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Status Report, Vol. 44, No. 3 | SPECIAL ISSUE: ROOF STRENGTH | March 24, 2009 Subscribe

Strong roofs also benefit small cars

Last year, Institute research indicated that stronger roofs on SUVs reduce the risk of driver injury in rollovers. Building on that, the Institute offers new evidence that drivers of passenger cars also benefit from stronger roofs.

Before the SUV study, there was no conclusive evidence tying vehicle roof strength to occupant protection in rollovers (see "Roof strength affects injury risk in SUV rollover crashes, study finds," March 15, 2008). In that study researchers tested 11 midsize SUVs in a procedure similar to what the federal government has required automakers to conduct to assess roof strength. Then the researchers related the findings to the real-world death and injury experience of the same SUVs in single-vehicle rollovers, finding that injury risk went down as roof strength increased.

In a follow-up, researchers subjected 12 small 4-door cars to the same roof crush tests and examined the results of 20,459 single-vehicle rollover crashes involving the same car models. The main finding is that the cars with the strongest roofs reduced ejection risk by as much as 39 percent compared with the vehicles that had the weakest roofs. The strongest roofs in this study reduced the risk of sustaining a fatal or serious injury by about 35 percent.

"There's no evidence that roof strength has a different benefit for people in SUVs compared with people in cars," says Institute president Adrian Lund. "While it's true that SUVs are more frequently involved in rollover crashes, cars roll, too, and the consequences often are deadly. Both types of vehicles benefit from stronger roofs that can prevent people from being ejected in the first place. For occupants who aren't ejected, stronger roofs reduce the chances that they'll be injured or killed."

Manufacturers have built cars to meet the same federal roof standard since 1973, while the standard for passenger vehicles other than cars took effect in 1994. The National Highway Traffic Safety Administration (NHTSA) has proposed changes to better protect people, including the occupants of pickup trucks and SUVs, in rollover crashes.

Because crush in a rollover can depend on vehicle weight as well as roof strength, Institute researchers calculated so-called strength-to-weight ratios for study vehicles. The researchers estimate that a 1-unit increase in the peak ratio, measured within 5 inches of roof crush, is associated with a 22 percent reduction in the risk of fatal or incapacitating injury in small cars. The ejection risk reduction in small cars was 24 percent. Comparable reductions for SUVs were 24 percent and 41 percent.

Among the small cars tested, the 1998-2002 model Toyota Corolla and the 2000-07 Ford Focus had the strongest roofs, while the 1997-2002 Ford Escort had the weakest. The roofs on the Corolla and Focus are more than 2.5 times stronger than current federal requirements.

To estimate what effect raising these requirements would have, the Institute looked at 2 hypothetical targets: a 2.5 strength-to-weight ratio, which is the level NHTSA proposes for a new safety standard, and a 3.9 strength-to-weight ratio representing the strongest roof among the study vehicles. Both targets were measured within 5 inches of roof crush, and researchers estimated that designing the roofs on every small car in the study to meet a minimum requirement of 2.5 would have prevented about 3 of the 228 deaths of drivers and right front passengers in these cars during 2007.

"Looking at the study vehicles, we noted that the car roofs tended to be stronger than the SUV roofs relative to their curb weights," Lund says. "So the minimum roof crush standard the government is proposing would result in only modest crash death declines for these car occupants but greater savings for the SUV occupants."

Far more lives would be saved if every car had roofs as strong as the strongest one the Institute tested. About 75 of the 228 front-seat occupant deaths in these cars during 2007 would have been prevented.

Researchers also examined whether stronger roofs make vehicles more prone to rolling over. This is a concern sometimes voiced by automakers who fear that vehicles will become top-heavy if they have stronger roofs. However, the Institute's analyses indicate that vehicles with stronger roofs aren't more likely to roll over. Single-vehicle rollovers as a proportion of all police-reported crashes were estimated to fall by 11 percent for a 1-unit increase in strength-to-weight ratio.

Why NHTSA'S estimate of lives saved is so low

The National Highway Traffic Safety Administration (NHTSA) argues that upgrading the roof crush standard will have a limited effect on rollover deaths and serious injuries. The agency says fewer than 45 lives would be saved — just a fraction of the deaths in rollovers each year. This lowball estimate contrasts sharply with the Institute's research.

"The agency made too many unproven assumptions about the relationship between roof strength and injury risk," says Institute president Adrian Lund. "For example, if someone was ejected during a rollover, NHTSA ignored the possibility that this wouldn't have occurred if the roof had held up better. Based on this one flawed assumption, the agency eliminated more than a third of all deaths as potential beneficiaries of stronger roofs."

NHTSA assumed that stronger roofs wouldn't help people who died in vehicles that didn't roll beyond the first side, possibly because the roll was arrested by a tree or other object (14 percent of deaths), or in vehicles that were arrested later in the roll (12 percent). Nor would stronger roofs help occupants who were unbelted (15 percent), who weren't in front seats or were younger than 12 years old (3 percent), or if their most serious injuries weren't related to roof crush directly over their heads (12 percent).

"It's a long list," Lund points out. "When NHTSA got to the end of it, hardly anyone was left who met the narrow criteria. The agency discarded 94 percent of rollover deaths."

Instead of trying to figure out what would have helped people who died in rollover crashes, as NHTSA did, the Institute compared the outcomes of all people in rollovers, not just those who died, and didn't limit the study to those who were belted or weren't ejected. Comparing the injury rates in vehicles with varying roof strengths, the Institute researchers determined how many people stronger roofs already are helping. A main finding is that such roofs decrease the risk of being ejected.

"We'll know more as we continue to test new vehicles," Lund adds.

IIHS launches roof strength tests

The Institute is releasing its first roof strength ratings to help consumers choose vehicles that offer the best protection in rollover crashes.

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