Where I drive, you simply can't leave any more distance when traffic is heavy: if you leave reasonable space between you and the car in front of you, someone will pull in. It's a bit nuts.
But the great thing about this tech is that, unlike me, it has the reflexes to always react safely and the ability to maintain that focus indefinitely. I rely on "looking upstream" to predict changes in traffic flow, and that works well enough, but it doesn't help with drivers who are just crazy, lose a tire, or other such unpredictable events. Now, I'm not sure what scope of events the car can react to, as it's early days yet for self-driving, but in principle it's great.
How close you drive to the car in front of you is a matter of reaction time. I expect we'll no longer be bound by the limits of the human nervous system, soon enough.