The Audi parking system (APS) makes parking easier. An intermittent warning tone indicates how far the car is from an obstacle. As the distance decreases, the warning tone becomes faster. It first sounds when the car is 1.6 metres from the obstacle and sounds continuously when only 20 centimetres remain. The APS system uses ultrasonic sensors that can be installed at the rear as well as the front of the car. It can be switched off at the front in special situations such as stop-and-go traffic. At the rear, APS switches on automatically when Reverse is selected.