FYI: I thought I had seen something about cars on trailers. This below is on the website of a shuttle company I regularly use (All Rivers out of Whitebird, ID - a very good, dependable operation)
Trailers with cars on them are considered a HAZMAT issue.
as the gas in your tank is considered hazardous.
Because of this, I can no longer shuttle trailers with cars on them. I apologize, but the fines are huge!