Actually, the highest you can be and go in all states legally is 13'6". I've trucked so long that going under anything lower than that, even in my car, makes me duck my head.
In a couple of states (most notably New York state), there is an "allowance" built into the posted bridge heights. If a bridge says 12'6" in New York, it's actually 13'6"... unless it says, clearly "actual clearance." If you see a sign that says "12'6" Statutory Clearance," that means that it's 13'6".
The one city you are most likely to have trouble in is Chicago, which has low bridges all over the place. There is actually a number truckers can call for routing into their destination.
Trucking to support the cats.