Are you saying that a rational driver will never apply maximum braking?
However, the fact that the computer in the following car can detect instantly that the front car is slowing, it can react instantaneously and (theoretically, with similar braking distances) the following car would stop exactly the same distance (give or take a few inches) from the leading vehicle as when they were driving at speed...
So how does data communication (or lack thereof) affect this?
The follower can instantly detect that the leader is slowing via sonar/lidar/radar, right?
Then the distance between the cars that they should hold should be calculable by enforcing braking performance on the automated cars...
Assuming its enough, a simple 2 carlength rule would 1) give enough distance to stop and 2) allow for merging easily (second car would slot into place, following cars would slow slightly to make the gap again) ...