Wash. Rev. Code § 47.68.340
A structure or obstacle that obstructs the air space above ground or water level, when determined by the department after a hearing to be a hazard or potential hazard to the safe flight of aircraft, shall be plainly marked, illuminated, painted, lighted, or designated in a manner to be approved in accordance with the general rules of the department so that the structure or obstacle will be clearly visible to airmen or airwomen. In determining which structures or obstacles constitute a safety hazard, or a hazard to flight, the department shall take into account those obstacles located at a river, lake, or canyon crossing, and in other low-altitude flight paths usually traveled by aircraft including, but not limited to, airport areas and runway departure and approach areas as defined by federal air regulations.
RCW 47.68.340