Wash. Rev. Code § 36.83.010
The legislative authority of a county may establish one or more service districts within the county for the purpose of providing and funding capital and maintenance costs for any bridge or road improvement or for providing and funding capital costs for any state highway improvement a county or a road district has the authority to provide. A service district may not include any area within the corporate limits of a city or town unless the city or town governing body adopts a resolution approving inclusion of the area within its limits. A service district is a quasi municipal corporation, an independent taxing "authority" within the meaning of Article VII, section 1 of the state Constitution, and a "taxing district" within the meaning of Article VII, section 2 of the state Constitution.
A service district shall constitute a body corporate and shall possess all the usual powers of a corporation for public purposes as well as all other powers that may now or hereafter be specifically conferred by statute, including, but not limited to, the authority to hire employees, staff, and services, to enter into contracts, to acquire, hold, and dispose of real and personal property, and to sue and be sued. All projects constructed by a service district pursuant to the provisions of this chapter shall be competitively bid and contracted.
A board of three commissioners appointed by the county legislative authority or county executive pursuant to this chapter shall be the governing body of a service district. The county treasurer shall act as the ex officio treasurer of the service district. The electors of a service district are all registered voters residing within the district.
RCW 36.83.010
County may fund improvements to state highways: RCW 36.75.035.