When it comes to air bags, more is not always merrier

Dear Car Talk: What’s the purpose of knee air bags? I was in an accident that resulted in the knee air bag doing major damage to my left leg. The right leg was on the brake and sustained bruising but not terrible problems.

Are the air bag manufacturers just good friends with the automakers? — Sondra A: Good question, Sondra. When they were introduced, the feeling about air bags was “the more, the merrier.” After all, right behind your dashboard is steel, and if we can provide a cushion between your knee and steel, why not, right?

The problem is that our federal safety regulators have a mandate to protect two different populations: the belted and unbelted.

So, when a car gets “crash tested,” they have to test it with both a dummy that’s wearing its seat belt, and a complete dummy that’s not wearing its seat belt. And in order to pass both of those tests, automotive engineers have to make compromises.

In the case of knee air bags, engineers figured out that an air bag at the knees could help keep an unbelted dummy in a more upright position during a crash, so he wouldn’t slide under the steering wheel and get crushed to death.

Unfortunately, that probably required a larger and more powerful knee bag than was necessary just to protect the lower legs of the belted majority of drivers.

So it seems knee air bags aren’t optimized for people like you and me, who take two seconds to put on our seat belts. And as a result, they can be problematic. A 2019 study by the Insurance Institute for Highway Safety demonstrates that.

IIHS studied real-world crash data from 14 states. And they found that for drivers and passengers wearing their seat belts, knee air bags barely helped prevent injuries — they decreased the overall injury risk by about half a percent — and in some types of accidents, they increased the risk of lower-leg injuries.

So what to do? It’s a public policy question that’s beyond the purview of this crash test dummy. But if it were up to me, I’d focus on the people who wear their seat belts and issue everybody else a football helmet and wish them the best of luck.

I’m really sorry you were injured, Sondra. I hope you heal up quickly and completely.