Effectiveness of knee airbags across two crash paradigms

July 2019

Objective: Knee airbags are increasingly common in the fleet, but their performance in real-world crash scenarios remains largely untested. The current study was designed to evaluate the effects of knee airbags in reducing injury rates among drivers involved in a frontal crash. Method: Analyses were conducted on two datasets. First, data from 414 frontal crash tests were used to calculate injury probability for 12 body regions. Injuries in each of these regions were compared across vehicles with knee airbags and vehicles without them. Second, police-reported crash data were compiled from 14 states and linked to knee airbag status by vehicle make and model. The injury rate for drivers of knee-airbag-equipped vehicles was compared against that for drivers of vehicles without knee airbags. Results: The presence of knee airbags did not significantly reduce overall injury risk for small or moderate overlap crash tests. In fact, knee airbags in the small overlap tests were associated with elevated injury risk for the upper and lower tibia on the right and left sides. Knee airbags were also associated with increased injury risk for the right femur in these tests, as well as a reduced injury risk for the head. Knee airbags in the moderate overlap tests did not affect injury outcomes for any body region. The police-reported crash data found knee airbags to slightly reduce overall injury risk (from 7.9% to 7.4%), but this effect was not statistically significant after controlling for vehicle weight, model year, and driver characteristics. Conclusions: The current study suggests that knee airbags do not confer a substantial safety benefit onto drivers. Both in crash tests and real-world crashes, vehicles equipped with knee airbags did not significantly reduce injury risk. Manufacturers should consider the limited safety benefit of knee airbags before expanding their fitment in the fleet.