Airbags have evolved to do a better job of protecting people in multiple kinds of crashes, and each generation has done a better job of this than the one before. That is, until now. A new Institute study suggests that frontal airbags designed to meet the latest federal standards haven't improved protection of adults and, in fact, appear to have reduced protection of belted drivers.
"The newest airbags appear to provide suboptimal protection for drivers who buckle up compared with the airbags that preceded them," says Institute president Adrian Lund. "It's a surprising finding. Based on our analysis of death rates in frontal crashes, belted drivers seem to fare better in vehicles that have many of the advanced features of current systems but weren't certified to the latest airbag safety standard."
Together with safety belts, airbags are the cornerstone of protection in frontal crashes. Ones to safeguard drivers and front-seat passengers have been standard in all passenger vehicles since 1999. They've saved more than 28,000 lives, the National Highway Traffic Safety Administration (NHTSA) estimates.
A big difference between today's airbags and first-generation ones is that they deploy with less force. Problems cropped up with the first generation of airbags in the mid-1990s. During crashes they were inflating with such force that they killed or seriously injured some children, small-stature adults, and other people who were too close to the bag when it inflated (see Status Report special issue: new federal airbag rule, June 17, 2000). NHTSA attributes 296 deaths to frontal airbags, including 191 children, 92 drivers, and 13 adult passengers as of Jan. 1, 2009. Nearly 90 percent of deaths occurred in vehicles made before 1998 when the agency first changed frontal crash safety standards in ways that promoted less forceful deployments.
"Early airbags as opposed to crash forces were the source of injury in some cases," Lund explains. "They saved many lives but at the same time put some vulnerable passengers at risk. When it became clear what was happening, NHTSA allowed automakers to redesign airbags, and once the fixes were in place, deaths dropped sharply."
Steps to address injuries
As a first step NHTSA modified safety rules in 1997 to encourage automakers to take energy out of the airbags. Depowering began with 1998 models. Manufacturers were given the option to use sled tests with unbelted dummies to certify that their vehicles met crash performance rules. Or they could continue to run barrier tests with both belted and unbelted dummies.
Most manufacturers picked sled tests, in which a whole or partial vehicle is attached to a moving platform that simulates vehicle crash decelerations and mimics the forces on occupants during crashes. The maximum sled accelerations NHTSA prescribed under this option were lower than typically occur in crash tests so airbags didn't need to deploy as quickly or forcefully to catch and cushion unbelted dummies. Airbags meeting this standard are called sled-certified. The Institute previously examined the impact of NHTSA's move to allow depowering and found an overall reduction in fatal crash risk associated with depowered airbags compared with earlier designs (see "Depowered airbags lessen fatality risk for drivers of most vehicles," March 6, 2004).
Other research has shown that the fatality risk among children in front seats decreased with sled-certified airbags (see "Redesigned airbags safeguard both children and grown-ups," June 9, 2008). At the same time a large-scale public education campaign encouraged parents to restrain children, especially infants in rear-facing restraints, in the back seat, where they're safest. Legislators in many states enacted laws requiring children to sit in the rear. Parents largely got the message.
Today most kids ride restrained in back seats. These and other changes plus increasing belt use have contributed to the drastic decline in frontal airbag-related deaths, the bulk of which occurred in vehicles made before 1998 (see "Evidence mounts that reducing airbag inflation force lowers injury risk," Aug. 1, 2004, and "Occupant deaths from inflating airbags have been all but eliminated," Aug. 6, 2005).
The sled test option was meant to be a stopgap until NHTSA could write a new standard to explicitly address airbag-induced injuries while also improving protection for a range of different-size people in various frontal crashes. During 2001 the agency issued a certified advanced airbag rule, with phase-in beginning with 2003 models.
Advanced airbags modify deployment patterns if weight sensors detect a small front-seat driver or passenger or a child safety seat. These airbags can be suppressed altogether or deploy with less force when passengers are small or out of position or if a crash isn't severe. They also can determine if occupants' safety belts are buckled. Certified-advanced airbags generally deploy at lower thresholds for people who aren't using belts.
