A car door restrictor is a mechanical device that prevents the car doors from being opened by more than about 50 millimetres when the car is outside the door zone. Its purpose is to stop passengers from forcing the doors and stepping into the shaft during an entrapment. EN 81-20 requires a restrictor on lifts where passengers could otherwise open the car door away from a landing. The device releases automatically when the car re-enters the unlocking zone, and can be overridden by rescue personnel from outside.