Any individual driver can still maintain that spacing. If a car pulls in in front of my Tesla S it will simply drop back slowly until it achieves the set spaci

Any individual driver can still maintain that spacing. If a car pulls in in front of my Tesla S it will simply drop back slowly until it achieves the set spacing (actually measured in time not distance) again. There are very few roads in my experience where long stretches are filled with high speed cars. The places that are filled with cars are usually around accidents and road works where the speed and hence separation distance are reduced. I don't think a three second separation has any meaningful impact on throughput. perhaps someone with the necessary expertise and tools could simulate it.