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This is the best US website I've seen on this topic but unfortunately it is still not particularly accurate.

The deciding factor in creating a truly safe gap depends upon the road surface conditions, not visibility (for example, in truly thick fog -- mentioned above -- you possibly could not even SEE nine seconds ahead -- that is when a very different rule comes into play). The key is the coefficient of friction of the road surface, for it is *this* that governs braking and stopping distances, which in turn dictate the required following distances.

The rule is three seconds for a dry, clean road surface; double it to six seconds for a wet road surface (irrespective of whether or not it is still raining -- remember it is the available GRIP that counts); and the triple or even quadruple it (i.e. 9-12 seconds) for extremely wet or otherwise slippery road surfaces -- everything from standing water and the risk of aquaplaning, through soft snow, to ice (and at the latter point look for the first opportunity to terminate your journey before somebody else's bad driving on sheet-ice does it for you).

The original rule was 2-4-10 seconds but this was extended to 3-6-12 as a result of research which showed that real-life reaction times are significantly slower than people expect them to be (largely because of complacency and a failure to concentrate).

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