martine wrote:A 2 second gap is a great and easy to judge way of keeping a reasonable gap...it's less than the highway code stopping distance - @ 70mph 2 seconds is 62m but it's considerably more than many drivers leave. It's a good guide and bearing in mind there are very few situations where the hazard ahead will be stationary (head-on collision, debris appearing underneath the vehicle ahead etc), it seems sensible.
The 2 chevrons sign is confusing but probably better than nothing.
It has less to do with the stopping distance and more to do with the reaction time. Now, reaction time for alert drivers are within 0.6 of a second to 1.5+ seconds. For the unalert driver, they can border on three whole seconds.
So it pays to keep a margin that allows for sensible reaction time and a little extra should the stopping capabilities of the "lead" vehicle be somewhat superior to the tracking one.
So, between two to three seconds depending on the condition of the driver. You'd also increase distance as conditions deteriorate to allow for an increase in the braking differential and a possible increase in reaction time.
Static hazards can be avoided if you also keep enough margin so that the vehicle ahead doesn't fill your visual field so much. You can therefore see ahead and spot debries or hazards that will cause the lead car to collide and halt faster than it could using its brakes.
Two seconds usually accomodate for this, but if you are following a larger vehicle at speeds below 40mph this need for forward vision tends to become the limiting factor and a margin of three to four seconds becomes suitable. This is also usually more than enough to stay out of tire spray.
This is also why we should increase our following distance to accomodate for the possibility of stopping distance differential in favor of the lead vehicle, but not so much in favor of our own vehicle when following a heavier vehicle.