With airbags, it may seem like the more the merrier — they are essential devices that have prevented tens of thousands of deaths in vehicle accidents, after all — but a new study by the Insurance Institute for Highway Safety suggests that one type, knee airbags, don’t do much in the way of preventing injury.
In some cases, they may even increase risk, researchers have found.
Knee airbags, which deploy from a car’s lower dashboard, are meant to distribute impact forces on an occupant’s legs in the case of an accident, thereby reducing leg injuries. Designed to control movement of the lower body, these airbags may also reduce impact forces on the abdomen and chest.
IIHS looked at data from more than 400 frontal crash tests it performed as part of its vehicle ratings program to see if injuries were less likely in knee-airbag-equipped cars.
In the driver-side small overlap front test — one of five intensive crash tests that factor into IIHS’ Top Safety Pick and Top Safety Pick Plus crashworthiness awards — knee airbags were associated with an increased risk for lower leg and right femur injuries. This test is designed to replicate what happens when the front left corner of a vehicle collides with another vehicle or object. Crash forces sometimes push the front wheel into the footwell, which could lead to serious leg or foot injuries.
The study also found that knee airbags had no effect on injury measures in the moderate overlap front crash test, which uses forces similar to those in a frontal offset crash between two vehicles.
Looking at real-world crash reports from 14 states, IIHS concluded that knee airbags did reduce overall risk of injury from 7.9 percent to 7.4 percent — a result that the organization said is not statistically significant.
If these knee airbags are, as the study suggests, doing more harm than help, why are they in vehicles in the first place? According to the IIHS, it is possible that knee airbags are beneficial to occupants in real-world crashes who aren’t wearing seatbelts. One of the reasons car manufacturers have been installing them is to help vehicles pass federally mandated tests with unbelted dummies. The IIHS said dummies are always belted in its vehicle ratings tests.