Traffic congestion will always be a thing. The real world is too unpredictable. You can't pack a highway full of cars and expect them to drive highway speeds. One animal jumping on the road and you'll have a massive accident because nobody can avoid anyone

Traffic congestion doesn't happen because of lack of coordination. Coordination prevents collisions. Our success rate could be a lot better but the system does pretty well with coordination limited to blinking lights, a noise that says "fuck off", the mutual knowledge that we all got the same training and the private knowledge that everyone else on the road is a suicidal idiot who slept through the training.

Congestion is a physical property of the road network. There's a physical limit to the number of cars you can push through a road in a given time period. Drive the cars too fast and they fly off the road when it bends, or cars slam into each other when a deer tries to commit suicide.

Traffic is well modeled as an incompressible slightly non-newtownian fluid, like runny oobleck. If you apply more pressure/increase vehicle volume the fluid starts to bind up as it becomes harder for the cars to maneuver without collision, regardless of coordination.

You reduce traffic by taking cars off the road, or scheduling utilization. Self driving cars won't help, and we can do both of those without them. Long term they're likely to make collisions worse because humans don't know what they're going to do, so the primary mode we have of coordination, "the theory of mind" will break down. Right now they're so uncommon that you assume any car is a human who reacts and prioritizes like you.

Tear down the freeways and replace them with trains.

I don't believe we will ever get coordinated autonomous vehicles. You will never be able to absolutely trust the information you are getting, whether it's because wireless sucks, auto manufacturers couldn't agree to or properly implement a universal standard, software bugs, or malicious behavior. If we want it to be safe, then the cars will have to drive cautiously and be prepared for behavior that was not properly communicated. This will not be a form of driving that is fundamentally different to what we have now.

And even if you could implement perfect coordination between autonomous vehicles, how would you be able to take advantage of it in the real world? Ban all human drivers? What about pedestrians and bikes? You still need crossings and traffic lights for an area to be livable by humans. You still need to drive slowly next to a pavement where someone could eg trip and fall onto the road.

Ha ha. Traffic will move to let billionaires through quickly.

Regular folks will be stuck in endless traffic trapped in driverless cars that charge them $5 a minute for the privilege all while playing the same loud and obnoxious audio video ads on repeat for hours on end.