NU Online News Service, Aug. 20, 11:02 a.m.EDT

The Insurance Institute for Highway Safety said its tests ofroof strength among micro and minicars rated the Smart Fortwostrongest and the Chevrolet Aveo weakest.

A Fortwo scored "good" in the IIHS test, while the Honda Fit,Hyundai Accent, Mini Cooper, and Toyota Yaris scored "acceptable".The Aveo was rated "marginal".

In order to rate "good," vehicles must have roofs that are morethan twice as strong as the current minimum federal safety standardrequires - able to withstand a force of four times the vehicle'sweight before reaching five inches of crush, the IIHS said.

The institute said this is called a "strength-to-weight ratio".For an acceptable rating, the minimum required ratio is 3.25. Amarginal rating value is 2.5 and anything lower than that ispoor.

A Fortwo withstood a force of 5.4 times its weight; the Aveowithstood a force of just over 3 times its weight, the saidIIHS.

About 10,000 people a year are killed in rollovers, according tothe IIHS, which added that occupants in rollover crashes benefitfrom stronger roofs.

The IIHS noted that manufacturers have strengthened roofs duringthe past few years with structural improvements to earn betterfront and side ratings in IIHS tests. These improvement have helpedhold up the roof in vehicles.

"Small cars should have an easier time with the roof strengthtest," said IIHS president Adrian Lund. "Their light weight meanstheir roofs don't have to work as hard to keep the structure aroundthe occupants intact in a rollover."

He added, "We anticipate that our roof strength test will driveimproved rollover crash protection the same way our frontal offsetand side tests have led to better occupant protection in thesekinds of crashes."