A lawyer says his quest to obtain relief for a client suffering serious injuries in a crash with a truck hauling U.S. mail highlights the difficulties of going up against this vast but little-known and minimally regulated field.

After filing suit on behalf of a man whose car was rear-ended by a tractor-trailer on Route 78 in Warren County, New Jersey, it took Tyler Hall nearly a year to learn that the truck was a  subcontractor hauling mail for the U.S. Postal Service.