Any motor vehicle in use on a highway shall be equipped with, and required signal shall be given by, a signal lamp or lamps or mechanical signal device 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 the motor vehicle exceeds twenty-four inches (24″), or when the distance from the center of the top of the steering post to the rear limit of the body or load of it exceeds fourteen feet (14?). The latter measurement shall apply to any single vehicle, and/or to any combination of vehicles.