I have the sense that US now is pretty consistent in trucking laws. In the sixties and seventies there was a lot of diversity in truck law. As example are the COE or forward control, And some of the strange combinations of straight truck with fifth when at the extreme rear, and a little bitty semi trailer. Made for some unique trucks. I get the under 26000 trucks now, but heavy singles seem odd. I know many municipal trucks that are primarily snow plows, secondarily, they haul gravel...sometimes. A single axle is more attractive to purchasing agents, more maneuverable, and hauls enough gravel to be somewhat useful. I presume for hire truckers are paid per yard. I'd think breaking even would be easier with either a tandem, or triaxle.
Willie