For cities, geometry is still the limiting factor. See http://humantransit.org/2012/09/the-photo-that-explains-almost-everything.html http://humantransit.org/2

For cities, geometry is still the limiting factor. See http://humantransit.org/2012/09/the-photo-that-explains-almost-everything.html http://humantransit.org/2012/09/the-photo-that-explains-almo... "The scarcity of space per person is part of the very definition of a city, as distinct from suburbia or rural area, so the efficiency with which transport options use that space will always be the paramount issue."