Wash. Rev. Code § 35.21.190
Any city or town council upon request of the board of park commissioners, shall have authority to designate such streets as they may see fit as parkways, park drives, and boulevards, and to transfer all care, maintenance and improvement of the surface thereof to the board of park commissioners, or to such authority of such city or town as may have the care and management of the parks, parkways, boulevards and park drives of the city.
Any city or town may acquire, either by gift, purchase or the right of eminent domain, the right to limit the class, character and extent of traffic that may be carried on such parkways, park drives and boulevards, and to prescribe that the improvement of the surface thereof shall be made wholly in accordance with plans of such board of park commissioners, but that the setting over of all such streets for such purposes shall not in any wise limit the right and authority of the city council to construct underneath the surface thereof any and all public utilities nor to deprive the council of the right to levy assessments for special benefits. In the construction of any such utilities, any damages done to the surface of such parkways, park drives or boulevards shall not be borne by any park funds of such city or town.
RCW 35.21.190