(a) Any stop or turn signal when required herein shall be given either by means of the hand and arm or by signal lamps except as otherwise provided in subsection (b).
(b) Any motor vehicle in use on a highway shall be equipped with, and required signal shall be given by, signal lamps when the distance from the center of the top of the steering post to the left outside limit of the body, cab or load of such motor vehicle exceeds twenty-four inches or when the distance from the center of the top of the steering post to the rear limit of the body or load thereof exceeds fourteen feet. The latter measurement shall apply to any single vehicle or to any combination of vehicles.