On the other hand if you could distribute traffic flow vertically the potential for scaling goes up considerably. It's a hard problem to solve (read: expensive) but building roads is not exactly cheap either.
BTW I'm not against autonomous vehicles. It would be great if they worked. But I think we're more likely to see near-term investment payoffs using it for incremental safety improvements, something that already seems to be happening.
Example: I currently drive to work 20km. As my wife needs occasionally as well, we have two. With an autonomous car we could forgo one and I send it back after I arrive at work. In the evening it'll pick me up again.
Self-driving cars can park far more densely without fear of being blocked in and can park inconveniently far away while still being reasonably available. Thus cities will need less on-street or near-street parking and can reclaim a lot of that space for traffic or transit or bikes or people or commerce.
If your wife only occasionally needs a car, couldn't she call a local on-demand one - an automated uber? That seems cheaper than paying to send an empty car on two daily 20km trips...
Lanes are as wide as they are and following distances are as long as they are because people are terrible at driving. A mostly-driverless 4-lane highway could more than double capacity just by reducing following distances when computers rather than human beings are making the decision when to brake and how hard to brake.
Humans also cause traffic jams by causing accidents or rubbernecking at accidents - that'll get better too.