Passive car safety features are devices which, in the event of a car crash, automatically deploy to protect the vehicle’s occupants. “Passive” refers in this instance to the driver. Unlike “Active” safety features – such as brakes – which require the driver to work to prevent an accident, passive safety features require no deliberate input by the occupants and remain dormant until the moment of the actual crash.
Passive safety features include crumple zones, airbags, seatbelt pretensioners and safety cells (reinforced body shells).
Crumple zones are areas of the car which are designed to deliberately deform when the car collides with a heavy object. The purpose of this is to absorb some of the energy in the collision, so that the extreme forces of the crash are not fatally transferred to the occupants. The force of a crash is determined by a car’s deceleration during the collision. If the entire area hit is rigid, the deceleration will be very sudden, and so the momentum of the car will be transferred to those inside it, killing them. If, however, a softer, crushable buffer zone is in place, the energy will be transferred to the crumpling of that area, reducing the rate of deceleration inside the car.
Airbags work on much the same principles, but applied to the occupants. During a crash, the people inside a car are thrown forward relative to the cabin as the rest of the car decelerates around them. When their bodies hit the frame of the car, their internal organs collide with their ribcage, puncturing both and causing death. The cushion of air again acts as a buffer zone to spread the victim’s deceleration over a larger space and time, so that velocity is greatly reduced, and so the internal impact on the ribcage is not as harsh. Modern airbags are controlled by sensors which determine the severity of the crash forces, and so the bags are fired at the precise time (to the nearest hundred milliseconds) for the optimum effect. Too late and they would be useless, too early and they may cause more damage than they prevent.
Pyrotechnic seatbelt pretensioners are used to restrain the occupants upon impact, and hold them in place so that the airbags may work correctly. If there is no restraint, the driver/passenger’s body will be flung forward, with the head violently striking the dashboard. Instead, pyrotechnic charges fire upon impact to suck the belt inwards, clamping the wearer flat against the seat so that they are in the correct position when the airbag deploys.
Government statistics report that the use of seatbelts saves around 2200 lives every year in the UK, while around 26000 lives of US drivers have been saved by airbags every year since their introduction.