A new study found that the strength of a sport utility vehicle's roof directly influences an occupant's injury risk.

A study by the Insurance Institute for Highway Safety said that injury risk went down as roof strength increased.

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

The IIHS said that about 35 percent of all occupant deaths occur in crashes in which vehicles roll over. This problem is worse in some kinds of vehicles than others. About 25 percent of occupant deaths in crashes of cars and minivans involve rolling over. The proportion jumps to 59 percent in SUVs.

Lund said electronic stability control in the vehicles would help, since it reduces rollover crashes by significant percentages.

"But until these crashes are reduced to zero, roof strength will remain an important aspect of occupant protection," Lund said.

Researchers estimate that electronic stability control reduces the risk of a fatal single-vehicle rollover by about 69 percent for all passenger vehicles and 72 percent for SUVs in particular. Side curtain air bags are expected to reduce the risk of death in the rollovers that still occur.

"These technologies are essential, but electronic stability control doesn’t completely eliminate rollover crashes, and side air bags aren’t the only protection occupants need if they do roll over," Lund said. "This is why we have to pay attention to the roof. If a vehicle’s roof is strong enough to absorb the energy of a rollover without caving in on its occupants, injury risk goes down."

The U.S. government requires that vehicle roofs withstand a force of 1.5 times the weight of the vehicle before reaching 5 inches of crush.

This requirement, in effect since 1973 for cars and 1994 for other passenger vehicles, is in the process of an upgrade. One of the government’s main proposals, issued in 2005, is to boost the specified force to 2.5 times the vehicle weight.