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Luxury cars often now come with ultrasonic proximity sensors to help drivers avoid collisions while parking. Now AB Automotive of Cardiff plans an add-on version that drivers can fit to older models (GB 2376075). The device is rather like an oversized bumper sticker, with a conductive strip stretching the length of the bumper. A control unit in the car feeds a low-voltage, high-frequency signal to the strip. Capacitance between the strip and the road below allows a certain amount of the signal to leak to earth. This capacitance changes as the strip on the bumper moves close to any solid object that is even slightly conductive, such as a brick wall or a pedestrian, and an alarm sounds.