Procedures for rating roof strength
Thousands of people are killed each year 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 had been used for testing under the original federal roof strength regulation since 1973, but with much higher requirements. Vehicles only needed a strength-to-weight ratio of 1.5 to meet the federal regulation, which just recently has been strengthened, phasing in with 2013 models.
Sample data comparing test results for vehicles rated GOOD and POOR