This would need some modeling to prove that you actually get that level of efficiency. Autonomous vehicles have a brownfield problem in that they would have to share the road capacity with conventional vehicles for a fair distance into the future. That consideration alone would likely reduce the efficiency gain considerably.

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.