(a) Any stop or turn signal when required in this chapter shall be given either by means of the hand and arm or by signal lights, except as otherwise provided in subsection (b) of this Code section.

(b) Any motor vehicle in use on a highway shall be equipped with, and a required signal shall be given by, signal lights 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 limits of the body or load thereof exceeds 14 feet. The latter measurement shall apply to any single vehicle and also to any combination of vehicles.