[Inception]

I would never get closer than a car's length away from the driver in front of me if I was driving 60 mph; why should I act any differently when I'm driving 30 mph? Speed is trivial - the gap must be maintained so drivers have the opportunity to react to cars around them without having to make any drastic changes in speed.

[sukilot]

Because following distance is irrelevant. Following time matters, and that is a function of speed.

[Inception]

Okay, but on-the-fly time calculations aren't exactly optimal. It's much easier to establish a lower limit on distance.