RCW 47.36.090: Cooperation with United States on road markers.

Standard federal road markers shall be placed on state highways in the manner requested by the department of transportation of the United States. The department of transportation of the state of Washington is authorized and empowered to cooperate with the several states and with the federal government in promoting, formulating, and adopting a standard and uniform system of numbering or designating state highways of an interstate character and in promoting, formulating, and adopting uniform and standard specifications for the manufacture, display, erection, and location of road markers and signs, for the information, direction, and control of persons traveling upon public highways.