(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, also to any combination of vehicles. On a combination of vehicles, the rear signal lamps shall be at the rear of the last vehicle in the combination. The provisions of this subdivision shall not apply to a vehicle, alone or in combination with other vehicles or implements, if such vehicle is used solely for agricultural purposes, and is not designed and intended primarily for use on the highway, or to fire vehicles, or to special purpose commercial motor vehicles registered under schedule F of subdivision seven of section four hundred one of this chapter.