The problem is the subway comes maybe once every 11 minutes when it's working properly, and if someone tries to kill themselves by jumping in front of a train in south Brooklyn (or trips or gets pushed) then trains stop working all the way up through The Bronx. And potentially on both the east and west sides of both Manhattan and The Bronx if it happens at one of the stations where the 2/3 and 4/5 overlap. And this isn't an unusual occurrence, it happens once or twice per week during rush hour alone. Every time some minor thing goes wrong ('sick passenger', signal malfunction, track fire, etc.) then the entire city's transportation basically gets shut down. I agree that cars are absurdly inefficient, but at least they're somewhat more resilient. If we could use self-driving technology to at the very least increase the bus coverage then that would be a pretty good outcome.