((a)) ** .—** 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)) ** .—** Nothing in this section shall affect the authority of a State or political subdivision of a State to regulate motorcycles for safety.