Well, the main issue is that most of the cars on the road during the busiest hours have just 1 person in them.
I think when the dust finally settles, the main form of passenger transit in urban-ish areas will basically be something that looks like autonomous vans.
Unlike public transit today, they won't have predefined timetables, nor predefined startpoints and endpoints. They'll pick you up wherever you are, whenever you want, and drop you off wherever you want, but they might take a roundabout way to get there to pick up and drop off other passengers too.
If they usually have 2+ people in them at a time, we'll be able to use our existing infrastructure (roads) at a much higher throughout.