Is there any interstate that is not at least 13'6" everywhere? I suppose out east there might be some exceptions.
No. Bridges on OTHER highways (including US highways) may be lower, but not any stretch of the Interstate and Military Highway System.
You made this very authoritative, but incorrect, statement on another thread, where I also contradicted you. Please, if you don't really
know what you are talking about, don't give authoritative-sounding advice -- you could get someone else in real trouble.
This statement is just plain wrong. The fact of the matter is that the standard for the Interstate Highway System mandates a minimum vertical clearance of 16' in rural areas, and 14' in urban areas, but, as with everything else, THERE ARE EXCEPTIONS.
Since Brian's question related to 13'-6" specifically, here is a PARTIAL list of places where Interstate Highways have obstructions lower than that:
I-395 through Washington, DC: 13'-0"
I-80 near Columbia, NJ 12'-3", and 13'-2"
I-278 near Linden, NJ 12'-0"
I-495 near Union City, NJ 13'-3 "
I-78 through the Holland tunnel, NY 12'-6"
I-90 near Albany, NY 13'-5"
I-190 near Buffalo, NY 12'-6'
I-478 through the Brooklyn Battery tunnel, NY 12'-9"
I-678 in New York City 11'-0", and 11'-1"
and there are possibly others. So, to Brian's question, no, you can not rely on, say, a 13'-6" clearance simply because you are on an Interstate. You must read the signs, or, better yet, consult a Motor Carriers Road Atlas, which will list all the truck routes and any low clearance you will encounter on those routes.
Here is a web site which also lists some low clearances, including on the Interstates:http://www.aitaonline.com/Info/Low%20Clearances.html