Paragraph 17

It shall be the duty of the person in immediate charge of the carrying out to any vehicle on a line of any operation of repair to ensure that, in every case in which the safety of a person engaged in that operation might be endangered as a result of the movement of a vehicle, a danger signal is displayed at or near each end of the vehicle under repair or (if that vehicle for the time being forms part of a train of vehicles) of that train (as the case may be), being an end which might be struck or moved by another vehicle.