Motorway 'safe' distance chevrons
Discussion
Toltec said:
The skill of the driver in front is not relevant, if you have a three second gap and get on to full braking within a second then, in a modern car, you will stop before hitting the car in front, even if it stops dead.
A modern car can brake from 70mph in 4 seconds without being on the limit, driver permitting.
Over that four seconds your average speed will be 35mph so you will travel the same distance as you would in two seconds at 70mph. That means two seconds of gap at 70 mph is enough braking distance to stop, then just add a second to decide you need to do an emergency stop and get on the brakes. Therefore if the traffic is heavy it is difficult to justify having a larger gap than three seconds, unless you are in the lhd lane and therefore not using an overtaking lane.
In light traffic where the road is not capacity limited then I do not see an issue with using a larger gap, it makes for a more relaxed drive and it allows other cars to move into your lane either to overtake a vehicle on the left or after overtaking you. In heavy traffic, you mentioned the M6 earlier so you know what WinstonWolf means about that, you just cannot expect to have a seven second chunk of road.
On the other hand, what I am behind matters, I once had to avoid a propshaft that came flying out from under a knackered old van and on another occasion only avoided an 8x4 sheet of plywood by swerving to the other side of the road and ducking.
In the first case the prop bounced into the air converting its forward momentum into vertical movement so effectively decelerating very quickly, in the latter, well an 8x4 sheet makes an effective air brake.
You make a lot of sense Toltec, I enjoy your posts. And others too.A modern car can brake from 70mph in 4 seconds without being on the limit, driver permitting.
Over that four seconds your average speed will be 35mph so you will travel the same distance as you would in two seconds at 70mph. That means two seconds of gap at 70 mph is enough braking distance to stop, then just add a second to decide you need to do an emergency stop and get on the brakes. Therefore if the traffic is heavy it is difficult to justify having a larger gap than three seconds, unless you are in the lhd lane and therefore not using an overtaking lane.
In light traffic where the road is not capacity limited then I do not see an issue with using a larger gap, it makes for a more relaxed drive and it allows other cars to move into your lane either to overtake a vehicle on the left or after overtaking you. In heavy traffic, you mentioned the M6 earlier so you know what WinstonWolf means about that, you just cannot expect to have a seven second chunk of road.
On the other hand, what I am behind matters, I once had to avoid a propshaft that came flying out from under a knackered old van and on another occasion only avoided an 8x4 sheet of plywood by swerving to the other side of the road and ducking.
In the first case the prop bounced into the air converting its forward momentum into vertical movement so effectively decelerating very quickly, in the latter, well an 8x4 sheet makes an effective air brake.
Edited by Toltec on Monday 13th January 21:10
I suppose some explanation is in order. I live in the extreme south west, close to Lands End in fact. I regularly travel long distances to pick up the cars I restore. I'm self employed and have the comparative luxury of being able to choose the times at which I travel and use the motorways. Often the M5 is empty, especially at night and I have the luxury of choosing when I travel. I realise that to have a 7 second gap in lane 3 of the M6 northbound is going to be difficult, also that 70 in those circumstances is near impossible whatever gap you leave. I'm able to tailor my own drives and the times at which I drive to suit my 'want's' rather than needs and accept that perhaps the majority don't have that luxury. I do feel that 2 seconds isn't enough at 70 mph, perhaps 4 which has been suggested is more like realistic for most of the drivers posting in this thread. Sure 7 is erring on the safe side, 2 I feel may be too much of a compromise for the majority, and yes drivers do frequently rear end other vehicles on the motorway because their alertness, special awareness, observation and following distances are inadequate, not giving figures about distances, just saying that frequently the distance chosen to follow at is inadequate, accidents may be caused by a combination of any of the above failings. I choose to follow at a distance I am comfortable with and travel at times wherever possible where I can do just that.
Just all do me a favour and think about stopping distances and following distances, maybe don't use my example of 7 seconds, it really isn't as far as you are thinking, try it sometime though to evaluate, but would it harm to perhaps add an extra second or two? Perhaps a 4 second gap as has been suggested. I think 2 can be done, but it might be cutting it a little fine, we all think we're invincible and have Senna's reaction times, but maybe add a little more breathing space just to make sure? It might save a life, it might even be your own.
