49 U.S.C. § 503

(a) Each motor carrier of migrant workers (except a motor contract carrier) and each motor private carrier shall designate an agent by name and post office address on whom service of notices in a proceeding before, and actions of, the Secretary of Transportation may be made. The designation shall be in writing and filed with the Secretary. The carrier also shall file the designation with the authority of each State in which it operates having jurisdiction to regulate transportation by motor vehicle in intrastate commerce on the highways of that State. The designation may be changed at any time in the same manner as originally made. (b) A notice of the Secretary to a carrier under this section is served personally or by mail on that carrier or its designated agent. Service by mail on the designated agent is made at the address filed for the agent. When notice is given by mail, the date of mailing is considered to be the time when the notice is served. If the carrier does not have a designated agent, service may be made by posting a copy of the notice in the office of the secretary or clerk of the authority having jurisdiction to regulate transportation by motor vehicle in intrastate commerce on the highways of the State in which the carrier maintains headquarters and with the Secretary. (c) Each of those carriers, including such a carrier operating in the United States while providing transportation between places in a foreign country or between a place in one foreign country and a place in another foreign country, shall designate an agent in each State in which it operates by name and post office address on whom process issued by a court with subject matter jurisdiction may be served in an action brought against that carrier. The designation shall be in writing and filed with the Secretary and with the authority of each State in which the carrier operates having jurisdiction to regulate transportation by motor vehicle in intrastate commerce on the highways of that State. If a designation under this subsection is not made, service may be made on any agent of the carrier in that State. The designation may be changed at any time in the same manner as originally made.