I work with a lady who's husband is a State Trooper, and assigned to the Motor Freight Carrier Division. After talking to him, anyone who pulls a trailer and has a gross vehicle weight of over 10,000 lbs is required to display DOT numbers on their truck. Along with this are other regulations as well, some of which are that you must carry a fire extinguisher, reflective triangles, inspection by a mechanic, etc. I have seen him stopped on the road with a few guys (one was an open mowing trailer) .. and though he usually doesn't write them a ticket for that offense, he gives them a good lecture and inspects their rig. He says most guys who use their truck for personal use usually have the numbers on magnetic signs and only run them when pulling the trailer. The numbers also have to be of a contrasting color from the vehicle, and visible from 50 ft. Still ... I see many, many without the numbers displayed. What do you do??