Wash. Rev. Code § 53.08.230
A port district may at its option file with the county auditor a plat of any of its properties or facilities, showing thereon such private streets, alleys, access roads, parking areas, parks and other places as the port district may wish to have treated as public for purposes of motor vehicle or other police regulations. Such plat may be amended at any time by the filing of an amendatory plat, and may be vacated at any time by the filing of a resolution of vacation. So long as any such plat or amendatory plat is on file and not vacated, the motor vehicle or other police regulations of the state, and the motor vehicle regulations of the city, town or county, as the case may be, in which the areas described in the plat are situated, shall apply to such areas as though they were public streets, alleys, access roads, parking areas, parks or other places, and it shall be the duty of all state and local law enforcement officers to enforce such regulations accordingly.
RCW 53.08.230