A lot of the decay is shockingly fast. The city of Pripyat, which was where the Chernobyl workers lived, is a ghost town already mostly reclaimed by nature. (The things I look at while writing in this fandom.)
The roads into Appalachia, with all that moisture and freezing, would be impassible without constant upkeep within a year or two, I'd think. And since no one is traveling on them, with everyone forced into fenced districts, who's going to see where the repairs need to be made?
The southwest would definitely be the treasure trove for old things... which probably explains why the Capitol ended up where it did.
Edited at 2017-01-13 01:32 am (UTC)