How about installing a giant metal claw on a swing connected to a bar at the height of the overpass about 100 meters before all overpasses.

If you are above the height of the overpass the claw grabs your trailer and tears a chunk out of your vehicle. It is at the drivers cost to repair their vehicle and replace the hook since it only occurs if your vehicle is over height and unlawful.

Much cheaper to replace the claw device than an overpass.