Cal. VEH § 25251

(a) Flashing lights are permitted on vehicles as follows:

(1) To indicate an intention to turn or move to the right or left upon a roadway, turn signal lamps and turn signal exterior pilot indicator lamps and side lamps permitted under Section 25106 may be flashed on the side of a vehicle toward which the turn or movement is to be made.

(2) When disabled or parked off the roadway but within 10 feet of the roadway, or when approaching, stopped at, or departing from, a railroad grade crossing, turn signal lamps may be flashed as warning lights if the front turn signal lamps at each side are being flashed simultaneously and the rear turn signal lamps at each side are being flashed simultaneously.

(3) To warn other motorists of accidents or hazards on a roadway, turn signal lamps may be flashed as warning lights while the vehicle is approaching, overtaking, or passing the accident or hazard on the roadway if the front turn signal lamps at each side are being flashed simultaneously and the rear turn signal lamps at each side are being flashed simultaneously.

(4) For use on authorized emergency vehicles.

(5) To warn other motorists of a funeral procession, turn signal lamps may be flashed as warning lights on all vehicles actually engaged in a funeral procession, if the front turn signal lamps at each side are being flashed simultaneously and the rear turn signal lamps at each side are being flashed simultaneously.

(b) Turn signal lamps shall be flashed as warning lights whenever a vehicle is disabled upon the roadway and the vehicle is equipped with a device to automatically activate the front turn signal lamps at each side to flash simultaneously and the rear turn signal lamps at each side to flash simultaneously, if the device and the turn signal lamps were not rendered inoperative by the event which caused the vehicle to be disabled.

(c) Side lamps permitted under Section 25106 and used in conjunction with turn signal lamps may be flashed with the turn signal lamps as part of the warning light system, as provided in paragraphs (2) and (3) of subdivision (a).

(d) Required or permitted lamps on a trailer or semitrailer may flash when the trailer or semitrailer has broken away from the towing vehicle and the connection between the vehicles is broken.

(e) Hazard warning lights, as permitted by paragraphs (2) and (3) of subdivision (a) may be flashed in a repeating series of short and long flashes when the driver is in need of help.