RCW 47.26.090: "Arterial" defined.

The term "arterial" as used in this chapter means any state highway, county road, or city street, in an urban area, that is functionally classified as a principal arterial, minor arterial, or collector street by the department in cooperation with the board, regional transportation planning organizations, cities, and counties. The board shall develop criteria and procedures for designating arterials in the incorporated cities and towns lying outside urban areas.