I've done ZIP code to great-circle distance calculations, and found a few wormy bits. Of course, the basic idea does have the rather major flaw that just looking at the distance between ZIP codes ignores the rather significant issue of water. Most of Long Island is within about 30 miles of the Connecticut shoreline (where I live), but getting there requires either a ferry ride (about $45, with car) or a route that passes through New York City. I'm sure that similar difficulties occur between the left and right banks of many of the rivers in the US.