(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 the 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 24 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 14 feet. The latter measurement shall apply to any single vehicle, also to any combination of vehicles.