Rollover poses real risks; IIHS announces new roof crush test

Rollover crash rates have been on the decline for several years, according to government figures, though they still pose a significant risk to motorists. About 10,000 of the nearly 40,000 annual fatalities on U.S roads involve rollovers. Even as recently as 2006, a quarter of fatally injured occupants were involved in a crash in which a rollover occurred. The decrease in rollover frequency is likely a result of the increasing popularity of electronic stability control (ESC), which reduces the chance of getting into a rollover situation in the first place, and side-curtain airbags, which reduce full and partial passenger ejection that makes rollover so lethal. But rollovers still take place.

There is no consensus among the safety community as to exactly how people die in a rollover. Clearly, some are non-belted and therefore ejected out of the vehicle. For those that are not ejected, injuries from impact against the vehicle's interior or other occupants is likely. Police reporting often does not include the level of detail required to determine the exact cause of death, but there is no denying, the risks are real.

Testing for roof crush
A NHTSA standard (FMVSS 216) dating back to 1973 dictates that a force, applied at a load of 1.5 times the vehicle weight should correspond to less than five inches of roof deformation. In 2005 NHTSA proposed to upgrade the standard to require a strength-to-weight ratio (SWR) of 2.5, up from 1.5. NHTSA's analysis concluded that vehicles with higher roof strengths were better able to maintain headroom during a rollover, and that in turn, increases survivability. (Read: "Raising the roof standard for rollover safety.")

According to NHTSA, 16 percent of the fatalities that resulted from a single-vehicle rollover in 2006 could have been avoided if the standard increased from a 1.5 to 2.5 SWR. Further increase of the SWR would have resulted in more lives saved, the research concludes. According to additional research by the Insurance Institute for Highway Safety (IIHS) an SWR of 4 can reduce fatal injuries by 50 percent compared to the existing standard of 1.5. While the NHTSA proposal has dragged on for four years, the IIHS decided to take action on the matter. In 2008, it subjected previous-generation Ford Explorer and Nissan Xterra to 10,000 pounds of force (2.5 SWR)—the Xterra's roof deformed 2 inches, while the Explorer caved in 10 inches—well into the occupant's head space. It deemed a correlation to real world findings was valid.

Today, the  IIHS released a second set of findings, with roof crush results for 12 small SUVs. The IIHS's test is more stringent than that proposed by NHTSA and subjects a given roof to four times the vehicle's weight before reaching five inches of deformation.

The poorest performer (Kia Sportage) was around 2.5 SWR (which also applies to its sister vehicle, the Hyundai Tucson). The best performer was the Volkswagen Tiguan at almost 6 SWR. After the test, the Tiguan looked as if it had only been scratched by a toll-booth barrier even after withstanding a force of 15,000 pounds. Also impressive was the Subaru Forester at 4.5. For 2010, the IIHS plans to remove its Top Safety Pick status for vehicles with an SWR of less than 4.0. 

Bottom line
Some critics argue that only a dynamic rollover test would be relevant. Given that rollovers are violent and unpredictable events, the process of finding a universal, agreeable and repeatable test is still in its infancy. It would also require developing a new generation of crash dummies since current ones have been designed for measuring frontal and side forces and not those associated with rollovers.

We hope that the increase in SWR won't come at the expense of visibility, as there is no federal standard for that. In order to make roofs stronger, the front and middle pillars of the vehicle (A and B pillars) have to be enhanced, and increased thickness can compromise visibility. But the Subaru Forester proves that such a tradeoff isn't a must. The Forester has excellent visibility and does well in the IIHS's new test. 

Learn more about car safety in "Crash Test 101" and "Rollover 101."

—Gabe Shenhar

Updated 3/30/09.