Wash. Rev. Code § 36.88.340
Except as they may establish continuing guaranty fund requirements, the board of county commissioners shall be the sole judges as to the extent of county road fund participation in any project under this chapter and the decisions of the board shall be final; the said board may receive grants from or contract with any other county, municipal corporation, public agency or the state or federal government in order to effect any construction or improvement hereunder, including the construction, installation, improvement, operation, maintenance of and furnishing electric energy for any street and road lighting system, and to effect the construction, installation, improvement, operation and maintenance of and furnishing electric energy for any such street and road lighting system, may contract with any private utility corporation.
RCW 36.88.340