I think this raises some really interesting ideas. Generally I would guess most of your trade routes will follow the natural geography more than current infrastructure.
The environmental consequences of war (long winters) and neglect would undo much of your infrastructure in the North, hot weather would have its impact on the south, and in the West you'd like to have a variety of more localized environmental challenges, not the least of which part of the road just being buried. Bridges, tunnels would vary in their condition.
In comparison- modern Zaire/Congo and much of West Africa lost much of its infrastructure rather quickly during the 1990s as the result of war and state collapse. The US would likely suffer much of the same.
Pre-war instrastructure would still be relevant. I would be curious if the railways would be in better condition than interstate highways. But population centers that survive the war would matter in terms of local care and upkeep.
This is a fascinating topic. Any re-builder would have to figure out the problems of economic organization- from resources to finished goods, and the movement of such goods, and there would be a political and social consequence at stake.
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