Wash. Rev. Code § 35.02.170
The right-of-way line of any public street, road or highway, or any segment thereof, may be used to define a part of a corporate boundary in an incorporation proceeding. The boundaries of a newly incorporated city or town shall not include a portion of the right-of-way of any public street, road or highway except where the boundary runs from one edge of the right-of-way to the other edge of the right-of-way.
RCW 35.02.170
Legislative finding, intent-1975 1st ex.s. c 220: "The legislature finds that the use of centerlines of public streets, roads and highways as boundaries of incorporated cities and towns has resulted in divided jurisdiction over such public ways causing inefficiencies and waste in their construction, improvement and maintenance and impairing effective traffic law enforcement. It is the intent of this act to preclude the use of highway centerlines as corporate boundaries in the future and to encourage counties and cities and towns by agreement to revise existing highway centerline boundaries to coincide with highway right-of-way lines." [ 1975 1st ex.s. c 220 s 1.]
Revision of corporate boundary by substituting right-of-way lines: RCW 35.21.790.