Best wishes all and hope we never have to put these theories to the test. That's all I can say now, I can't add anything so further input would just be repetition.
25NAD90TUL said:
Isn't it? Those people couldn't stop in the distance they could see to be clear.
The smoke may have provided nucleation sources that gave a localised, rapid increase in the density of the fog. So the distance they could see changed very quickly and there happened to be something hidden in it. It is difficult to see fog through fog so one moment they can see the car in front and the next they cannot. If you are relying on the car in front to brake and they do not have chance or do so or at least very late at the same time as they disappear then you are stuffed.So what do you do? Slow down and let the car in front pull away? Without them as an early warning system you have to slow even more and start worrying about someone hitting you up the rear. Before anyone starts stone throwing how many of you drive at night on a motorway at a speed that allows you to stop in the distance you can see with your dipped beam? Yes, you can use light from cars in front or on the opposite carriageway to extend the 40 metres or so you get on dip, however if there is no one in front and you enter a lhd bend reducing the illumination from the offside do you slow down?
V8Ford said:
Am I only one here that just judges the gap in front based on experience, speed and conditions, or does everyone spend their journey counting up in seconds every time the car in front goes past a landmark?
I occasionally calibrate my judgement of distance either by counting or by comparing my following distance to the 100m marker posts on motorways. And very occasionally (ie not at all often) I try hard braking from different speeds to see how far it takes me to stop once I start braking.Toltec said:
The smoke may have provided nucleation sources that gave a localised, rapid increase in the density of the fog. So the distance they could see changed very quickly and there happened to be something hidden in it. It is difficult to see fog through fog so one moment they can see the car in front and the next they cannot. If you are relying on the car in front to brake and they do not have chance or do so or at least very late at the same time as they disappear then you are stuffed.
So what do you do? Slow down and let the car in front pull away? Without them as an early warning system you have to slow even more and start worrying about someone hitting you up the rear. Before anyone starts stone throwing how many of you drive at night on a motorway at a speed that allows you to stop in the distance you can see with your dipped beam? Yes, you can use light from cars in front or on the opposite carriageway to extend the 40 metres or so you get on dip, however if there is no one in front and you enter a lhd bend reducing the illumination from the offside do you slow down?
Good points. I live down in Cornwall as you know and often you will be driving along in conditions of very good visibility usually at night and suddenly the road crosses a stream or some damp ground and you find yourself in very severe fog, it may be clear half a mile ahead but inside these dense patches the visibility is down to less than a few metres, I've slowed to walking pace before and haven't been able to see where the road actually is, it is scary, must have been terrible that night on the M5, strange in that the M5 from my experience can be very quiet, not that night though sadly.So what do you do? Slow down and let the car in front pull away? Without them as an early warning system you have to slow even more and start worrying about someone hitting you up the rear. Before anyone starts stone throwing how many of you drive at night on a motorway at a speed that allows you to stop in the distance you can see with your dipped beam? Yes, you can use light from cars in front or on the opposite carriageway to extend the 40 metres or so you get on dip, however if there is no one in front and you enter a lhd bend reducing the illumination from the offside do you slow down?
Would I have been able to avoid trouble? I don't know for sure but I do think with good forward observation and a (clearly in my case) extra safety margin, I would like to think so but who knows really?
I enjoy these forums, I get a lot out of them, I'm not so foolish that I think I know it all, hell I have a lot to learn and am still very much learning, therefore I take a lot out of the discussions, my main priorities when driving are staying safe, trying my best to not compromise others safety and trying not to get in the way of others often because I'm driving unusual vehicles with 1950s technology.
My thinking on the following distance is that if using a 2 second gap at 30 mph, then surely this gap should be increased as speed increases? That's all it is, I do leave larger gaps than most drivers I see.
25NAD90TUL said:
My thinking on the following distance is that if using a 2 second gap at 30 mph, then surely this gap should be increased as speed increases? That's all it is, I do leave larger gaps than most drivers I see.
