Procedures for rating roof strength(3)

More than 10,000 people a year are killed in rollovers. The best way to prevent the deaths is to keep vehicles from rolling over in the first place. Electronic stability control is significantly reducing rollovers, especially fatal single-vehicle ones. When vehicles do roll, side curtain airbags help protect the people inside, and belt use is essential. However, for these safety technologies to be most effective, the roof must be able to maintain the occupant survival space when it hits the ground during a rollover. Stronger roofs crush less, reducing the risk that people will be injured by contact with the roof itself. Stronger roofs also can prevent occupants, especially those who aren't using safety belts, from being ejected through windows, windshields, or doors that have broken or opened because the roof has deformed.
In the Institute's roof strength test, a metal plate is pushed against 1 side of a roof at a constant speed. To earn a good rating, the 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 strength-to-weight ratio is 3.25. A marginal rating value is 2.5. Anything lower than that is poor.
The Institute's test method is the same one that has been used for testing under the federal roof strength regulation since 1973, but with much higher requirements. Vehicles only need a strength-to-weight ratio of 1.5 to meet the federal regulation. While the actual roof strengths of vehicles may surpass this minimum level by a large amount, this information has not been available to consumers. Institute research has found that a vehicle with a roof strength-to-weight ratio of 4.0 has an estimated 50 percent reduction in the risk of serious and fatal injury in single-vehicle rollover crashes compared with the minimum level of 1.5.

Video roof strength test

Sample data comparing test result for vehicles rated good and poor