"Only a fool breaks the 2 second rule."

I was taught to repeat that phrase, at a normal steady pace, when I saw the back of their car go past something, to use as a marker (a signpost, the end of one of the lines on the road, whatever).

If you finish the phrase after the front of your car has gone past the same marker, then you don't have a big enough braking distance and need to ease off a bit.