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Status Report, Vol. 39, No. 3 | March 6, 2004 Subscribe

Government rollover ratings now include results of dynamic tests

Rollover ratings from the National Highway Traffic Safety Administration (NHTSA) for the first time include information from vehicle handling tests. The fishhook maneuver test simulates a driver overcorrecting after swerving to avoid an obstacle in the roadway at speeds up to 50 mph. This test doesn't replace the static measurements the agency previously used to indicate a passenger vehicle's top-heaviness. Instead, the test adds a new element to the agency's rollover rating system.

Since 2001 NHTSA has provided rollover ratings from one to five stars as part of the New Car Assessment Program. Each vehicle's rating, which assesses rollover risk in a single-vehicle crash, was based solely on static measurements — specifically the ratio of a vehicle's track width to twice the height of its center of gravity. Vehicles with high centers of gravity such as SUVs and pickups typically earn fewer stars than vehicles with lower centers of gravity such as passenger cars.

NHTSA estimates that a five-star vehicle has a less than 10 percent chance of rolling over in a single-vehicle crash. A four-star vehicle has a 10 to 20 percent chance, etc.

Under the new rating system, a vehicle can improve its score based on static measurements if it successfully undergoes the fishhook maneuver without tipping up on two wheels. Vehicles are tested with a simulated five-passenger load starting at 35 mph and then in 5 mph increments up to a maximum of 50 mph.

Regardless of tipping, 2004 and later models are rated according to slightly different formulas than those NHTSA used in previous years, before dynamic testing. For this reason, a 2004 model may be assigned a different rating from the 2003 version, even without any design changes.

Fishhook maneuver test

Star rating system before and after dynamic testing (higher ratio indicates less likelihood of rolling over)
Rollover risk in single-vehicle crash  Old rating system: based on static stability factor only New rollover rating system
Vehicle doesn't tip in dynamic test Vehicle tips
5 stars: less than 10% > 1.44  > 1.45 > 1.55
4 stars: 10 to 20% 1.25-1.44 1.18-1.45 1.23-1.55
3 stars: 20 to 30% 1.13-1.24 1.08-1.17 1.11-1.22
2 stars: 30 to 40% 1.04-1.12 1.02-1.07 1.05-1.10
1 stars: more than 40% < 1.04  < 1.02 < 1.05

"The inclusion of dynamic testing in the rollover ratings is an important step, but it hasn't achieved much because the test results don't carry enough weight in the rating system to allow vehicles to distinguish themselves from the rest of the pack," says Susan Ferguson, Institute senior vice president for research. "The static measurements of some vehicles place them so solidly in the middle of a star rating that, whether they tip up or not, their scores aren't likely to be affected. For example, the two-wheel-drive Jeep Liberty and four-wheel-drive Toyota Tacoma are rated the same based on static measurements. The Tacoma tipped up in the fishhook maneuver, but the Liberty didn't. Still these vehicles keep their same three-star ratings."

Almost all of the 2004 SUVs and pickups NHTSA tested earned three or four stars. The exception is the two-wheel-drive version of the Ford Explorer Sport Trac, which earned two stars. Only the Sport Trac and four-wheel-drive Toyota Tacoma extended cab tipped up on two wheels during the test.

In addition to testing 19 SUVs and pickups, NHTSA released rollover results for three cars (Ford Focus wagon, Subaru Outback wagon, and Toyota Echo). None of these cars tipped up during the fishhook maneuver, and each earned a four-star rating. The tests of cars were conducted primarily for comparison. NHTSA doesn't plan to test the majority of cars because most have such high static stability values that there's little likelihood of tipping up in the dynamic test.

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