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How airbags know when to save your life

They don't always get it right, but it's a miracle they work at all.

One of our viewers in New York, Gerald T., told me he is fascinated by crash-test videos but puzzled by the different ways airbags deploy in various cars, even when subjected to the exact same crash. The answer lies in the the brain of the airbag.


The Airbag Control Module is a hardened computer in your car with a single purpose: Decide when airbags need to be fired and get that signal to them.


That brain is the Airbag Control Module, or ACM, a hardened computer in each car that keeps communicating with airbags even when the car around them is disintegrating. It gets constant motion data from a series of accelerometers around the vehicle and must decide if and when airbags should be deployed. If the answer is yes, it must be done during the key 20 milliseconds of what is only about a 100 millisecond impact. Talk about threading the eye of a needle.

Now playing: Watch this: Why air bags don't always deploy

But why airbags don't just deploy liberally in every impact comes down to three factors:

  • Prevent injuries and save lives: This is the main goal, of course, and airbags are said to prevent some 2,800 auto fatalities per year. But doing so requires the ACM software consider the nature of an impact, its direction and rotation, the crumple characteristics of the car its installed in, and the distance between each airbag and occupants. Its like taking an AP Calculus exam in a fraction of a second. The answer isn't just "deploy airbags" because of the next two factors:
  • Minimize airbag injuries: They are not completely benevolent devices and often cause their own fractures, bruises, and burns as they do their job. ACM programming is carefully tuned to only use them when the alternative is even worse.
  • Minimize airbag replacement costs: Airbags cost roughly $1,000 apiece and can only be used once. In some cases, airbag replacement can make up the lion's share of collision repair costs, so deploying them is not taken lightly otherwise a lot more cars would be totaled by insurers.

Many people are surprised that there isn't a standard airbag scheme used in every car, but that's because airbags are as smart as seat belts are dumb. Maybe we need a new bumper sticker:


Deploying airbags is like taking an AP Calculus exam in a fraction of a second with lives in the balance if you get one answer wrong.