This.
Self-driving cars would open up an entire new kind of public transportation. Instead of having large buses which are only ever used at capacity for very small time slots and which must follow pre-planned routes that are almost invariably inefficient for the users of said buses, you could have extremely granular transit where people could just enter their desired destination and time, and they'd get processed into the network of self-driving cars for the most efficient route possible. Minimal time lost, minimal fuel usage (if the car's not used, it just parks instead of looping like a bus), almost-ideal saturation, precise start and end points.
As it is now, I'm taking the bus and it's taking between 25 and 75% longer than by car. Moreover, I still have to drive to the bus stop because it's much too far to walk and connections are hilariously bad (think adding an hour for what's a 5 minutes ride because of mismatched departure times). Public transportation based on self-driving cars would mean I'd be able not to own a car at all while being nearly as fast as if I had one. You'd see a much faster adoption if public transit worked that way.