New research by the National Highway Traffic Safety Administration backs up the Institute's findings about the importance of roof strength in protecting vehicle occupants in rollover crashes.
The study compares the results of roof strength tests of various vehicle models with real-world rollover crashes and finds a direct correlation between the test results and the number of centimeters a vehicle's roof is pushed into the occupant area in an actual crash. Previous research by the agency showed a direct relationship between the amount of roof crush and the severity of injuries to the head, neck, and face. Taken together, the two studies confirm Institute research that shows injury risk in real-world rollovers goes down as roof strength measured in the laboratory goes up (see "Institute launches new roof strength rating," March 24, 2009).
Roof strength is measured by pushing a metal plate into the roof of a stationary vehicle. How much force the roof can withstand before it caves in 5 inches relative to the vehicle's weight is the strength-to-weight ratio.
The new study compared 38 roof strength test results to 931 real-world rollovers of similar vehicles. After controlling for the number of times the car flipped, whether the roof hit anything besides the ground, and whether other vehicles were involved, the researchers found that a 1-unit increase in strength-to-weight ratio translated into a 5.9-centimeter (2.3-inch) decrease in roof crush.
Currently, federal rules require a roof-to-strength ratio of only 1.5 for vehicles with gross weight ratings up to 6,000 pounds (a gross weight rating is the vehicle's weight when it has a full load of passengers and cargo).
Last year, the National Highway Traffic Safety Administration announced it was doubling that requirement and mandating a ratio of 1.5 for vehicles with ratings from 6,000 to 10,000 pounds. The standards will be phased in beginning in 2012. The agency says 135 lives will be saved each year by the change. The Institute believes this is an underestimate because it excludes unbelted occupants and others at risk of ejection, who also are likely to benefit (see "Rollover safety gets a big boost in new roof rule," June 11, 2009). Vehicles must have a roof strength-to-weight ratio of at least 4 in Institute tests to earn Top Safety Pick.