The New De Facto Roof Strength Standard? IIHS Raises the Bar

Reprinted from The Safety Record, V6, I1

WASHINGTON, D.C. - As the National Highway Traffic Safety Administration's effort to write a new roof strength standard drags into its fourth year, the Insurance Institute for Highway Safety has gone ahead and created one that is far more stringent than anything the agency has proposed.

Beginning in 2010, automakers who want IIHS's coveted Top Safety Pick designation will have to build vehicle roofs with a 4.0 strength-to-weight ratio - far above the timid 2.5 ratio the government has been contemplating for its amended standard. The IIHS estimated that vehicles that could meet this new strength standard could reduce injury risk to occupants by 40-50 percent. In January, the insurance advocacy group informed manufacturers about its new requirement for vehicle roofs to win its highest honor. The industry greeted the news with the "can't-do" spirit that characterizes its reaction to nearly every safety improvement.

"A number have said to us the 4.0 strength-to-weight ratio is a very hard standard to meet," says IIHS's Adrian Lund.
(Based on NHTSA data, the Volvo XC90, the 2006-2009 Honda Civic, Volkswagen Jetta 2005-2009; Toyota Camry 2007-2009 and Toyota Tacoma 2005-2009 already meet or exceed that standard.)

But the IIHS has heard it all before. When it introduced its new 40 mph frontal offset crash tests in 1995, automakers protested that their vehicles couldn't pass such a tough test. Today, virtually 100 percent of new vehicles earn a good rating in that test. In 2003, when the IIHS upped the ante on side-impact crashworthiness, by using a barrier more representative of an SUV than the sedan-type barrier used in the federal compliance test, manufacturers complained again. The IIHS reports that automakers are rapidly rising to that challenge, with 64 percent earning a "good" rating in that test in 2009.

"The main point is: It's hard when you start, but obviously, it can be done," Lund says. "I think we will get some movement on roof strength. They are going to try to do it - this one isn't rocket science."

The IIHS decided to move forward on roof strength, after conducting two studies on mid-sized SUVs and small sedans showing that roof strength was strongly related to occupant injury risk. In conducting its research, the IIHS cleverly sidestepped the chicken-and-egg debate of whether occupants sustain injuries in a rollover because they "dive" into the roof or because the roof crushes into occupants. Instead, it compared injury figures from real world crashes with the roof strength ratios of the 11 models in those crashes, as measured by the Federal Motor Vehicle Safety Standard 216 quasi-static compliance test.

The first study, published in March 2008, focused on SUVs. The IIHS culled 22,817 rollover crashes from the State Data System - police-reported crashes submitted to NHTSA - in 12 states that had data available for some part of calendar years 1997-2005, had a mechanism to identify single-vehicle rollovers, and had sufficient VIN information to determine vehicle make, model, and model year. The 12 states that met these criteria - Florida, Georgia, Illinois, Kentucky, Maryland, Missouri, New Mexico, North Carolina, Ohio, Pennsylvania, Wisconsin, and Wyoming - used KABCO injury coding, in which "K" represents fatal injuries and "A" represents incapacitating injuries as assessed by the investigating police officer.

The IIHS used the crash data to select the mid-sized SUVs most represented in fatal crash data and the models most represented on the road to ensure a sufficient sample size. Eleven models were used as the basis of comparison. General Testing Laboratories, under contract with IIHS, subjected eight midsize SUVs - six of which were used vehicles - to the FMVSS 216 quasi-static tests. The maximum force required to crush the roof to 2, 5, and 10 inches of plate displacement was recorded. (The IIHS used NHTSA roof strength data for three models.)

The IIHS found that in all cases, "increased measures of roof strength resulted in significantly reduced rates of fatal or incapacitating driver injury after accounting for vehicle stability, driver age, and state differences." Researchers estimated that a one-unit increase in peak strength-to-weight ratio within five inches of plate displacement was estimated to reduce the risk of fatal or incapacitating injury by 28 percent.

This finding contradicted other studies on the relationship between roof strength and injury risk, but the IIHS defended its study as having more tightly controlled potential confounding factors. Also, the IIHS estimated number of lives saved by increasing the regulated SWR to 2.5 is considerably higher than the estimated 13 and 44 lives saved indicated in NHTSA's 2005 NPRM, despite the fact the agency's estimates cover the entire passenger vehicle fleet.

This winter, the organization conducted a second study, using the same methodology, with small sedans. The as-yet unpublished study confirmed the results of the SUV project - roof strength was highly correlated with injury risk, and the benefits of stronger roofs were substantial.

Lund said that the IIHS's research results were too definitive to wait for the agency to finally move on an amended roof crush standard. First introduced in August 2005, the proposed amendment would increase the roof strength-to-weight ratio from the current standard of 1.5, established in 1973, to 2.5 times a vehicle's weight in a rollover crash. The maximum 5-inch plate displacement limit would be replaced by a requirement that the minimum strength be achieved prior to head-to-roof contact for an ATD positioned in the front outboard seat on the side of the vehicle being tested. In January 2008, NHTSA issued a supplemental notice of proposed rulemaking announcing that it would delay the adoption of a new standard while it considered testing a sequential two-sided test for possible adoption. The agency was required to revamp the standard by July 2007, but has delayed further action until April.

IIHS is scheduled to start testing roofs soon and will be releasing small SUV roof strength ratings in the spring. Roof strength will officially be among the Top-Safety-Pick criteria in 2010 models beginning this Fall, Lund said.

Copyright @ Safety Research & Strategies, Inc., 2009

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