There is a throughput issue. Driverless cars with 4 people a car in a lane (or a bus lane) will do ~6000/people/hour if you are optimistic. A train 'lane' will on the other hand will do 32'000/people/hour. Non-bus lanes do 1500/hour, so a train is equivalent to a 21 lane highway, one way. [0]

I think driverless cars are a great adapter / collector last mile technology for suburbs to go to the train station, but if somewhere has traffic problems now, it will have traffic problems with driverless cars too. After a certain point you need to put in trains.

[0] https://www.theurbanist.org/2016/05/26/the-supply-and-demand...