49 U.S.C. § 31504: Identification of motor vehicles

(a) General Authority.—The Secretary of Transportation may—(1) issue and require the display of an identification plate on a motor vehicle used in transportation provided by a motor private carrier and a motor carrier of migrant workers subject to section 31502(c) of this title, except a motor contract carrier; and (2) require each of those motor private carriers and motor carriers of migrant workers to pay the reasonable cost of the plate. (b) Limitation.—A motor private carrier or a motor carrier of migrant workers may use an identification plate only as authorized by the Secretary.