The invention relates to a device (8) to be arranged on a motor vehicle (2). Said device (8) comprises a speedometer (4) for ascertaining a relative speed between an object and a device (8), at least one warning mechanism (6) for emitting a warning signal in the direction of the object, find an electric controller which is designed to emit a warning signal via the at least one warning mechanism (6) when the ascertained relative speed exceeds a predefined threshold value.