[danvet]

.2 is way to short. Only works when you're anticipating something and have all your muscles prepped for action. The realistic figure is 1 second for an attentative driver, a bit less if there is some signal ahead to anticipate the situation, way more if distracted by something (e.g. looking in the mirror for a lane change).

And to further your point even more, machines are much better at keeping the required distance for safely stopping. Stressed-out/aggressive humans tend to tail-gate ...