Reversing Aids and Parking sensors use bumper mounted sensors to check the distance to obstacles - generally mounted on the rear bumper. They are a great add-on for longer vehicles like estates, and those with limited rear visibility, like many MPVs and 4x4s.

The more sensors the kits has, the more accurate results you will get, as there will be fewer blind spots. Sensors will be connected to either and audible warning device or some kind of display on the dashboard.