Wash. Rev. Code § 46.61.430
Notwithstanding any law to the contrary or inconsistent herewith, the secretary of transportation shall have the power and the duty to fix and regulate the speed of vehicles within the maximum speed limit allowed by law for state highways, designated as limited access facilities, regardless of whether a portion of said highway is within the corporate limits of a city or town. No governing body or authority of such city or town or other political subdivision may have the power to pass or enforce any ordinance, rule, or regulation requiring a different rate of speed, and all such ordinances, rules, and regulations contrary to or inconsistent therewith now in force are void and of no effect.
RCW 46.61.430
Federal requirements-1977 ex.s. c 151: See RCW 47.98.070.