I suspect fuel efficiency based on load factors is a very bad proxy for space usage, as fuel efficiency will be calculated as an average, whereas infrastructure

I suspect fuel efficiency based on load factors is a very bad proxy for space usage, as fuel efficiency will be calculated as an average, whereas infrastructure is (necessarily) dictated by peak usage. This is true for all modes, of course, but because mass-transit is much more efficient at the high load factors that occur during peaks, it ends up being much significantly more space efficient as well. [Especially in the U.S., automobile infrastructure is actually often excessive even for peak usage, due to things like ill-considered minimum parking regulations and local politics ("I widened roads! Vote for me!"). But that's a cultural/political problem, not a technical one.] Another factor in favor of mass transit is that it tends to result in much more focused development around transit networks, resulting in denser and more walkable communities. The fact that it "can't go everywhere" is actually a benefit when it comes to the urban form...