I think it could help solve congestion as a problem too.
Maybe cars could book their routes, and the road brokers will maximise for #cars/destination instead of allowing people to keep piling in even after the useful capacity is exceeded.
Cooperation would already go a long way to solving congestion with just local cooperation though:
* higher road density (cars can pretty literally travel bumper-to-bumper)
* better handling of congestion (car train can smooth local slowdowns over all its component cars)
* much, much improved merging and splitting of roads (including local flow analysis e.g. a small road merging into a bigger interstate with a 3:1 traffic ratio could be managed into a "perfect" 3:1 merge frequency)
* simplified urban planning (if only cooperative automated cars are on the road, you can get rid of red lights)