Washington Revised Code Title 81. Transportation § 81.70.330. Vehicle identification

(1) It is unlawful for a charter party carrier or excursion service carrier to operate a motor vehicle upon the highways of this state unless there is firmly affixed to both sides of the vehicle, the name of the carrier and the certificate or permit number of the carrier. The characters composing the identification must be of sufficient size to be clearly distinguishable at a distance of at least fifty feet from the vehicle.

(2) A charter party carrier or excursion service carrier holding both intrastate and interstate authority may identify its vehicles with either the commission permit number or the federal vehicle marking requirement established by the United States department of transportation for interstate motor carriers.