Minn. Stat. § 169.52

When 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 when lighted lamps on vehicles are required in this chapter, a red light or lantern plainly visible from a distance of at least 500 feet to the sides and rear. The light or lantern required under this section shall be in addition to the rear light required upon every vehicle. At any time when no lights are required there shall be displayed at the extreme rear end of such load a red, yellow or orange flag or cloth not less than 16 inches square.