an interesting idea; i've got databases that have a lat/longitude for each county, so you could calculate where nearby counties are < 40 miles or something; but that's not what you were after, right?
what happens if the county in question borders on another state? do you want all counties contiguous with that county, regardless of state, or just contiguous AND in that state?
this would be a project i'd throw at an intern to gather the data; it's kind of neat, although i don't know how it would be of value yet.
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