49 U.S.C. § 14506: Identification of vehicles

No State, political subdivision of a State, interstate agency, or other political agency of two or more States may enact or enforce any law, rule, regulation standard, or other provision having the force and effect of law that requires a motor carrier, motor private carrier, freight forwarder, or leasing company to display any form of identification on or in a commercial motor vehicle (as defined in section 14504a), other than forms of identification required by the Secretary of Transportation under section 390.21 of title 49, Code of Federal Regulations. under the International Registration Plan under section 31704; under the International Fuel Tax Agreement under section 31705 or under an applicable State law if, on October 1, 2006 , the State has a form of highway use taxation not subject to collection through the International Fuel Tax Agreement; under a State law regarding motor vehicle license plates or other displays that the Secretary determines are appropriate; in connection with Federal requirements for hazardous materials transportation under section 5103; or in connection with the Federal vehicle inspection standards under section 31136.