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.