The Insurance Institute for Highway Safety said its tests of roof strength among micro and minicars rated the Smart Fortwo strongest 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 more than twice as strong as the current minimum federal safety standard requires – able to withstand a force of four times the vehicle's weight 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. A marginal rating value is 2.5 and anything lower than that is poor.

A Fortwo withstood a force of 5.4 times its weight; the Aveo withstood a force of just over 3 times its weight, the said IIHS.

About 10,000 people a year are killed in rollovers, according to the IIHS, which added that occupants in rollover crashes benefit from stronger roofs.

The IIHS noted that manufacturers have strengthened roofs during the past few years with structural improvements to earn better front and side ratings in IIHS tests. These improvement have helped hold up the roof in vehicles.

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

He added, “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.”

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