Two points here, this one, from the pages of Governing magazine, and this one, my comment on an Invisible Cyclist blog post.
It’s obvious in NYC that noxious environmental conditions stemming from highways and excess traffic conditions degrade residential neighborhoods in the vicinity, however the point you make, that the systematic process of slum development has also affected the ability of locals to get around without automobiles, is not often made.
Funny, because it seems even more obvious. If you build a large highway through original neighborhoods, spillover traffic from that highway will make it tough for anyone to get around without driving.
You see this in the Bronx. The highway development there divides neighborhoods, creates unappealing choke points along crossings, generates excess noise and pollution, breaks up the grid to make it harder for traffic to flow smoothly around obstacles, streams extra cars onto city streets from off the highways, blocks access to waterfront areas, and alters mental geography to make relatively close-by places seem very distant.