Oh, and also: > This effect cannot be caused by population increases because most of the 20th century had large increases without this zoning pattern emerging.

Oh, and also: > This effect cannot be caused by population increases because most of the 20th century had large increases without this zoning pattern emerging. It's important to remember that the rise of the automobile happened over this same period, which raised the level of marginal land (and delayed conflict over overpopulation, for at least a few generations).