Wash. Rev. Code § 47.04.060
The department is authorized and directed to act for and on behalf of the state of Washington, and any political subdivision of the state, in all things pertaining to the selection, construction, and maintenance of highways and roads under the provisions of the act of congress approved July 11, 1916, and any and all acts amendatory thereto; and to enter into such agreement with the secretary of transportation or other duly authorized agent of the United States as may from time to time be desirable or necessary to secure the money or aid for any section of state highway, county road, or city or town street selected by law for construction or improvement through an appropriation for the period in which the construction or improvement is to be made. The money shall be added to and expended in connection with the appropriation aforesaid; and shall apply thereto, as may be required, cooperative expenditures from the motor vehicle fund, which may have been appropriated by the state legislature, and from any highway, road, or street fund of any political subdivision, and which are available for the construction and maintenance of any section of state highway, county road, or city or town street selected as aforesaid for such aid and improvement.
RCW 47.04.060