Wash. Rev. Code § 47.04.020
All public highways in the state of Washington, or portions thereof, outside incorporated cities and towns shall be divided and classified as state highways and county roads. All state highways and branches thereof shall be established by the legislature of the state of Washington by appropriate general location and termini. Any prior distinctions between highways as primary or secondary are hereby abolished. All powers granted to, or duties imposed upon, the department with regard to either primary or secondary state highways shall be construed to relate to all state highways. Whenever these terms are used, either jointly or independently, each shall be construed to include all state highways. All public highways in the state of Washington, or portions thereof, outside incorporated cities and towns, not established as state highways, are hereby declared to be county roads.
RCW 47.04.020
Highway designation system-Signs: RCW 47.36.095.