In any case where a vehicle is loaded with any material in such a manner that any portion of the load extends toward the rear four feet or more beyond the rear of the bed or body of the vehicle, there shall be displayed at the extreme end of the load, at the times and under the conditions hereinbefore specified, in addition to the ordinary rear or tail light hereinbefore required to be displayed on the vehicle, a red light plainly visible under ordinary atmospheric conditions at least two hundred feet from the rear; provided that at other times while the vehicle is upon the public highway, a red flag or cloth not less than sixteen inches square shall be displayed at the extreme rear of the load.