Leaving core city areas for cheaper housing in the suburbs is one of the few strategies available to lower income people. Thing is, when they get out to the suburbs public transit is scarce and car ownership sometimes mandatory. The financial requirements of getting around, especially reaching a workplace, could easily soak up any gains from the cheaper housing.
These two links are to short items on Wired blogs. They mention a Brookings Institution report into the matter and a recent American civil rights conference which concluded that reasonable access to transportation is actually a human right.
Ever wait in snow up to your ankles for a bus at 5:30 in the morning? Ever have the timing belt snap on a fifteen year old Honda Civic in an industrial park after getting off the afternoon shift? If so, you know what it’s all about.