One particular airbag could be doing more harm than good, according to a study by the Insurance Institute for Highway Safety.

After looking at recorded "injuries" in more than 400 small- and moderate-overlap front crash tests, researchers at the institute turned the spotlight on a seldom-thought-of safety device: the knee airbag. While these types of crashes are most likely to lead to lower-leg injuries (read up on the Dodge Challenger if walking away from a crash is something you value), the presence of a cushion for that all-important leg joint didn't do much to alleviate the risk of injury.

Surprisingly, the presence of knee airbags in some cases actually upped the risk of leg injury. The IIHS researchers then pored over real-world data to see if the same outcomes showed up in actual crashes.

From the IIHS:
Knee airbags had only a small effect on injury measures recorded by dummies in IIHS driver-side small overlap front and moderate overlap front crash tests. 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 airbags had no effect on injury measures in the moderate overlap test.

In the analysis of real-world crashes, knee airbags reduced overall injury risk by half a percentage point, from 7.9 percent to 7.4 percent, but this result wasn't statistically significant.
The institute suggests the proliferation of knee airbags is primarily aimed at helping manufacturers pass tests conducted with unbelted crash test dummies. In collisions where the occupant is not restrained, such an airbag might prove valuable. When you're a loose pebble in a tin can, everything helps.

For secure front seat occupants, the value in having a knee airbag is much less clear.

"There are many different design strategies for protecting against the kind of leg and foot injuries that knee airbags are meant to address," said Becky Mueller, IIHS senior research engineer and the study's co-author, in a statement. "Other options may be just as, if not more, effective."

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