When we speak of the legalities of towing double on the Interstates, I believe we need to be careful to differentiate between commercial loads (regulated by FMCSA) and non-commercial loads (such as our RVs in private use). The Federal government has regulatory authority over interstate commerce, but non-commercial vehicles are a different bag of worms. Federal influence on state non-commercial vehicle statutes comes through NHTSA, and their only real enforcement mechanism is the ability to withhold Federal transportation funding from the states (ref. helmet laws, seat belt laws, .08% blood alcohol DUI threshold, etc.) I don't know that NHTSA has addressed towing double for non-commercial vehicles; I believe that's still within each state's jurisdiction.
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