I think the key is that computer controlled vehicles could synchronize their stopping and starting. You wouldn't have to wait for the few feet moved by the car in front to bubble down the line as drivers notice the extra space. Instead, all stopped cars could coordinate to advance simultaneously.
You wouldn't even have to explicitly sync them; it'll take just few milliseconds for a self-driving car to notice another car is beginning to accelerate. Machines can keep precise control to the level simply unavailable to and unperceivable by humans.
