We have given a number of reasons that autonomous vehicles will reduce congestion.
“In particular, vehicle automation, once it gets critical mass, should greatly increase road capacity, both because of shorter following distances and because of narrower lanes. New, narrow vehicle forms designed for a single passenger (which is how most cars are used) will become more widespread with automation, as safety fears diminish, and will also increase person throughput.”
I have thought of one more reason autonomous vehicles will reduce traffic congestion. Currently most drivers do not take the shortest path (though we don’t really know why that is so, we have speculations). If we take humans out of the navigation decision, cars will be more likely to find the shortest path. This may not be system optimal, but will be a significant improvement over current routing decisions.
Not only should we not let people drive, we probably shouldn’t let them route themselves if we want an efficient system.