RCW 47.24.010

The department of transportation shall determine what streets, together with bridges thereon and wharves necessary for use for ferriage of motor vehicle traffic in connection with such streets, if any, in any incorporated cities and towns shall form a part of the route of state highways and between the first and fifteenth days of July of any year the department shall identify by brief description, the streets, together with the bridges thereon and wharves, if any, in such city or town which are designated as forming a part of the route of any state highway; and all such streets, including curbs and gutters and street intersections and such bridges and wharves, shall thereafter be a part of the state highway system and as such shall be constructed and maintained by the department from any state funds available therefor: PROVIDED, That the responsibility for the construction and maintenance of any such street together with its appurtenances may be returned to a city or a town upon certification by the department to the clerk of any city or town that such street, or portion thereof, is no longer required as a part of the state highway system: PROVIDED FURTHER, That any such certification that a street, or portion thereof, is no longer required as a part of the state highway system shall be made between the first and fifteenth of July following the determination by the department that such street or portion thereof is no longer required as a part of the state highway system, but this shall not prevent the department and any city or town from entering into an agreement that a city or town will accept responsibility for such a street or portion thereof at some time other than between the first and fifteenth of July of any year.