Whenever the load on any vehicle extends more than four feet beyond the rear of the bed or body thereof, there shall be displayed at the end of such load in such position as to be clearly visible at all times from the rear of such load a red flag not less than twelve inches both in length and width, except that between sunset and sunrise, there shall be displayed at the end of any such load a red light plainly visible under normal atmospheric conditions at least two hundred feet from the rear of such vehicle.