Why would self-driving cars reduce traffic congestion? If my car drove itself, I'd be less incentivized to use public transportation. I'd probably be willing to put up with a slightly longer door-to-door time than if I used public transportation because it would be more convenient: I'd rather surf the web in my own car than have to pay attention to stops, schedules, transfers, etc. Congestion would probably be differ…
There is a possibility to optimize transport better using self driving cars but we may not actually take advantage of it. Even things like sharing self driving cars may not work in areas with larger sprawl because the probability of two people looking for the "same" ride within a certain time window and geographical area is small. In the city it's more likely but there are already solutions such as bicycles and public transport that work fairly well in dense areas.
It is very difficult to predict the impact. Perhaps impossible.