Knee Airbags: Are Their Safety Claims A Bunch Of Hot Air?

Bonus! A rudimentary history of the airbag. 

The idea of the airbag - a gas-inflated cushion to cushion the blow of major car accidents - is a great idea, by and large. Invented in 1952, technology was insufficient to make the airbag practical at the time. By the 1970s, the technology was there, and General Motors offered dual front, dual stage airbags from 1973-1976. Called ACRS, Air Cushion Restraint System, it was a great idea for the time, with a flaw - it was designed to work without, not with, seat belts. Then, in the early 1980s, Mercedes Benz came out with a system, called "SRS" for Supplemental Restraint System, that was designed to supplement the seat belt in a major crash. At first, it had only a driver airbag, but passenger airbags soon followed. Soon, luxury manufacturers began to adopt the bags, and news articles began to appear touting their safety benefits. Crash tests also proved the safety benefits of the airbags. By the early 1990s, airbags were irresistible. They were saving lives. By 1993, most new cars in the US had a driver airbag, and by 1995 most were equipped with dual airbag.

Side airbags followed shortly thereafter. Pioneered by Volvo in 1994, most luxury cars offered them as standard by 1998, and even economy cars such as the Toyota Corolla could be had with them by then. By 2008, they were ubiquitous on new cars. Curtain airbags - pioneered by BMW in 1997 - also saw quick adoption. A stringent new crash test which simulates an SUV or a pickup striking the side of a vehicle was launched by the IIHS in 2003, and test after test proved that some sort of head protection was needed to get even a halfway decent score in the test. Real world data also proved the curtains' amazing benefit: 45 percent reduction in driver deaths in driver side impacts was observed in a 2003 IIHS study. Today's curtains are far better at cushioning heads and blocking debris than the late 1990s-early 2000s curtains seen in that study, so the fatality reduction would be expected to be even greater. 

Almost all cars sold since about 2009 (and most from a few years before then) come with a set of 6 airbags standard. You get a driver front airbag, a passenger front airbag, a side airbag in each front seat, and side curtain airbags on each side. On top of this common set, knee airbags, rear seat side airbags, and even seat cushion airbags are cropping up in the race to cram more 'bags in our rides. 

Knee airbags are the most common. A large percentage of cars, maybe half, have at least a driver knee airbag. Passenger knee airbags, fortunately, remain rare. While a driver knee airbag certainly doesn't hurt things, I've found that it offers limited benefits at best. 

Wha?!?! An airbag that doesn't help much?

Knee airbags are commonly touted to provide cushioning to the knees and legs, hence the term "knee airbag". The idea gives me a bit of heebie-jeebies, since I sit with next to no distance between my knees and the dash. (I'm 6' tall with fairly short legs for my height). But my car doesn't have knee airbags - it has only the common six airbag system. So I wouldn't know the ergonomics of actually sitting behind a knee airbag. For all I know, they could have it set up so that it's more comfortable to sit with your knees farther back from the lower dash. 

It is the structure of the car that primarily determines what can happen to the legs and knees in a frontal crash, not the presence or absence of a knee airbag. The new IIHS small overlap is a gold mine for what can happen during structural collapse; there are a number of structural failures both with and without knee airbags. Some cars from Lexus, Toyota, Ford, GM, even Mercedes and BMW, equipped with knee airbags earned poor ratings for leg protection in the new test. That's not to say that all cars with knee airbags will crush your legs in a small overlap crash test - there were a few good performers equipped with them. And some cars did seem to show some benefit from the knee airbags, such as the 2012-present Toyota Camry, which suffered structural collapse but left the legs relatively uninjured.

So, overall, knee airbags don't really help or hurt. And they generally don't make up for a bad structure. Six airbags are enough.