But there are other factors in determining whether the airbag should deploy - what if you are stationary and hit by something? You still want the bags to deploy.
Depends, if you really want to think about it, there are numerous times one may, or may not, want the bags to deploy. While I wouldn't worry about the front bags, I'd certainly want the side bags to deploy at low speeds or even when stopped if the impact was more than a good thump! if you're sitting in a car, with the ignition/engine on, you could be expected to be wearing seat belts and so if a minimum speed for the bag for a head on collision is 30kph, then a car hitting you would have to be doing just over 42kph for the same energy, which would be a rare event.
Some manufacturers use two stage bags, one for lighter impacts and a stronger one for big impacts - with seat pressure sensors, they may also be used for light and heavy people, reducing the risk for children and smaller people who may be closer to the bag when it goes off.
For all that, some experts have suggested that the best thing to improve safety would be a big spike in the middle of the steering wheel - with that as a reminder, drivers could be expected to be VERY careful when driving!
