One other thing is that in many cases there's no particular reason the journey has to be completed end-to-end in a self-driving car. For longer journeys it might make sense to take public transport for the majority of the route, and just use taxis for last-mile at either end. This might even be preferable for taxi companies, since they probably make more money on many short trips, and they don't waste time driving empty cars to/from remote areas.

This could work really well if Google integrated payment alongside their directions, so when you search for directions, you get a total price, and a button to pay for that journey immediately.