This changed the way auto manufacturers test vehicles for compliance. It introduced a range of tests, including head-on and offset frontal crash tests plus out-of-position tests of airbags using different-size dummies. For the first time, the automakers were directed to use dummies representing 5th percentile females and children 1, 3, and 6 years old, in addition to the standard 50th percentile male dummy.
Crash test speeds also changed. For belted male dummies, the rule phased in a 5 mph speed increase, to 35 mph, beginning with 2007 model vehicles. Rigid-barrier tests for unbelted occupants were reinstated, but the crash test speed was lowered from 30 mph to 25 mph (see Status Report special issue: new federal airbag rule, June 17, 2000).
Anticipating the design changes that an advanced airbag standard would require, some automakers added new features ahead of the rule. These included dual-stage inflators, belt status sensors, seat position sensors, and occupant size and weight sensors. Some new airbag systems also had sensors to detect rear-facing infant restraints in front seats and prevent airbags from deploying. Many of these systems closely resemble certified-advanced airbags.
"The advanced features automakers added changed the game," Lund says. "Instead of tailoring protection and deployment for one group — average-size men in a typical crash — manufacturers were able to design airbag systems to provide better protection for a range of people in a variety of crash situations."
What the changes mean
What hadn't been known is how advanced airbags compare with the previous designs. To find out, Institute researchers recently compared mortality rates in frontal crashes among front-seat occupants in vehicles with certified-advanced airbags — the latest generation of airbags — with sled-certified ones with advanced features. They also looked at mortality among front-seat occupants of vehicles with sled-certified airbags with advanced features versus those without advanced features. The researchers analyzed the effects of airbag design changes by driver age, gender, and belt use. They also looked at mortality rates for children in front seats. The study included 1998-2006 model vehicles in crashes during 2004-07.
Some people were benefiting from advanced airbag features even before airbags were certified as advanced. Mortality rates were 16 percent lower for drivers of vehicles with sled-certified airbags with advanced features than for people who drove vehicles with sled-certified airbags without advanced features. The benefit was 17 percent for adults riding in front passenger seats.
Death rates were lower for both male and female drivers ages 15-59, as well as for men older than 60. Unbelted male drivers had a 38 percent lower death rate in vehicles with sled-certified airbags with advanced features compared with vehicles with sled-certified airbags lacking such features.
Results for certified-advanced airbags don't follow the same pattern. Although children benefited from both kinds of advanced airbag systems, drivers didn't. People who drove vehicles with certified-advanced airbags had a higher mortality rate than drivers of vehicles equipped with sled-certified airbags with advanced features.
Belted drivers had the biggest uptick in the risk of death — 21 percent — compared with drivers of vehicles with sled-certified airbags with advanced features.
"This finding puzzles us because these drivers had otherwise done everything right in terms of buckling up," Lund says. "It suggests there might be potential problems with the way manufacturers are required to certify airbags as advanced because the technology introduced in vehicles during the sled test era seems to work. But when the new standard is fully in effect we don't see an improvement."
The agency's 2001 decision to reintroduce a rigid-barrier crash test for unbelted occupants was controversial. Automakers contended the unbelted test would prompt a return to overly aggressive airbags. The Institute initially objected to reinstating the unbelted barrier test, while other safety groups favored it (see "Advanced airbags are focus of new NHTSA rulemaking," Oct. 9, 1998, and "30 mph unbelted crash test shouldn't be reinstated for airbags," Feb. 6, 1999).
The maximum test speed sparked debate, too. NHTSA at first proposed 30 mph but settled on 25 mph in the final rule, a change the Institute supported (see "Unbelted crash test speed is subject to legal challenge," March 15, 2003). Some safety groups, however, feared 25 mph would provide inadequate protection for large occupants, particularly unbelted men. Public Citizen and the Center for Auto Safety sued NHTSA, but a federal appeals court upheld the agency (see "Two court decisions both serve safety," Aug. 1, 2004).
"Automakers may have had a point," Lund concedes. "Airbags may be too aggressive because of the rigid-barrier test requirements for unbelted dummies. It's also possible that advanced deployment algorithms result in some airbags not deploying at all when they would be beneficial. NHTSA needs to look at our study and try to understand if the new standard missed the mark on striking a balance between protection for both belted and unbelted occupants. In particular, belted drivers aren't reaping the benefits we expected."