Wash. Rev. Code § 47.52.105
Whenever, in the opinion of the department, frontage or service roads in connection with limited access facilities are not feasible either from an engineering or economic standpoint, the department may acquire private or public property by purchase or condemnation and construct any road, street, or highway connecting to or leading into any other road, street, or highway, when by so doing, it will preserve a limited access facility or reduce compensation required to be paid to an owner by reason of reduction in or loss of access. The department shall provide by agreement with a majority of the legislative authority of the county or city concerned as to location, future maintenance, and control of any road, street, or highway to be so constructed. The road, street, or highway need not be made a part of the state highway system or connected thereto, but may upon completion by the state be turned over to the county or city for location, maintenance, and control pursuant to the agreement as part of the system of county roads or city streets.
RCW 47.52.105