Self-driving cars are the magic pixie dust of transportation planning, brought out to justify noninvestment in public transit.
As a mode of transportation, self-driving cars already exist--they're basically a taxicab service, the main difference being that some people hope that self-driving might magically make the cost of providing a taxi service cheaper.
> Instead of busses and semis, we'll have small pods for smaller cargo and small parties. Highways will turn into logistics corridors, and we'll route people and goods seamlessly.
"Lots of small things going point-to-point" is a much more difficult problem to route, especially at high throughput, than "bundle things into large containers that get broken apart near their destination." In the space of transit, your idea is known as Personal Rapid Transit (PRT), and PRT systems have invariably underwhelmed every time they've been built, as they struggle to live up to their promise.
Rail transit is incredibly efficient at moving large numbers of people--a metro line can easily move a dozen lanes of highway traffic--and there is nothing that you can do to roads to make them approach that level of efficiency, in part because the routing problems are insurmountable.