In article , Tom
Anderson wrote:
Then they're travelling too close together. Unless the traffic's
flowing at 40 mph, at which speed 2 seconds is almost enough.

Your arithmetic is wrong in that during the period you are stopping
your average speed is only going to be about half the initial speed,
so you would need to double your calculated times. What you ignore,
and what lies behind the 2 second rule on faster roads with good
visibility is that the car in front is not going to stop dead -
unless something really catastrophic happens.

Being a old guy who passed his test in imperial days the formula for
stopping distance was m(1+m/20) feet: the first term is basically the
distance travelled whilst your brain is getting your foot to press
the brake, the second the actual time needed to stop.

So if the car in front is doing 60 (88ft/sec) the total stopping
distance is 240ft - 60ft thinking, 180ft to actually stop. You're 2
seconds (176ft) behind the car in front when you see his brake
lights. He comes to a halt 180ft further down the road - 356ft from
where you are at the point when you see his brake lights. Assuming
your reactions and brakes are equal to the Highway Code you stop in
240ft from this point, 100 ft or so to spare.

--
Tony Bryer