The parking sensors warn you of obstacles behind your car and for some models about ones in front. An intermittent warning tone is activated by ultrasonic sensors in the bumpers when the car comes within 1.6 meters of an obstacle. As your car gets closer, the warning tone speeds up. When you have just 20 cms of maneuvering space left, a continuous warning tone sounds. The rear parking distance warning system is switched on automatically when the car is put in reverse.