I may be mistaken on how low those overpasses were because I just did a quick search on the txdmv site for low bridges and the lowest ones I see are about 13'. The ones I am thinking of were in the 11' or so range I think. The last one was somewhere outside Brownwood TX that I whistled under but commented someone would have problems because the sign on the bridge over the road read 11' xx. It was a very old bridge. I was not towing. Maybe I was dreaming, my eyesight failed me...or, my memory just thinks up what it wants to....
I have seen other 11-12' signs I'm sure in my travels (I think??)but that's the only one that comes readily to mind.