The stuff above about airbag deployment is not correct.
I was the test manager of an electronic contract manufacturer in Hillsboro, Oregon for 2 years. We made automotive electronics for NEC who supplies Honda.
The facts are as follows:
1) If you are very light and short, say under 90 pounds, the airbag won't deploy at all. The seat has a capacitive antenna in it (all makes and models of all cars sold in the U.S.A.) and knows how tall you are.
2) If you are a bit heavier and taller, maybe 120 pounds and 3/4 of the way up the seat back, then half of the propellant fires and that's it.
3) A bit heavier and taller yet, then half of the propellant fires, followed by a time delay, followed by the other half of the propellant.
4) Big, tall guy or gal, it all fires at once.
5) Will NOT fire below a certain "g" level. If that "g" level was exceeded (probably not since you are alive and well), and if it did not fire, then the g-sensor is likely a bad unit. There is a VERY low probability of that, on the order of less than 1/10 of 1%.
So, if you are short and light, it won't fire. if you are medium, it SHOULD fire if you exceed the threshold "g" force. If you did, and if it didn't fire, you were probably very heavily injured and barely survived. If not, you most probably didn't exceed the required "g" force to fire the unit.
A relatively light tap, like you hitting a big pothole or a fender bender, should NEVER cause the airbag to fire. If it does when it doesn't need to do so, it can injure you badly. So, they monitor the "g" force and your height very closely.
I used to run the department that maintained the test equipment for manufacturing these things that went into Hondas, and the above is TRUE.
Other makes and models have to pass the SAME standards with the SAME reliability, no exceptions, for all cars sold in the U.S.A. It's Federal Law.
By now, 2008, every new car sold in the U.S.A. knows how tall you are and how much you weigh as soon as you sit in the seat and turn on the ignition key. Actually, it doesn't know if you are taller than the seat, but it knows if you are shorter than the seat. Weight is simple to measure.
The real failure rate is astronomically low since the circuits are simple and reliable. Most so-called "failures to deploy" are accidents that did NOT exceed the "g" force necessary to deploy the airbags.
Think of the lawsuit if you could injure someone by stepping in front of a car with a driver and/or passenger and simply hit it with a a sledge hammer causing injury to driver or passenger. That would NOT be a good product. It needs to recognize false "wrecks."
The threshold of deployment is really the threshold of bodily injury to a human being of average size and health, seated and belted properly. The car manufacturers have no need to protect you if your seat belt is not fastened since a fastened seat belt is the law if youa re driving. They also have an issue if the airbag deploys accidentally ... and they work to see that does NOT happen.