There is a Federal Motor Carrier Safety Administration which is mostly about commercial vehicles.
What is legal or not concerning vehicles is mostly regulated individually in each state. There are a few states where tarping a load isn't required. Some states are more aggressive in enforcing this. Fines for noncompliance may vary from nothing to very expensive.
Again the FMCSA has regulations for commercial carriers.
How much a load can extend out the back of a vehicle or beyond the front or sides for noncommercial vehicles is another vehicle coade regulated state by state > https://motorandwheels.com/truck-sti...aws-per-state/
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