The other factor is that roads simply cannot handle the volume of people that trains can. One subway train can easily result in hundreds of cars not being on the road.
Same goes for parking, especially in cities like San Francisco. (And if someone wants to counterargue that self-driving cars can drive away to park somewhere else -- that just makes the traffic even worse, since now there's twice as many cars entering and leaving the city around rush hour.)
I saw a transportation guy that said what transit systems like BART are good at is getting people on and off quickly. Better than buses and far better than taxi's.
