163Any stop or turn signal when required herein shall be given either by means of the hand and arm or by a signal lamp or lamps or mechanical signal device, but when a vehicle is so constructed or loaded that the distance from the centre of the top of the steering post to the left outside limit of the cab, body or load exceeds sixty centimetres or that the distance from the centre of the top of the steering post to the rear limit of the body or load of the vehicle, or the last vehicle where more than one vehicle is included in a single combination, exceeds four hundred twenty centimetres, then the vehicle shall be equipped with, and the said signals shall be given by such a lamp or lamps or signal device.