Wash. Rev. Code § 47.28.140
When in the opinion of the governing authorities representing the department and any public agency, instrumentality, municipal corporation, or political subdivision of the state of Washington, any highway, road, street, or urban public transportation system will be benefited or improved by constructing, reconstructing, locating, relocating, laying out, repairing, surveying, altering, improving, or maintaining, or by the establishment adjacent to, under, upon, within, or above any portion of any such highway, road, street, or urban public transportation system, by either the department or any public agency, instrumentality, municipal corporation, or political subdivision of the state, and it is in the public interest to do so, the authorities may enter into cooperative agreements wherein either agrees to perform the work and furnish the materials necessary and pay the cost thereof, including necessary engineering assistance, which costs and expenses must be reimbursed by the party whose responsibility it was to do or perform the work or improvement in the first instance. The work may be done by either day labor or contract, and the cooperative agreement between the parties must provide for the method of reimbursement. In the case of some special benefit or improvement to a state highway derived from any project that assists in preventing or minimizing flood damages as defined in RCW 86.16.120 or from the construction of any public works project, including any urban public transportation system, the department may contribute to the cost thereof by making direct payment to the particular state department, agency, instrumentality, municipal corporation, or political subdivision on the basis of benefits received, but such payment may be made only after a cooperative agreement has been entered into for a specified amount or on an actual cost basis prior to the commencement of the particular public works project.
RCW 47.28.140
Findings-Intent-1991 c 322: See note following RCW 86.12.200.
Urban public transportation system defined: RCW 47.04.082.