Problem is with the unexpected lane changes, last minute mergers and variable speeds of motorists jockeying for position. Traffic would flow much better when lane changes are minimal. When merging is done proactively and intelligently. And when adequate speeds and distances are maintained.
It might help some, but at what cost? I’ve been to Chicago during rush hour. The problem is 60 year old interchanges that can’t handle the traffic flow. Self driving cars won’t solve that problem alone. They have to add more lanes and improve interchanges.