PDFWAC 204-21-070
Supplemental high-mounted stop and rear turn signal lamps.
Supplemental high-mounted stop and rear turn signal lamps must meet requirements of SAE J1957 and J2068.
(1) Supplemental high-mounted stop and rear turn signal lamps must:
(a) Be single-faced.
(b) Be actuated in the same manner and at the same time as the required stop lamps or turn signal lamps.
(c) Be mounted not lower than thirty-five inches nor higher than fifty-five inches. Except that standard stop or turn signal lamps not combined with tail lamps or reflex reflectors may be used respectively as supplemental lamps in which case they must be mounted at any height not lower than fifteen inches or higher than seventy-two inches.
(d) Not be used in lieu of required stop and turn signal lamps.
(2) Supplemental turn signal lamps and combination stop-and-turn signal lamps must be mounted in pairs facing the rear with one lamp near each side of the vehicle, at the same height and equally spaced from the vehicle centerline.
(3) Supplemental stop lamps must be mounted in pairs as specified above or with not more than two lamps on or adjacent to the centerline of the vehicle.