Arlington, Virginia – The Smart Fortwo has the strongest roof and the Chevrolet Aveo the weakest among micro and minicars recently tested by the U.S. Insurance Institute for Highway Safety (IIHS). In a test of 2009 model-year vehicles, Smart earned the highest rating of “good,” while the Honda Fit, Hyundai Accent, Mini Cooper and Toyota Yaris rated “acceptable,” and the Aveo rated “marginal.”

The rating system, based on IIHS research, shows 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.  “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,” said IIHS president Adrian Lund. “Small cars should have an easier time with the roof strength test. Their light weight means their roofs don’t have to work as hard to keep the structure around the occupants intact in a rollover.”

Roofs have gotten stronger during the past few years, the IIHS said, partly because automakers have made structural improvements to earn better front and side ratings in IIHS tests. Strong A and B pillars help prevent intrusions in these types of crashes, but as they also hold up the roof, they improve roof strength.

In the test, a metal plate was pushed against one side of a roof at constant speed. To earn a “good” rating, the roof must withstand a force of four times the vehicle’s weight before reaching five inches (127 mm) of crush. This is called a strength-to-weight ratio. For an “acceptable” rating, the minimum required ratio is 3.25, while a “marginal” rating value is 2.5. Anything lower than that is rated “poor.”  The Smart withstood a force of 5.4 times its weight, while the Aveo withstood a force of just over three times its weight.

“Compared with the current federal standard of 1.5, a strength-to-weight ratio of four reflects an estimated 50 per cent reduction in the risk of serious or fatal injury in single-vehicle rollover crashes,” Lund said.

Cars have been built to meet the same federal roof crush standard since 1973. The rule was extended in 1994 to include all passenger vehicles up to a gross weight rating of 6,000 pounds (2,721 kg). Many SUVs and pickup trucks are heavier and so are exempt. In April 2009, the National Highway Traffic Safety Administration (NHTSA) unveiled a new rule that doubles the current roof strength requirement on vehicles rated up to 6,000 pounds, will require vehicles with weight ratings of 6,000 to 10,000 (2,721 to 4,535 kg) to withstand a force equal to 1.5 times their unloaded weight,  and will require that roofs maintain sufficient headroom during testing. For the first time, the government will also 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, so we plan to continue rating vehicle roof strength for the foreseeable future,” Lund said. “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 choice.”

For 2010, the IIHS will include a “good” roof strength rating as a new requirement to earn its Top Safety Pick Award. This is the second time criteria have been tightened since the first winners were announced in 2005. The availability of electronic stability control became a requirement starting with model-year 2007. “Adding roof strength to Top Safety Pick criteria means we’re going to see fewer winners in 2010,” Lund said. A record 84 vehicles have qualified for the 2009 award so far.

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