In good weather, on dry roads, two seconds is a safe gap. It's actually quite a sensible way of doing things really, as the faster you're travelling, the further you go in two seconds, so the gap increases with velocity.
But the last thing you want to do is take your eyes off the road to look at a stopwatch when you're doing 71.3 miles per hour down the M62.
So us instructors have a little phrase.
"only a fool breaks the two second rule". Find a bridge or an orange roadside telephone, and say it.
It should take two seconds to pronounce.
But does it?
No. It took 2.594 seconds. It's good to include a safety margin, but is there a more accurate way of doing this?
The other way I've encountered is to count elephants.
So, "one elephant, two elephant..." is a safe gap, according to this method.
Clicking on any of these pictures, by the way, will open them up full size. So if you can't see, try clicking on them for a better view. You should find that the light blue boxes at the top of the picture contain a number. This is the length of the sound wave, in milliseconds.
1729 milliseconds is just over 1.7 seconds. That's 15% less than a safe gap. At 70 miles per hours, that 0.3 second difference translates to about an extra 30 feet travelled before you've come to a halt, and that, you tailgating fool, puts you right into the boot of that Volvo you were following.
Not good. It doesn't matter that he slammed on, because a dickhead in a BMW suddenly changed lanes and then immediately braked. Hitting someone from behind is pretty much always considered to be your fault (you must have been either too close, or going too fast, or you weren't paying attention) you wouldn't have a leg to stand on. Especially if it had been amputated because it had been badly mangled in the collision.
So we need something a bit longer than an elephant.
An anaconda should do it.
Nope. At 2.319 seconds, it's a third of a second too long.
So I'm looking for something longer than an elephant, but shorter than an anaconda.
. . .
This went on for quite a while but then I found that my willy was exactly the right length. It's just slightly longer than an elephant, but a bit shorter than an anaconda.
See! Exactly 2000 milliseconds!
So next time you're wondering if you'll be able to stop in time as you travel down some dry road, just recite, "one my willy, two my willy", or "one Paul's willy, two Paul's willy" if you prefer.
Remember though, on wet roads, you need twice the distance.