Seems like the safe distance in front of a robot car would be significantly less than for that of a car driven by a human. Of course, most humans I know think that the safe distance is 2 feet, which is probably less still.
They probably have to assume the human co-pilot may still need to assume control and avert disaster, and give them enough extra time to do that (beyond the time they'd need after starting out in control). With no need for failover, the robot should reliably react faster, though it's probably only reacting to immediate events rather than predicting drivers' (mis)behavior.
