RCW 46.61.115: When overtaking on the right is permitted.

(1) The driver of a vehicle may overtake and pass upon the right of another vehicle only under the following conditions:

(a) When the vehicle overtaken is making or about to make a left turn;

(b) Upon a roadway with unobstructed pavement of sufficient width for two or more lines of vehicles moving lawfully in the direction being traveled by the overtaking vehicle.

(2) The driver of a vehicle may overtake and pass another vehicle upon the right only under conditions permitting such movement in safety. Such movement shall not be made by driving off the roadway.