As others have said, know your exact measurements when hooked to your tow vehicle, and measure at every high point. My rig is 13’4” at the front AC, but 13’5” at the rear. I assume no “fudge” factor on marked overpasses. Remember that if roads have been resurfaced, the bridge marking may not have been remeasured. We have seen several where crews were out measuring again after roadwork had been performed. Generally, if I’m on a road and see 18 wheelers on that road, I figure I’m safe. To date we have never had to detour because of a low bridge.