RCW 47.52.110

After the opening of any new and additional limited access highway facility, or after the designation and establishment of any existing street or highway, as included the particular highways and streets or those portions thereof designated and established, shall be physically marked and indicated as follows: By the erection and maintenance of such signs as in the opinion of the respective authorities may be deemed proper, indicating to drivers of vehicles that they are entering a limited access area and that they are leaving a limited access area.