Wash. Rev. Code § 81.70.290
A charter party carrier or excursion service carrier of passengers, authorized to transport persons for compensation on the highways and engaging in interstate, or interstate and intrastate, operations within the state of Washington which is or becomes qualified as a self-insurer with the federal motor carrier safety administration of the United States department of transportation in accordance with the United States interstate commerce act applicable to self-insurance by motor carriers, is exempt from RCW 81.70.280 relating to the carrying or filing of insurance policies or bonds in connection with carrier operations as long as the qualification remains effective.
The commission may require the charter party carrier or excursion service carrier to prove the existence and continuation of qualification with the federal motor carrier safety administration by affidavit in a form the commission may prescribe.
RCW 81.70.290