Wash. Rev. Code § 47.52.020
The highway authorities of the state, counties, and incorporated cities and towns, acting alone or in cooperation with each other, or with any federal, state, or local agency, or any other state having authority to participate in the construction and maintenance of highways, may plan, designate, establish, regulate, vacate, alter, improve, construct, maintain, and provide limited access facilities for public use wherever the authority or authorities are of the opinion that traffic conditions, present or future, will justify the special facilities. However, upon county roads within counties, the state or county authorities are subject to the consent of the county legislative authority, except that where a state limited access facility crosses a county road the department may, without the consent of the county legislative authority, close off the county road so that it will not intersect such limited access facility.
The department may, in constructing or relocating any state highway, cross any county road at grade without obtaining the consent of the county legislative authority, and in so doing may revise the alignment of the county road to the extent that the department finds necessary for reasons of traffic safety or practical engineering considerations.
RCW 47.52.020