(a)    Any rural letter carrier’s motor vehicle may be equipped with a warning device for the purpose of warning the drivers of other vehicles of the presence of a vehicular traffic hazard requiring the exercise of unusual care in approaching, overtaking, and passing, and, when so equipped, may display the device in addition to any other warning signal required or permitted by law.

(b)    The warning device shall include two bi-directional lights mounted at each side of the vehicle on the roof, with two amber lenses facing the rear and two amber lenses facing the front.

(c)    Each light shall be at least 4 inches in diameter and shall be powered by a bulb of at least 21 candlepower, with reflectorization sufficient to assure visibility for at least 500 feet in front of and to the rear of the vehicle.

(d)    The lights shall be wired with one “on-off” switch and one “flashing” switch, which shall activate both lights simultaneously, and which may be wired to operate from the vehicle’s stoplight switch.

(e)    Between each light on the assembly of the device a sign shall be mounted with the words “U.S. Mail” in black letters on a white background, facing both to the front and the rear.

(f)    The lettering on the sign shall be at least 4 inches in height, and the width of each stroke shall be at least 3/4 of an inch.

(g)    The sign shall be constructed to permit it to be folded down out of vision when not in use.

(h)    The warning device may be used only during the discharge of a rural letter carrier’s official duties.

(i)    Before the first stop on the route and following the last one the lights shall be darkened and the sign folded down out of vision.

(j)    The warning device may not be installed or operated until an application has been submitted to and approved by the Administration.

(k)    The application shall be made by the individual rural letter carrier on forms furnished by the Administration.