Wash. Rev. Code § 47.36.050
The department shall erect and maintain upon every state highway in the state of Washington suitable and proper signs, signals, signboards, guideposts, and other traffic devices according to the adopted and designated state standard of design, erection, and location, and in the manner required by law. The department shall erect and maintain upon all state highways appropriate stop signs, warning signs, and school signs. Any person, firm, corporation, or municipal corporation, building, owning, controlling, or operating a railroad that crosses any state highway at grade shall construct, erect, and maintain at or near each point of crossing, or at such point or points as will meet the approval of the department, a sign of the type known as the saw buck crossing sign with the lettering "railroad crossing" inscribed thereon and also a suitable inscription indicating the number of tracks. The sign must be of standard design that will comply with the plans and specifications furnished by the department. Additional safety devices and signs may be installed at any time when required by the utilities and transportation commission as provided by laws regulating railroad-highway grade crossings.
RCW 47.36.050