My wife and I love the back roads too and avoid Interstate highways anytime we can. In Texas we're lucky because the TxDOT has a website that lists (by region) low vertical clearance bridges/underpasses: TXDMV.GOV - Structures with Low Vertical Clearance
If we're heading out on a new route, we check to make certain there are no obstacles. And, we've avoided some misery by doing so.
Last summer we spent 3 weeks traveling Florida. We took Interstates most of the way out there, but coming home we took the back roads. The first 100 miles took us three days simply because we found so many great places to stop.
Did I mention...we LOVE retirement!! :-)