[JandC]

As someone already mentioned, you need to keep a Trucker's Atlas and consult it. They list all low clearance overpasses for each and every location by State. If you are staying on Class I designated highways (Interstates and most U.S. Highways) you will almost never run into an overpass less than 13'6".

We run a Rand RV GPS but we also double check the route with our Trucker's Atlas prior to travel. It only takes a couple of minutes.