While cars that drive themselves are not the final solution they could be a necessary stepping stone to mass transportation.
Imagine a future where cars are not owned by individuals but are owned by few companies (or maybe even government, the way public transport is) and participate in what is essentially a global taxi network.
Instead of using your own car you would just step out of your home, enter destination address into your phone and press a button. That would be sent to a server which given your location, position and destination of all existing cars in route would pick one that is the closest to you that has available seat and goes in that direction. That car would just pull up and you would be notified in real time how far it is from you.
If a car is not available, it would be dispatched from the closest holding area.
Think ZipCar except with cars that come to you, drive themselves and you can share with other people while it drives itself (or you can have it for yourself, if you prefer, except at a higher cost).
This kind of a network would drastically reduce number of cars needed to drive people around and given global knowledge of where people are and where they want to go, it could plan the absolutely most efficient route.
For popular routes (e.g. if it detects a pattern that a lot of people commute from San Francisco to Mountain View around 9 AM on Mondays through Fridays) it could deploy bigger cars (i.e. buses), further reducing traffic.
When you think about it, it's not surprising that car companies will not work on a technology like that since it would drastically reduce demand for cars and if this comes anywhere close to reality, expect lots of legislative and political battles (after all, this will mean that Americans in Detroit will loose jobs).