Two Chevron rule

[Astraist]

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.

[Smeeagain]

It's not rocket science is it? Two chevrons need to be visible between you and the car in front ALL the time. Not at one point in time.

The sign says KEEP

Yes its not rocket science but please think about what Ive said - there are many ways to interpret what youve said and what the sign says and one of thise interpretations does not hive enough stopping distance.

Smee

[Smeeagain]

It's not rocket science is it? Two chevrons need to be visible between you and the car in front ALL the time. Not at one point in time.

The sign says KEEP

Yes its not rocket science but please think about what Ive said - there are many ways to interpret what youve said and what the sign says and one of thise interpretations does not hive enough stopping distance.
My point about "at one point" as you put it was as a snapshot. You could keep moving at 42 metres apart in your scenario and be compliant with your response, be compliant with the sign (which is what Average Joe tries to do), but you wont have enough stopping distance.
Smee

[Mr Cholmondeley-Warner]

If you know how to maintain the correct stopping distance yourself, then do so. If you are worried about others, then you've yet to realise that you can't really affect what they do, however "advanced" you might be yourself. All you can do is modify your own behaviour when you see what looks like risky interpretation of safety on their behalf.

Sticking rigidly to numbers - whether speed limits, stopping distances, seconds in "rules" or any other absolute, is not the mark of the true advanced driver. They think for themselves.

Your latest edit only reinforces your rigid belief in the importance of numbers which may or may not apply.

[Smeeagain]

The 2 chevrons sign is confusing but probably better than nothing. I wonder if there is any evidence to show it reduces collisions?

Yes it would be interesting to know. It was really illuminating to see how many colleagues interpreted in different ways, although that said pretty much all of them also said that they tended to slow down when they say that sign

Smee

To slow down? Why on earth? I thought for the vast majority speed choice on motorways was almost always about attitudes to the legal limit.

Yes to slow down. Why on earth? Simples. Because the sign made them realise they were too close to the vehicle in front ....

[Smeeagain]

Your latest edit only reinforces your rigid belief in the importance of numbers which may or may not apply.

Errr no .... I said tvat was what Average Joe did and how the sign could be interpreted.

So we just ignore others when they mention something in conversation in a controlled non driving scenario, that is patently wrong when we have the knowledge, experience etc to try and help them improve ????

[Mr Cholmondeley-Warner]

Define "knowledge and experience"... (hint - do they involve reading books and memorising figures, or thinking logically?)

Now describe circumstances in which the Highway Code definitions of total stopping distance apply to following another vehicle at the same speed on a motorway (warning - we've been here before). You might like to consider the consequences of an alert driver using the full braking capacity of their vehicle to stop as quickly as possible in a fast-flowing stream of traffic on a motorway, believing that what they're doing is "advanced"

Now explain how you will use that to "help them improve" (they, presumably, being your work colleagues or friends in the pub), without danger of your arguments being exposed to criticism (or possibly ridicule), or losing friends

[Smeeagain]

It's not rocket science is it? Two chevrons need to be visible between you and the car in front ALL the time. Not at one point in time.

The sign says KEEP

I am with the OP, I think it is extremely confusing. The French system is much clearer - long lines with short gaps between and it says keep at least two lines apart. I am happy that it is actually asking you to keep at least 80m apart, but it could be so much clearer.

Question - do you notice people leaving longer following distances on stretches of m'way which have these markings? I don't think I do.

Thank you. Yes I do but ...... only momentarily. They realise they were too close then slow down to increase the gap, give it five seconds if that, and then they drift off again .....

[Mr Cholmondeley-Warner]

Thank you. Yes I do but ...... only momentarily. They realise they were too close then slow down to increase the gap, give it five seconds if that, and then they drift off again .....

I agree. They are generally better behaved - for a while.

[GTR1400MAN]

It's not rocket science is it? Two chevrons need to be visible between you and the car in front ALL the time. Not at one point in time.

The sign says KEEP

Yes its not rocket science but please think about what Ive said - there are many ways to interpret what youve said and what the sign says and one of thise interpretations does not hive enough stopping distance.

Smee

No! Your one interpretation is a point in time interpretation, which less than a second later is no longer valid.

Keep = ALL the time. Not at one point in time. There is no other interpretation.