s likelihood of rolling over in a single-vehicle crash, NHTSA also reports the chance of a rollover and how the vehicle ranks compared to others in its class. It also tells whether a vehicle started to tip up on two wheels. A note about NHTSA’s Web site: If you are looking at “Five Star Crash Test and Rollover Ratings,” which provides ratings from all the NHTSA tests, you must double-click on the rollover star rating to open up a page with the in-depth data. Visit www.nhtsa.dot.gov.

NHTSA says the rollover star ratings can be used to compare vehicles in different classes and weights. If two vehicles have the same star rating, then compare the chance of rollover, which is expressed in a percentage, between the vehicles. The lower the percent, the less likely the chance of a rollover.

While consumer safety groups were happy when NHTSA began dynamic testing, some criticized the emphasis on the static stability factor. For example, the 2004 Toyota Tacoma tipped up on two wheels in the handling test, but still got four out of five stars.

“How can the Tacoma tip up and get four stars?” asked R. David Pittle, senior vice president for technical policy at Consumers Union, the publisher of Consumer Reports. Mr. Pittle retires from Consumer Union today..

NHTSA responded of the overall score is based on the static stability factor because 95 percent of real-world rollovers are caused by “tripped” rollovers, which the static stability factor predicts.

“We won’t recommend a vehicle that tips up in the government test. I don’t care how good it is in other ways,” Mr. Pittle said. “Our advice is to look for one that didn’t tip up and then look for one with the highest static stability factor.”

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