The signals required by this division shall be given either by means of the hand and arm or by a signal lamp or mechanical signal device, but when the body or load on any vehicle or combination of vehicles projects 24 inches or more to the left of the center of the steering wheel, or under any condition when a hand and arm signal would not be visible both to the front and rear of the vehicle or vehicles, then the vehicle or vehicles shall be equipped with, and signals shall be give by, a signal lamp or device.