The Insurance Institute for Highway Safety (IIHS) has concluded that knee airbags provide little safety benefit to drivers and front-seat passengers.

To study the effectiveness of knee airbags, the IIHS compiled crash reports for more than 400 frontal accidents across 14 states in the US. They then compared the injury risk in vehicles with knee airbags to vehicles without them. The conclusion? Knee airbags aren’t particularly useful.

“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,” the institute states. “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.”

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In addition, the IIHS’s investigation found that while knee airbags reduced overall risk injury from 7.9 percent to 7.4 percent, that’s not enough of an improvement to be statistically significant.

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

The IIHS says many car manufacturers install knee airbags to pass federal safety tests using unbelted dummies.They suggest it is possible knee airbags would be more effective in preventing injuries of those not using seat belts, but the IIHS study didn’t look specifically at crashes where people weren’t using their seat belts.