23 U.S.C. § 102: Program efficiencies

(a) Access of Motorcycles.—No State or political subdivision of a State may enact or enforce a law that applies only to motorcycles and the principal purpose of which is to restrict the access of motorcycles to any highway or portion of a highway for which Federal-aid highway funds have been utilized for planning, design, construction, or maintenance. (b) Savings Provision.—Nothing in this section shall affect the authority of a State or political subdivision of a State to regulate motorcycles for safety.