Autonomous cars (if used exclusively) over time should decrease safe separation, increasing capacity, but there's a limit to the gains from that.
Some additional gains if they are networked at routing is optimized to utilize all available roads rather than just the usually-best route.
For example I doubt we'll see any benefits of decreased safe separation in cities where there are cyclists and pedestrians and other factors that are hard to predict.
My prediction would be that the efficiency gains will be marginal and will be dominated by other factors that affect congestion.
Well, yeah, that applies to roads with exclusive use of autonomous vehicles, which clearly doesn't apply to roads shared with cyclists and pedestrians, but limited access freeways. But even within cities those can serve as arterial routes.
Platooning https://youtu.be/t9uEPnGI6zA?t=54s
Shuttling to and from stations
Road pricing to get people to share
More road available as there would be less parked cars