> The only other state(ish) borders that has a major city on a state border and might have as many people crossing would be the states bounding the District of Columbia.

What about Chicago, Illinois? It's just across the border from Gary, Indiana. South Bend and Elkhart aren't too far away. I imagine there's quite a few Hoosiers who cross the border each day to work at jobs in the Windy City. I have no idea how the population numbers would compare, though. I just thought I'd bring it up.