Wash. Rev. Code § 36.89.020
The legislature finds that the open spaces, park, recreation and community facilities, public health and safety facilities, stormwater control facilities and highways within any county of this state, whether located partly or wholly within or without the cities and towns of such county are of general benefit to all of the residents of such county. The open spaces, park, recreation and community facilities within such county provide public recreation, aesthetic, conservation and educational opportunities and other services and benefits accessible to all of the residents of such county. The public health and safety facilities within such county provide protection to life and property throughout the county, are functionally inter-related and affect the health, safety and welfare of all the residents of such county. The stormwater control facilities within such county provide protection from stormwater damage for life and property throughout the county, generally require planning and development over the entire drainage basins, and affect the prosperity, interests and welfare of all the residents of such county. The highways within such county, whether under the general control of the county or the state or within the limits of any incorporated city or town, provide an inter-connected system for the convenient and efficient movement of people and goods within such county. The use of general county funds for the purpose of acquisition, development, construction, or improvement of open space, park, recreation and community facilities, public health and safety facilities, stormwater control facilities, or highways or to participate with any governmental agency to perform such purposes within such county pursuant to this chapter is hereby declared to be a strictly county purpose.
RCW 36.89.020