ARLINGTON, VA --- The Insurance Institute for Highway Safety said the Smart Fortwo proved to have the strongest roof among the 2009 micro and minicars recently tested for roof strength. 

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

The rating system is 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 requirements. The ratings, products of the institute's new roof strength testing program, add to consumer information tests that rate vehicles for front, side and rear crashworthiness. The roof test is designed to help consumers pick vehicles that will help protect them in rollover crashes, the institute said. 

"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 Institute president Adrian Lund. 

Roofs have gotten stronger during the past few years, IIHS research shows. Part of the reason is that automakers have made structural improvements to earn better ratings in front and side crash tests. Strong A and B pillars help prevent intrusion in these types of crashes. They also help hold up the roof. 

"Small cars should have an easier time with the roof strength test," Lund said. "Their light weight means their roofs don't have to work as hard to keep the structure around the occupants intact in a rollover." 

About 10,000 people a year are killed in rollovers. When vehicles roll, their roofs hit the ground, deform and crush. Stronger roofs crush less, reducing the risk of injury from contact with the roof itself, IIHS said. Stronger roofs also can prevent people, especially those who aren't using safety belts, from being ejected through windows, windshields or doors that have broken or opened because the roof deformed. Roofs that don't collapse help keep people inside vehicles as they roll. 

In the institute's test, 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 five 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 explained. 

The Smart withstood a force of 5.4 times its weight. 

Cars have been built to meet the same roof crush standard, Federal Motor Vehicle Safety Standard 216, since 1973. The rule was extended in 1994 to include all passenger vehicles up to a gross weight rating of 6,000 pounds. Many SUVs and pickup trucks are heavier, so they're exempt. 

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 said, "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." 

A "good" roof strength rating will be a new requirement to earn the Institute's Top Safety Pick award for 2010.