This is a slogan, mantra or chant often taught in the UK when instructing drivers about safe following distance. The idea is that when the car in front passes an easily-identifyable point (like a bridge or a lamppost), the driver starts saying "only a fool...", which takes approximately two seconds at normal recitation speed. If the driver's car passes the above-mentioned point before the chant is completed, the gap between the cars is too small.
Obviously, this is only a rough guide, and poor weather conditions and speed need to be taken into account, but as most drivers here tend to drive closer than 2s apart, it's a start. At least we're not as bad as France, where many people drive closer than 1s at over 100mph.