There are non-rational things like animals, kids, and adults with their face in their phones to watch for, so it will never be fully rational...

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...