1) Inter-vehicle communications is standardized. This allows for each vehicle to communicate to each other and signal important metrics (speed,direction,intent...), also allows to complement each vehicles 'knowledge' and context of what is going on on the road at any given moment. Communication only needs to happens for vehicles clustered in a given radius (depending on speed and flow of traffic).
2) A critical mass of autonomous vehicles are on the roads.
Once you have these 2 conditions it will be easier for the 'swarm' behavior and flow of traffic to be automated fully. It will also remove a lot of the 'unknowns' stemming from human drivers and smoothly direct traffic.