The device has an alarm lamp operated in hazardous distance by a set of sensors i.e. ultrasonic sensors, and a cut-off unit provided to switch off the alarm lamp at a rear front side of a vehicle (1). Another set of sensors (5) is resilient secured at the vehicle. The vehicle is moved backwards in a plane. The alarm lamp is arranged on each supporting body and optionally in a cab (2). The alarm lamp has a bright white LED, a yellow LED and a red LED. The alarm lamp is activated by the sensors and switched off through the cut-off unit. An independent claim is also included for a trial-and-error method.
