Whenever the load upon any vehicle extends to the rear four feet or more beyond the bed or body of such vehicle there shall be displayed at the extreme rear end of the load, at the times specified in section 321.384, a red light or lantern plainly visible from a distance of at least five hundred feet to the sides and rear. The red light or lantern required under this section shall be in addition to the red rear light required upon every vehicle. At any other time there shall be displayed at the extreme rear end of such load a red flag or cloth not less than sixteen inches square.