[ptaipale]

Not bad, but you need an element from the square of velocity (for kinetic energy/braking distance) there to be safe. 1 second is fairly okay at low speeds, but at 60 mph (~100 km/h, 27 m/s) two seconds is shortish and if you are doing 200 km/h (as on a German Autobahn) then 2 seconds (100 m) is not safe in my opinion.

(Although the penalty line in Germany is just this, half the speed in km/h to get meters, i.e. driving closer than 100 meters if going 200 km/h gives you a ticket for tailgating)

[sib]

Completely agree with you. Should have stated that i was taught in the US, where it would be ... slightly atypical to be going 200km/h ;) At the time that I took drivers education classes, the US national speed limit was still 55mph, about 88km/h. At that speed, 2 seconds is probably a good rule of thumb for something that's pretty easier for a driver to observe.

[ptaipale]

True of course. At low speeds, the braking distance is negligible, and the margin is needed for driver reaction. When exceeding 60-80 km/h, the amount of kinetic energy and correspondingly the braking distance will start to dominate what is necessary for safety margin.