There are other actors on the road, namely pedestrians and cyclists. Unless we're going to remove the right of citizens to the street at all, they will be present and will need to be accommodated for.
My point is that unless you get rid of them completely, traffic flow is not going to increase. You can't have free-flowing traffic everywhere and also have pedestrians safely crossing the street, and the lower bound on a traffic light cycle today is the amount of time it takes an old person to cross the street from curb to curb.
That neglects limited access roads and wide boulevards. In either case, even if pedestrians aren’t banned, they are rare enough to be an exception rather than a rule. For example, the old guy trying to make their way across 8-lane Chang-an Road in Beijing...well, there is a reason Chang an road forces pedestrians onto bridges.
By and large, today the congestion problem is not really on the massive roads themselves, but where they interface with low-capacity local networks. Most destinations are not located on a limited access road by definition, and the busy exit into city center with lots of pedestrians is not going to get any less congested, AVs or not.
In modern Western urban planning, large car-capacity streets aren't considered a good thing to be running through urban areas anyways. Most of the large cities (Paris, Berlin, New York) are taking steps to reduce road widths to more equally distribute the road between pedestrians, cyclists and transit users. As it turns out, building pedestrian overpasses at a convenient enough spacing for pedestrians is extremely expensive.
That's only going to work on roads that are AI-only.