If i remember Fallout 3 correctly, the infrastructure is hardly intact. Large areas are completely devastated, while others are standing,, but quite damaged. But remember that Fallout is an alternate reality, where radiation isn't a great problem, and where nuclear devices are used as infantry support. It seems that they have miniaturised the bombs, so who know what kind of yield we really are looking at.
Also, the height on which the bomb is detonated matter a lot. Both when it comes to radiation and physical destruction.
Best game of the last century. I might have enjoyed the sequel Fallout: New Vegas a bit more, but mostly because I needed a Fallout fix so badly.