Only one time in all the years we've been traveling around have we encountered a low overhead. We were following our TomTom GPS in South Carolina and were about 10 miles from the campground. TomTom wanted us to go down a country road. When turning off the highway, I caught a slight glimpse of a low-overhead sign, but caught just enough of it to realize the trailer was too tall. I wasn't sure how far ahead it was either. We turned around in a church parking lot that was right there. Saw the sign indicated the low-overhead was ahead about 8 or 9 miles.
We studied the paper map then and compared with Google Maps on our SmartPhones and found a way around. Next day, after the trailer was unhitched, we were driving the opposite direction site seeing, and we ended up on the same road. We found the low overhead! I had to stop and get photos. It's the only one I've ever encountered.
I guess the moral of the story is to REALLY pay attention to your road signs once you get off the main highways and Interstates. Know the height of your rig, so if you see something indicating a height clearance, you know immediately to take evasive action if it's too low:
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