RCW 58.08.050: Official plat—Platted streets as public highways.

Whenever any city or town has been surveyed and platted and a plat thereof showing the roads, streets and alleys has been filed in the office of the auditor of the county in which such city or town is located, such plat shall be deemed the official plat of such city, or town, and all roads, streets and alleys in such city or town as shown by such plat, be and the same are declared public highways: PROVIDING, That nothing herein shall apply to any part of a city or town that has been vacated according to law.