Wash. Rev. Code § 46.61.195
All state highways are hereby declared to be arterial highways as respects all other public highways or private ways, except that the state department of transportation has the authority to designate any county road or city street as an arterial having preference over the traffic on the state highway if traffic conditions will be improved by such action.
Those city streets designated by the state department of transportation as forming a part of the routes of state highways through incorporated cities and towns are declared to be arterial highways as respects all other city streets or private ways.
The governing authorities of incorporated cities and towns may designate any street as an arterial having preference over the traffic on a state highway if the change is first approved in writing by the state department of transportation. The local authorities making such a change in arterial designation shall do so by proper ordinance or resolution and shall erect or cause to be erected and maintained standard stop signs, or "Yield" signs, to accomplish this change in arterial designation.
The operator of any vehicle entering upon any arterial highway from any other public highway or private way shall come to a complete stop before entering the arterial highway when stop signs are erected as provided by law.
RCW 46.61.195
City streets subject to increased speed, designation as arterials: RCW 46.61.435.
Stop signs, "Yield" signs-Duties of persons using highway: RCW 47.36.110.