Not sure if its true or not. A Hyman driver told me that they weren't allowed to run them thru port of entry.Jifflox? I've never seen one other than that one occasion, but I've never seen anything in the law suggesting they're illegal. A-trains aren't as common here, so I could see a DOT cop questioning why part of your truck comes off. But to my knowledge they are legal.
I was a little confused when I saw a tandem Big R, until I saw the second pair of mudflaps.