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Status Report, Vol. 46, No. 4 | April 26, 2011 Subscribe

New ejection rule may spur changes in side airbags

Side curtain airbags that deploy in rollovers as well as side impacts are expected to become the norm, thanks to a new federal standard aimed at decreasing ejections from passenger vehicles. The rule, issued in January by the National Highway Traffic Safety Administration (NHTSA), also provides an incentive for manufacturers to use laminated glass on side windows, something the Institute backs.

While supporting the goals of the regulation, Institute researchers point out that it requires some changes to current airbags that aren't proven to be needed. It's unclear how those changes will affect crash protection.

"Research shows that side curtain airbags are very effective at reducing injury risk in side crashes," notes Matthew Brumbelow, a senior research engineer for the Institute. "We hope the changes required by the new regulation don't diminish this."

Rollovers make up about 3 percent of all crashes but more than a third of passenger vehicle occupant deaths. Nearly half the people killed in rollovers are thrown from vehicles. NHTSA has estimated that occupants who are fully ejected are 56 percent more likely to die than those who remain in a vehicle during a rollover. Most ejected people go through side windows, and the vast majority are unbelted.

Side airbags that deploy in a rollover can play a key role in keeping occupants inside the vehicle. They deploy from the roof downward in response to sensors that measure a vehicle's sideways movement and tilt.

Most new vehicles come with side airbags to protect the head and torso, and the equipment is on its way to becoming universal because of new standards for side protection. Manufacturers now are expected to modify side airbags in response to the new ejection rule. In addition to deploying in a rollover, the airbags will need to cover more of the side windows and stay inflated longer than airbags designed solely for side impacts.

Airbags in some vehicles already do all that, but most of them still will have to be changed to meet the standard. That's because NHTSA is requiring systems that prevent the equivalent of an unbelted adult's head from moving more than 4 inches past the side window opening. Most current side airbags allow more movement than that, and it's unclear that such a strict standard is necessary.

The 4-inch rule is intended to prevent a situation in which an airbag gets pushed outside the vehicle so far that the bottom of it falls out, creating a gap through which a person could be ejected. NHTSA says the 4-inch rule is used in standards concerning certain aspects of vehicle doors and bus windows and in international building codes for stair and balcony railings. But those standards all have to do with the width of openings, while in the new regulation the 4-inch limit refers to how far the test device can move outside the window as it pushes on the airbag, not to the size of a resulting opening.

Of the 24 vehicles with rollover airbags NHTSA tested ahead of the rule, only 1 met the requirement. The others will need changes, probably stiffer airbags that inflate more powerfully. The effects of these changes in other crashes are unknown, but it's possible that stiffer airbags could be less effective at preventing head or neck injuries in some cases. Added force when an airbag deploys also could cause injuries. Up to this point, injuries from side airbag deployments have been rare, in part because of voluntary guidelines manufacturers adopted to minimize inflation injuries (see "Consumers get new information on side airbags with reduced injury risk," Sept. 13, 2004).

In announcing its decision, NHTSA cited 6 real-world cases in which rollover airbags didn't prevent ejection. In none of the cases was there evidence an occupant pushed past a too-weak airbag — the issue the 4-inch rule is meant to address. Instead, the airbags may have had deployment problems or simply didn't cover enough of the window.

"Although there have been some ejections from vehicles with rollover airbags, there's no compelling evidence that the 4-inch rule will fix that," Brumbelow says. The Institute expressed reservations about the 4-inch limit in a comment to NHTSA last year.

The Institute got a more favorable response to its suggestion that NHTSA encourage the use of shatter-proof laminated glass for side windows as a complement to side airbags. The rule allows side windows to remain rolled up for 1 of 2 required tests. That way, the window can help keep side airbags in place during the test, providing an incentive for stronger glass.

The standard will be phased in beginning in 2013, with all new vehicles required to meet it by the 2018 model year. NHTSA estimates that when fully implemented, the requirement will prevent 373 deaths and 476 serious injuries every year. Institute researchers believe those estimates are high because NHTSA undercounts the benefit of a more stringent roof strength standard to be phased in by 2017. Institute studies have shown that stronger roofs reduce the risk of occupant ejection (see Status Report special issue: roof strength, March 24, 2009).

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