Different laws apply to 18-wheelers depending if they are interstate trucks (semi-trucks that cross state boundaries), or if they are intrastate, (trucks than only travel within one state).

If a truck is involved interstate commerce, it must comply with the Federal safety regulations and Operating Authority rules, in addition to State and local requirements.

 Trucks traveling only must comply with applicable State and local regulations. The only Federal regulations that are applicable to intrastate operations are: the commercial driver's license (CDL), for drivers operating commercial motor vehicles as defined in 49 CFR 383.5; controlled substances and alcohol testing for all persons required to possess a CDL; and minimum levels of financial responsibility for the intrastate transportation of certain quantities of hazardous materials and substances.

For more information on interstate vs. intrastate trucking laws, read this article.