The safety feature that inflates in a crash to protect occupants is an airbag.
An airbag is a supplemental restraint system: a sensor-controlled fabric cushion that deploys during qualifying collisions. It creates an energy-absorbing barrier between a person and the steering wheel, dashboard, side structure, roof lining, or windshield, reducing the severity of some impacts. It is designed to work with—not replace—a seat belt.
Crash sensors send information about impact direction and severity to an electronic control unit. If deployment thresholds are met, an inflator rapidly produces gas and fills the cushion in roughly 0.04 seconds. Vent holes then let the gas escape as the occupant loads the bag, helping spread deceleration over more time and body area.
Airbags are not always supposed to deploy in every crash, and they can cause injuries if an occupant is too close or unbelted. Modern vehicles may have frontal, side, curtain, knee, seat-mounted, and pedestrian airbags. A seat-belt pretensioner tightens the belt, while a crumple zone absorbs crash energy in the vehicle structure; neither is an inflating occupant cushion.