IIHS Releases Roof Strength Ratings for Micro- and Mini-Cars; Smart Fortwo Ranks Number-One
August 20, 2009

The Smart Fortwo has the strongest roof and the Chevrolet Aveo has the weakest among 2009 micro- and mini-cars recently tested by the Insurance Institute for Highway Safety (IIHS), according to the latest reports from the group.

The Smart earned the highest rating of good compared with acceptable for the Honda Fit, Hyundai Accent, Mini Cooper, and Toyota Yaris. The Aveo was rated marginal.

The rating system was developed based on IIHS research showing that occupants in rollover crashes benefit from stronger roofs.
Vehicles rated good must have roofs that are more than twice as strong as the current minimum federal safety standard requires. The ratings, part of the IIHS's new roof strength testing program, were designed to help consumers pick vehicles that will help protect them in rollover crashes.

"We anticipate that our roof strength test will drive improved rollover crash protection the same way our frontal offset and side tests have led to better occupant protection in these kinds of crashes," says IIHS president Adrian Lund.

In the IIHS test for roof strength, a metal plate is pushed against one side of a roof at a constant speed. To earn a good rating, a roof must withstand a force of four times the vehicle's weight before reaching 5 inches of crush. This is called a strength-to-weight ratio. For an acceptable rating, the minimum required ratio is 3.25. A marginal rating value is 2.5. Anything lower than that is poor.

"Compared with the current federal standard of 1.5, a strength-to-weight ratio of 4 reflects an estimated 50 percent reduction in the risk of serious or fatal injury in single-vehicle rollover crashes," Lund explains.

The Smart withstood a force of 5.4 times its weight. The Aveo withstood a force of just over three times its weight.

In April the National Highway Traffic Safety Administration ended numerous delays by unveiling a new rule that doubles the current roof strength requirement (strength-to-weight ratio of 1.5) for vehicles with weight ratings up to 6,000 pounds. Roofs on vehicles with weight ratings 6,000 to 10,000 pounds will be required to withstand a force equal to 1.5 times their unloaded weight. Another requirement is that roofs maintain sufficient headroom during testing. For the first time, the government also will require the same performance on both sides of the roof when tested sequentially. Phase-in begins in September 2012, and all vehicles must comply by September 2016.

"The federal government's leisurely phase-in of the new standard means roofs won't have to get stronger right away," Lund says, "so we plan to continue rating vehicle roof strength for the foreseeable future. We want to reward manufacturers who are ahead of their competition when it comes to providing protection in rollover crashes. We want to help consumers identify the safest vehicle choices."

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