According to the Insurance Institute for Highway Safety, about 35 percent of all occupant deaths occur in rollover crashes. Only 25 percent of the deaths in cars and minivans occur in rollovers, while that number jumps to 59 percent in sport utility vehicles.

With so much at risk, the IIHS tested 11 midsize SUVs and compared the results to 23,000 real-world rollovers involving the same vehicles from 1997 to 2005. The results showed that SUVs with the strongest roofs are associated with injury risks 39 to 57 percent lower in rollovers than SUVs with the weakest roofs.

“We don’t know just what happens to people in these crashes or what the injury mechanisms are. What we do know from the new study is that strengthening a vehicle’s roof reduces injury risk and reduces it a lot,” said IIHS President Adrian Lund.

The IIHS test results back the 2005 proposal by the National Highway Traffic Safety Administration that roof strength standards be increased. The current standard, Federal Motor Vehicle Safety Standard 216, calls for a roof to withstand a force of 1.5 times vehicle weight before reaching 5 inches of crush. The proposed standard would call for a vehicle to withstand a force of 2.5 times its weight before 5 inches of crush. The IIHS believes this change would be beneficial and an even more aggressive standard would save more lives.

The vehicles evaluated were the 1996-2004 Chevrolet Blazer, 2002-2005 Chevrolet TrailBlazer, 1998-2003 Dodge Durango, 1996-2001 Ford Explorer, 2002-2004 Ford Explorer, 1996-1998 Jeep Grand Cherokee, 1999-2004 Jeep Grand Cherokee, 2002-2005 Jeep Liberty, 1997-2004 Mitsubishi Montero Sport, 2000-2004 Nissan Xterra and 1996-2000 Toyota 4Runner. The SUVs excluded electronic stability control and side-curtain air bags, both of which can reduce the risk of rollover injury.

In the study, the IIHS used a metal plate at a constant speed to subject the 11 SUVs to forces that crushed the roofs 2, 5 and 10 inches. Of the vehicles tested, the Nissan Xterra had the strongest roof. The Xterra withstood a force of 11,996 pounds, 3.16 times the weight of a two-wheel-drive model or 2.93 times the weight of a four-wheel-drive model, before it crushed 5 inches. The weakest roof belonged to the 1999-2004 Jeep Grand Cherokee. It withstood a crushing force of 6560 pounds, 1.64 times the weight of a 4WD model or 1.72 times the weight of a 2WD model before 5 inches of crush. All SUVs tested met the minimum federal requirement.

The IIHS says the relationship between roof strength-to-weight ratio and injury risk is stronger at 2 and 10 inches than at 5. According to IIHS predictions, at 5 inches of crush the risk of injury for people in SUVs with roofs as strong as the Xterra’s would be 39 percent lower than for people in SUVs with the roof strength of the 1999-2004 Grand Cherokee. At 2 inches, the IIHS predicted a 51 percent lower injury rate.

Based on the results of its study, the IIHS estimates that if the roofs of all the SUVs tested were as strong as the Xterra’s, about 212 of the 668 rollover deaths that occurred in these vehicles in 2006 would have been prevented. The IIHS also estimates that highway safety administration’s proposed increase to a 2.5 times roof strength-to-weight ratio from the current standard would reduce the risk of serious and fatal injury in a rollover crash by 28 percent. When it proposed a higher standard in 2005, the federal agency believed the increase would save only 13 to 44 lives per year across all types of vehicles.

Of course, roof strength isn’t the only factor that can reduce the risk of rollover death or injury. The IIHS estimates that electronic stability control reduces the risk of fatal single-vehicle rollover by 69 percent for all passenger vehicles and 72 percent for SUVs. It does so by eliminating a certain percentage of rollovers altogether. ESC monitors vehicle response to steering inputs. If it detects the vehicle is not responding properly, it uses the anti-lock brake system to apply brakes to individual wheels to get the vehicle back on its intended path.

The IIHS conclusion is in contrast to at least two studies previously conducted by other entities that have concluded the strength of a vehicle’s roof has little or no effect on the likelihood of injury. The IIHS believes its study shows a definite connection between roof strength and injury risk in rollovers, but the institute still thinks more research is needed, including a dynamic test with instrumented dummies in rollover situations.