K.S. LaVida's post of June 1, 2009, at the DAILY KOS notes that a small portion of the population in the USA lives at densities that can begin to support transit, " . . . under 12% -- only about 35 million people (out of over 300M) -- live in ZIP codes with a density over 7000 per square mile. In those sorts of places, mass transit lines are likely to be well-utilized. About 44% live in areas with densities between 700 and 7000. At the high end, that's amenable to some mass transit, but for the most part it's car country. . . "