Density benefits

Fred Foldvary: "With economic activity most intense in a city center, rents are highest there, because those at the fringes have to pay transit costs to obtain the same income. "

Fred, I'd add also synergy. People on the fringe also suffer opportunity costs, the networking and accidental encounters that take place in downtowns that pay off unexpected benefits. Every prime vacant lot is not only a hole in land use efficiency it's also a wound in the business fabric.