Trains can be a superior alternative depending on what country you're in. In the U.S. we're far behind some other countries in high speed rail infrastructure, so over long distances trains are usually slower than driving.
Edit: As an example of political will for the established and proven solution of mass public transit system, over the unproven non-existent solution of autonomous highway driving lanes. Look at the steam (pun intended) the idea of establishing high speed rail between Portland OR, and Vancouver BC is getting from the public and politicians alike.
They have similar physics problems as cars.
1. They can't go very fast nor carry a lot of weight before they become uneconomical or unsafe.
2. They especially can't go very fast for very long if they are electrical.
3. They need a driver which makes them expensive to run often or with few passengers.
If you get to choose, what you want is something like:
A. Medium speed trains. Cheaper than high speed ones, but still twice as fast as cars with more comfort and less attention. But they need to have good infrastructure, so they can go that speed well and consistently.
B. Local buses with fixed routes.
C. Better golf carts for local transport.
Eventually you would automate all of them. Which would be relatively easy because the fast trains go on tracks, the buses go known routes and the cars, that are the most complex to automate, go slow. Slow also wouldn't affect for example automated deliveries, or repositioning, over longer distances.
Also even before automation as the cars would be "underbuilt", relative to today, they are cheap. So the don't get the sunk cost of a car. And since they don't go outside the local area, each municipality can choose their own infrastructure more freely.
Of course I don't see it happening as things are today, but this is in my opinion more inline with what should be discussed. Since things relative to physics isn't likely to change quickly.
With less cars in cities buses could be competitive with trains since they are street level and can go in different directions, but the would probably still have to be electric and automated for that to be true. (Of course you would still need subways and commuter trains anyway, but you wouldn't be as dependent on them). Trams could probably also be an option, but I am not entirely sure on the future of self-driving trams.
The drive from Las Vegas to Los Angeles is about 3.5 hours. It's a 90 minute flight.
If you try to do it by Bus it takes 12 (often for about 1/2 the price of the plane ticket), and for some reason a train with NO STOPS took 16 hours. Which was discontinued just a couple years ago, because no one was using it.
Maybe someday we will get a high speed Mag Lev train for this commute, because mass transit has completely failed this commute.
Since this is a comment on a thread about the possibility of building the infrastructure required for highway lanes reserved for autonomous cars, i.e. a thread about how things "could work" I see no problems with giving my self the same leeway for the established technology as the parent does for any future technology.
Although not strictly automated, they are externalized. That is you are not the one doing the driving, and therefor you are free to sleep, read, work, etc, while you wait to get to your destination.
The beauty about busses is that they exists. There is no unsolved technology problems with busses. And they can be (and most likely already are) implemented without any or minimal additional infrastructure in your area.
If busses are overcrowded your local government can simply buy more, so they scale really well. If they become congested, we also have a solution called trains. Trains also solves your problems with driving and as a bonus get you to your destination far quicker then driving.