A stop motion device for use in preventing or arresting movement, more especially as a safety device for a vehicle when reversing comprises detection means (26 or 51, 53, 50) moveable between an inoperative or retracted position on the rear of a vehicle to a rearwardly extending activated or operative position. On detection of an obstacle by the detection means immediate automatic application of the vehicle brakes is caused to take place and also simultaneous retraction of the detector means back to the rear of the vehicle. Thus, for a reasonably cautious reversing speed for which the device is intended, braking distance of the vehicle is accommodated before it strikes the obstacle. Therefore, only light initial contact of the detection means (26) with the obstacle occurs. Where the detection means (51, 53) provides a transverse infrared detection beam (50), there is no actual physical contact with the obstacle.