Motorway 'safe' distance chevrons

[waremark]

Do you know the French system, 'Un trait danger, deux traits securite'? The white line marking the nearside edge of the carriageway is broken, conceivably about every 40 m, and the words that I have quoted which appear on signs tell you to leave a following gap equal to the length of two sections of white line - so probably 80 m. I find this French system both less ambiguous than the English system and easier to use to assess your actual following distance.

I am fairly sure that the English system intends to set a following distance of 80 m. Personally, I used to have a car with a dashboard display of my following distance, compared with a 2 second following distance, and I felt comfortable with a 2 second gap. I accept that a 2 second gap is less than the full HWC stopping distance above about 50 mph, but I take the view that the main reason for the 2 second gap is not because that gives you enough space to stop but because it gives you time to react and perhaps to avoid an obstruction.

Who actually goes for a 4 second following distance in the rain, as is supposedly good practise? Not me.

[Tosh]

At 70 mph it would take you 4.6 seconds to stop. That is based on the industry standard figure of 15 feet per second braking time applying a medium to firm braking force. Throw in another second for you to realise what is happening up ahead and get your foot on the brake and we're up to 5.6 seconds. That's on a dry road with good tyres.

So how does the two second rule in any way provide anyone any hope of being able to stop in the distance you can see to be clear, the fundamental principle of safe driving?

[gannet]

the 2s gap is NOT designed to be your stopping distance, it's your reaction time. If you are looking far enough ahead, ie not transfixed on the car in front you will see how the cars ahead are reacting to things in the road so the 2s rule works...

[waremark]

At 70 mph it would take you 4.6 seconds to stop. That is based on the industry standard figure of 15 feet per second braking time applying a medium to firm braking force. Throw in another second for you to realise what is happening up ahead and get your foot on the brake and we're up to 5.6 seconds. That's on a dry road with good tyres.

So how does the two second rule in any way provide anyone any hope of being able to stop in the distance you can see to be clear, the fundamental principle of safe driving?

The time you take to stop is not significant. For example, part of that time is spent travelling at 1 mph. What is significant is the distance you take to stop, and 'the two second rule' can help estimate distance.

[GJD]

At 70 mph it would take you 4.6 seconds to stop. That is based on the industry standard figure of 15 feet per second braking time applying a medium to firm braking force. Throw in another second for you to realise what is happening up ahead and get your foot on the brake and we're up to 5.6 seconds. That's on a dry road with good tyres.

So how does the two second rule in any way provide anyone any hope of being able to stop in the distance you can see to be clear, the fundamental principle of safe driving?

At anything above about 40mph the two second rule gives a separation distance closer than the highway code stopping distance (thinking time plus braking time). The two second rule seems to be pretty much universally advocated the government, by DSA, by IAM, by RoADAR and by just about anybody else who advocates such things. So either they are all a load of useless bloody loonies or you are trying to shoehorn into the rule a meaning it never had.

The car ahead of you doing 70mph is constrained by the same laws of physics as your car, so from the point the driver starts to brake, by your calculation it will continue to move for the next 4.6 seconds, or by HC braking distances it will go 245 feet further down the road before it stops. You said you threw in one second for you to realise what was happening and get on the brakes yourself. Well the two second rule gives you twice as much time as that. 100% contingency isn't a bad buffer - for example, it gives you some time to spend looking in other places, rather than having to continuously watch the car ahead.

Of course, the car ahead will stop quicker than that if it runs into, say, the back of a stationary lorry. If the car ahead fails to slow for a stationary lorry in its path, only a useless bloody loony would continue to follow as close as two seconds.

[MGF]

This suggests you should increase your following distance when you can't see beyond the vehicle in front, which is not uncommon.

[Ralge]

This suggests you should increase your following distance when you can't see beyond the vehicle in front, which is not uncommon.

This explains neatly and precisely where you see chevrons painted on the m/w I.e. where the m/w is long and straight.
So, for me, any rule about separation distance is not just about braking/stopping distances but chiefly about what you can see of the hazards ahead. If you can't see the 'big picture' take more space.

[GJD]

So, for me, any rule about separation distance is not just about braking/stopping distances but chiefly about what you can see of the hazards ahead. If you can't see the 'big picture' take more space.

Indeed. You don't violate the two second rule by being more than two seconds behind, only by being less than two seconds behind.

[7db]

Remember that the two second rule can be expressed as four seconds from the car in front to the car behind. Helpful if you are dealing with those who are a little close.

[waremark]

This suggests you should increase your following distance when you can't see beyond the vehicle in front, which is not uncommon.

..... and reminds you to work to get a view beyond the vehicle in front. I am never comfortable if I am without a view beyond the vehicle in front for more than a very short time.

[jameslb101]

Remember that the two second rule can be expressed as four seconds from the car in front to the car behind. Helpful if you are dealing with those who are a little close.

I've heard you say this before, and I like it.

[7db]

Remember that the two second rule can be expressed as four seconds from the car in front to the car behind. Helpful if you are dealing with those who are a little close.

I've heard you say this before, and I like it.

you'll go far.

[Mr Cholmondeley-Warner]

Bweeeeeeeeeeeeeeeeeeecchhhhhhhh