How roofs are evaluated

How roofs are evaluated: In the Institute's test, a metal plate is pushed against 1 side of a roof at a constant speed. To earn a good rating, a roof must withstand a force of 4 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 3 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.