Knee airbags, which deploy from the lower dashboard and are intended to reduce leg injuries, have a negligible effect on injury risk and may even increase it in some cases, according to a study released Wednesday by the Insurance Institute for Highway Safety.
"A lot of people have this idea that more is better," Becky Mueller, an IIHS senior research engineer and co-author of the study, told Automotive News. But this preliminary study indicates that idea may not be true, she said.
The IIHS study examined data from more than 400 frontal crash tests and real-world crash reports from 14 states.
Of the two kinds of frontal crash tests, "small overlap" indicates 25 percent of the front of the vehicle interacts with a barrier, while a "moderate overlap" test indicates 40 percent interaction with a barrier, Mueller said.
"In the small overlap test, knee airbags were associated with increased injury risk for lower leg injuries and right femur injuries, though head injury risk was slightly reduced," the study said. "The airbags had no effect on injury measures in the moderate overlap test."
Analysis of real-world crashes showed knee airbags reduce overall injury risk by half a percentage point, from 7.9 percent to 7.4 percent.
"This result wasn't statistically significant," the study said.
"We think knee airbags may be effective at preventing some kinds of leg injuries but could also be putting people's legs at risk for other kinds of leg injuries that aren't being seen in vehicles without those knee airbags," Mueller said.
She said the study indicates redesigns of lower instrument panels, to prevent leg contact, could be "just as effective at preventing leg injuries" as knee airbags.
"Other kinds of airbags like the front airbag in your steering wheel or the side curtain airbags typically focus on fatal injuries," Mueller said. "Head and chest injuries can result in death, so it's easy for us as researchers to say the airbag saved a life or could have saved a life. Leg injuries aren't going to necessarily kill people, so it's a tougher problem to study how effective this technology is."
Since cars have been equipped with knee airbags for about 10 years, Mueller said, there is just enough data to begin studying them.
Mueller said the study did not investigate the effect of knee airbags on unbelted occupants but that other forms of testing could show knee airbags "may have more benefits … than [test modes] we chose for this study." IIHS said one reason some automakers have installed knee airbags is to help vehicles pass federally mandated tests with unbelted dummies. IIHS said dummies are always belted in its vehicle ratings tests.