Do Driver Knee Airbags Provide Any Benefit? - Part 1: Background

As I've been doing my "Airbag Timeline" series, I've noticed that front airbags spread rapidly on cars. For instance, the first driver airbag on a midsize car was made standard for the 1990 model year. Three years later, most midsize cars had a driver airbag standard, and by 1995, all but one had dual airbags standard. It only took a few years for side and side curtain airbags to undergo a similar transformation; in 2005, only 32% of vehicles had standard side and curtain airbags. By 2010, this number was 77%. The safety benefits of front, side torso, and side curtain airbags are clear. These airbags can quite literally save your life. 

Knee airbags haven't spread quite as fast. Even in early 2007, a number of non-luxury cars, such as the Toyota Camry, Kia Sorento, Mitsubishi Lancer, and Scion tC were equipped with driver knee airbags. Almost a decade later, only 58% 2014-2017 of vehicles tested by the IIHS have a driver knee airbag, and just 26% have a passenger knee airbag. 

Some vehicles, such as the aforementioned Kia Sorento, offered knee airbags in the past but no longer offer them. In fact, knee airbag availability seems to be stagnating in recent years; while 63% of the 2014-2015 vehicles tested had a driver knee airbag, just 52% of the 2016-2017 vehicles did. Passenger knee airbags were in 24% of 2014-2015 vehicles tested and 30% of 2016-2017 vehicles tested.

The benefits of knee airbags aren't as clear. Ostensibly, the bags' purpose is twofold: to protect the occupant's legs, as well as to prevent the occupant from sliding forward, keeping him in a better posture during the crash. 

To find out if knee airbags worked, I looked at the moderate overlap crash test results for all 95 vehicles that the IIHS has tested so far for model years 2014-2017. All 95 of these vehicles earned a Good overall rating, with the structures holding up well in every one and no life-threatening injury risk to any. Since all of the vehicles did well, this is going to be a battle of force levels and small percentages of injury risk. After all, every tenth of a percent less injury risk and every pound less force your body experiences in a crash count. 

The tests are conducted on the driver's side with a driver dummy; therefore, the 55 vehicles equipped with a driver knee airbag are compared to the 40 without a knee airbag. For leg injury, the individual measures for the legs (knee displacement, tibia index, and tibia axial force) are compared, while for whole-body injury, the overall injury risk score (explained in this post) is compared. The overall injury risk score encompasses femurs, chest, neck, and head - basically everything above the knee.