I think self-driving cars could more than double road capacity, and almost eliminate congestion. There are so many exciting permutations.

Major causes of congestion could be almost entirely eliminated (breakdowns and accidents)

Vehicles could be routed more efficiently, and dynamic road tolls used to keep traffic flowing on popular routes.

On-street parking could be eliminated, as fewer people will own their own cars, or their cars will be able to park themselves elsewhere.

Meanwhile the absence of human drivers (and exhaust emissions, once we move to all-electric vehicles) will see a step-change in the take-up of cycling and walking in urban centres.

This is just scraping the surface. Self-driving cars are going to change everything.