I don't expect self-driving cars to drive much more densely than current cars, though, because all or almost all the same reasons we drive with separation still apply for self-driving cars. Where you might think you get some advantages, they will also have disadvantages, too, like being able to compute exactly how risky their behaviors are. People making the cars and writing the car software aren't going to want to stand in court and explain why their car crashed the 50-car dense train because their car software left no possibility of the brakes going out at that exact moment (i.e., no previous sign of brake failure), and caused a huge crash because none of the cars had any buffer for failure.

Personally I expect self-driving cars to drive a great deal more conservatively than humans as a result, and I think self-driving cars are going to end up demanding a lot more space and buffer on the freeways. And if they don't at first, one day you'll way up and they suddenly all will, because somebody won some lawsuit and a software update was pushed overnight to double the buffers.