Except as provided in subsections (b) and (c) of this section, each required stop or turn signal shall be given: By hand and arm in conformity with § 21-606 of this subtitle; or By signal lamps. Each motor vehicle in use on a highway shall be equipped with and the required signal given by signal lamps, if 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 vehicle is more than 24 inches. Each motor vehicle in use on a highway shall be equipped with and the required signal given by signal lamps if, for any vehicle or combination of vehicles, the distance from the center of the top of the steering post to the rear limit of the body or load is more than 14 feet.