[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.