"Only a fool breaks the two-second rule!"

Takes about two seconds to say. Whatever speed you're doing, note the vehicle infront of you pass a fixed mark. If you don't have time to say the above before reaching the fixed mark then you are too close.
Obviously the faster you're going, the further you'll travel in those two seconds, so the greater gap you should be leaving. The two seconds however remains constant!

CHEERS
Nigel