Whenever the load upon any vehicle extends to the rear four feet [121.92 centimeters] or more beyond the bed or body of the vehicle there must be displayed at the extreme rear end of the load, at the times specified in section 39-21-01, a red light or lantern plainly visible from a distance of at least six hundred feet [182.88 meters] to the sides and rear. The red light or lantern required under this section must be in addition to the red rear light required upon every vehicle. At any other time there must be displayed at the extreme rear end of a load a red flag or cloth not less than twelve inches [30.48 centimeters] square and so hung that the entire area is visible to the driver of a vehicle approaching from the rear.