In my mind, something cheaper and simple like a wire down the centre of each lane, or maybe plop some rfid chips every 20 yards to identify each lane, so each car knows where it is at all times, then add some communications between cars, so they all know where every other car is within a mile or whatever makes sense. Most cars will have a planned destination, and planned route to get there, then run it like a train system, so they aren't trying to interpret visually or with lidar to figure out what all the other cars are doing. Trying to simulate a human brain and eyes is not cheap or possible, so far.
I don't think the computing for a huge train system would be that difficult? Maybe it has to be distributed between vehicles, but computing power is cheap when it doesn't have to be tiny or all that efficient..
Making the system hacker proof is probably the biggest challenge?
Even old "dumb" cars would need the position tracking and communication system installed, to let every other automatic car know what the car with the crazy human is doing.
Maybe some of our resident programmers could give some insight in the challenges in programming the operating system for something like this?