Air bags generally deploy when a car hits a solid, fixed object at 8 to 14 miles per hour or a movable object at about 16 to 28 miles per hour. Some newer cars have higher thresholds for airbag deployment due to the risk of injury from airbag deployment.
Newer frontal air bags are designed to deploy only in accidents that may result in serious head injury. They should not deploy in minor accidents, since the injuries from the air bag deployment may be more severe than the accident would have caused. Frontal air bags may also deploy in some side-impact crashes because rapid deacceleration similar to that of a frontal crash may occur. They also may deploy in a rear impact crash if the car is pushed into another car at a high rate of speed.