The whole point of my post was that even the most idiotic muppet out that can understand that at 30 metres per second when hes driving 5 metres behind someone at 70mph hes got less than 1 second gap. There is no mental arithmetic needed.
Or you could just count the seconds between the car in front and your own car, without needing to know speed or distance in any units. Don't you think that is easier than mentally dividing the estimated (and very likely incorrect) distance between cars by your speed?