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Vehicle Rollover Risk

Vehicle Rollover Risk

By Jeff Youngs, February 24, 2012
While accidents that involve vehicle rollovers are relatively rare, you should be aware of the risk-especially if you drive a sport utility vehicle (SUV). Statistics indicate that SUVs are three times more likely to be involved in a rollover accident than passenger cars. And, if a rollover does occur, occupants riding in SUVs are most at risk.

Some SUVs pose a greater risk than others. As a result, the National Highway Traffic Safety Administration (NHTSA) has developed Rollover Resistance Ratings (www.safercar.gov) to supplement the existing frontal and side-impact crash test data that the government organization provides. While the Insurance Institute for Highway Safety (www.iihs.org), which is not affiliated with the federal government, also conducts frontal and side-impact crash tests, as well as low-speed bumper tests, currently only NHTSA assesses rollover risk.

New test procedure leads to more accurate ratings

The agency originally assigned rollover ratings to vehicles based on a mathematical calculation that took into consideration a vehicle's weight, width, and center of gravity to create a statistical likelihood of a rollover. The measurement, which NHTSA called the Static Stability Factor, was widely criticized because it did not simulate real-world driving situations. Some 2003 model-year and older vehicles have a rollover rating based solely on this mathematical calculation.

In 2004, NHTSA began using a dynamic test for rollovers, and the star ratings it currently assigns are the result of a combination of both static and dynamic tests. The dynamic test was developed to assess a vehicle's overall risk in real-world situations.

To begin the test, NHTSA adds weight to the subject vehicle to simulate a five-passenger load and a full tank of gas. The vehicle's stability control system is activated if the model being tested sells at least 50 percent of its units equipped with that feature. Then, a remote-control steering system simulates an emergency lane change (sometimes referred to as a "fishhook" maneuver). As a result, the vehicle turns sharply and suddenly in one direction, and then quickly returns in the opposite direction. To ensure consistency in testing all models, the steering system exactly duplicates the fishhook maneuver for each vehicle, thus ensuring that the test is conducted the same way every time. If two of the vehicle's tires lift at least two inches off the pavement simultaneously, it is considered to have "tipped up," a precursor to a rollover.

Some 2004 model-year vehicles and all 2005 and newer vehicles are rated using the results of both the static and dynamic measurements.

NHTSA's rating system for measuring rollover risk ranges from one star (40 percent or greater risk of rollover) to five stars (10 percent or less risk of rollover).

Rollover risk varies widely among SUV models

While SUVs have the greatest risk of a rollover, the risk varies quite widely from one SUV model to the next. Overall, the SUVs that have been tested have between a 13 percent and a 34 percent chance of rolling over, versus a range of 7 to 15 percent for passenger cars, indicating that SUVs are more likely to roll over than passenger cars.

Examining the details of the rollover ratings reveals even more information than the star rating alone. For example, while 19 different 2005 model-year SUVs received a four-star rollover rating, the risk of a rollover associated with those vehicles ranged from 13 percent to 19 percent.
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