I once drove across the USA and tried to find VEGETARIAN things to eat along the way, and it was pretty much hopeless outside of Oregon and California (and even then, near I5, it was difficult enough). Try finding vegetarian food anywhere near a major freeway in Oklahoma or Texas (maybe outside of Austin?) or Nebraska or Tennessee! As Harry said, anywhere near an interstate freeway, food is pretty much identical everywhere. Lots of fast food, Red Robin, Chili's, Cracker Barrel (in the Midwest and South), Buffalo Wild Wings, etc.
I spent a couple of weeks in Montreal years ago, and though it was a beautiful, elegant city with a lot of great restaurants, ethic foods I was accustomed to such as Mexican or Thai food were virtually nonexistent.
Also when I visited Philadelphia and spent some time with Saul Stokes and his wife who were living there at the time, I seem to recall there were no Mexican restaurants, but plenty of Thai and Indian restaurants.... though all the kitchen help in the Indian restaurant we tried were Mexicans! There was a cheesesteak sandwich place and a deli on every corner, though, no matter whether you were downtown or in the suburbs.
Anthony Bourdain's No Reservations TV show does a great job of exploring geographic diversity in food customs. It's one part travel show and one part food show, and the most interesting episodes are when he visits some locale whose cuisine is not well known in other areas. When he visits Thailand and finds Thai food, or find French cuisine in Paris, it's no big revelation, but seeing how people eat in Cleveland or Iceland or Singapore is interesting.