Washington Revised Code Title 46. Motor Vehicles § 46.61.428. Slow-moving vehicle driving on shoulders, when

(1) The state department of transportation and local authorities are authorized to determine those portions of any two-lane highways under their respective jurisdictions on which drivers of slow-moving vehicles may safely drive onto improved shoulders for the purpose of allowing overtaking vehicles to pass and may by appropriate signs indicate the beginning and end of such zones.

(2) Where signs are in place to define a driving-on-shoulder zone as set forth in subsection (1) of this section, the driver of a slow-moving vehicle may drive onto and along the shoulder within the zone but only for the purpose of allowing overtaking vehicles to pass and then shall return to the roadway.

(3) Signs erected to define a driving-on-shoulder zone take precedence over pavement markings for the purpose of allowing the movements described in subsection (2) of this section.