They are not designed to inflate in rear-end or rollover crashes or in most side creshes. Generally, air bags are designed to deploy in crashes that are equivalent to a vehicle crasing into a solid wal at 8 to 14 miles pre hour.
Air bags inflate when a sensor detects a front-end crash.
In 1998, an estimated 1.043 lives were saved by air bags. From 1987 to 1998 an estimated 3,706 lives were saved. There is no guarantee of safety in a crash, with or without an air bag. However, most of the people killed by air bags would not have been seriously injured if they had followed these safety tips. Since the risk zone for driver air bags is the first two to three inches of inflation, placing yourself ten inches from your driver air bag provides you with a clear margin of safety.