Every city street would have 4 lanes without the need for free car storage. Maybe make one of them a bike lane or widen the sidewalks and have 3 lanes for 30% more capacity. Also, traffic engineering could be optimized to a much greater extent since you wouldn't have to worry about all the affordances which keep humans from getting confused, keep them from getting aggressive, keep them from speeding, etc. Also, most congestion is caused by drivers causing turbulence by switching lanes, stopping each other from switching lanes, getting in the wrong lane, etc. A city of only AVs would probably flow much more smoothly.