Wash. Rev. Code § 47.28.010
Whenever the general route of any state highway shall be designated and laid out as running to or by way of certain designated points, without specifying the particular route to be followed to or by way of such points, the department shall determine the particular route to be followed by said state highway to or by way of said designated points, and shall be at liberty to select and adopt as a part of such state highway, the whole or any part of any existing public highway previously designated as a county road, primary road, or secondary road or now or hereafter classified as a county road. The department need not select and adopt the entire routes for such state highways at one time, but may select and adopt parts of such routes from time to time as it deems advisable. Where a state highway is designated as passing by way of a certain point, this shall not require the department to cause such state highway to pass through or touch such point but such designation is directional only and may be complied with by location in the general vicinity. The department is empowered to construct as a part of any state highway as designated and in addition to any portion meeting the limits of any incorporated city or town a bypass section either through or around any such incorporated city or town.
RCW 47.28.010
Effective date-2006 c 334: See note following RCW 47.01.051.