Nope, the roads can't handle it. There is a chicken-and-egg problem getting actually-useful subway service (under 10 minutes apart, 24x7 including holidays) in

Nope, the roads can't handle it. There is a chicken-and-egg problem getting actually-useful subway service (under 10 minutes apart, 24x7 including holidays) in a place that is currently low-density. Without the density of Manhatten/Tokyo/London you can't afford good service. Without good service, anything above suburban density is misery. BTW, SF does not have good service. It's much worse than Boston even.