California Code, Vehicle Code - VEH § 25260.3

Any vehicle having personnel aerial lift equipment, actually engaged in the construction, removal, maintenance or inspection of any building, structure, or appurtenances thereto, including the cutting or trimming of trees immediately adjacent thereto, may display flashing amber warning lights to the front, sides, or rear when necessarily parked on a highway or when moving at a speed slower than the normal flow of the traffic.