California Code, Vehicle Code - VEH § 24604

(a) Whenever the load upon any vehicle extends, or whenever any integral part of any vehicle projects, to the rear four feet or more beyond the rear of the vehicle, as measured from the taillamps, there shall be displayed at the extreme end of the load or projecting part of the vehicle during darkness, in addition to the required taillamp, two red lights with a bulb rated not in excess of six candlepower plainly visible from a distance of at least 500 feet to the sides and rear. At any other time there shall be displayed at the extreme end of the load or projecting part of the vehicle a solid red or fluorescent orange flag or cloth not less than 18 inches square.

(b) There shall be a single flag or cloth at the extreme rear if the projecting load is two feet wide or less. Two warning flags or cloths are required if the projecting load is wider than two feet. Flags or cloths shall be located to indicate maximum width of loads that extend beyond the sides or rear of the vehicle.