And this is what is flawed... as I stated above, the two second gap is reaction time, not any increased safety margin for braking. Once you're all on the brakes, the rate of deceleration is broadly similar - even for 1950s technology.StressedDave said:
25NAD90TUL said:
My thinking on the following distance is that if using a 2 second gap at 30 mph, then surely this gap should be increased as speed increases? That's all it is, I do leave larger gaps than most drivers I see.
And this is what is flawed... as I stated above, the two second gap is reaction time, not any increased safety margin for braking. Once you're all on the brakes, the rate of deceleration is broadly similar - even for 1950s technology.V8Ford said:
Am I only one here that just judges the gap in front based on experience, speed and conditions, or does everyone spend their journey counting up in seconds every time the car in front goes past a landmark?
I tend to consider what vehicle I am in and base it on that. It has stood me in good stead for the sudden stops. Keeping further back also gives me a better view further up the road. I don't rely soley on the brake lights in front of me and even keep an eye out behind me for the plonker that hitches to my bumper. And then of course the lanes around me. Saved me a few times, or rather I was able to safely exit the developing scenario where I knew the bloke behind would not have stopped in time.Tonsko said:
StressedDave said:
25NAD90TUL said:
My thinking on the following distance is that if using a 2 second gap at 30 mph, then surely this gap should be increased as speed increases? That's all it is, I do leave larger gaps than most drivers I see.
And this is what is flawed... as I stated above, the two second gap is reaction time, not any increased safety margin for braking. Once you're all on the brakes, the rate of deceleration is broadly similar - even for 1950s technology.Toltec said:
You mentioned this earlier and it made me consider it. If the gap is to react then why would it need to be greater in the rain? I think the simple answer is that two seconds would still work, but you need to be going slower.
Simply because the visual clues which flick the switch from 'I'm fine' to 'I really need to be doing something' are obscured by spray and the like and are harder to pick up.a good thread, and interesting rants......all I know is that driving along the M4 on sunday night away from Wales in the rain, that the "comfortable "gap I left in front of me was constantly being "taken", often by cars causing me to have to lift off or brake. The amount of cars "undertaking" to make progress was shameful, and I'm supprised there are not more accidents. All i know is that if there had been a "shunt" by a c%&t in front of me that I would have been able to stop in time.......however would the C*7T's behind me been able to avoid hitting me.
Best driving slogan to learn is "Only a c86t hit's the car in front".......it is always the car following that is at fault if they run into the car ahead.....so keep a distance you can stop in safely, mindful of the twit behind who you might also have to influence his ability to react to your brake lights....then be able to stop.....if you have good tyres/brakes/reactions/fit/alert/sober, it does not mean that everyone else around you has good tyres/tread/brakes that work/tired/drunk/on drugs/angry/hand up girlfriends skirt/insured/car registered to them/legally on the road etc...have a nice drive home tonight!
Best driving slogan to learn is "Only a c86t hit's the car in front".......it is always the car following that is at fault if they run into the car ahead.....so keep a distance you can stop in safely, mindful of the twit behind who you might also have to influence his ability to react to your brake lights....then be able to stop.....if you have good tyres/brakes/reactions/fit/alert/sober, it does not mean that everyone else around you has good tyres/tread/brakes that work/tired/drunk/on drugs/angry/hand up girlfriends skirt/insured/car registered to them/legally on the road etc...have a nice drive home tonight!
StressedDave said:
25NAD90TUL said:
My thinking on the following distance is that if using a 2 second gap at 30 mph, then surely this gap should be increased as speed increases? That's all it is, I do leave larger gaps than most drivers I see.
And this is what is flawed... as I stated above, the two second gap is reaction time, not any increased safety margin for braking. Once you're all on the brakes, the rate of deceleration is broadly similar - even for 1950s technology.SK425 said:
And the other flaw that's going on in this discussion is that we're working with half a mantra. It's not "always be able to stop in the distance you can see to be clear", it's "always be able to stop in the distance you can see to be clear and can reasonably expect to remain clear". As long as I can see further up the motorway that there is no immovable object for the vehicle ahead to hit (and almost all the time I can see that), it is very reasonable for me to expect that it will not stop instantly, and so I do not need my following distance to be as great as my stopping distance in order to comply with the mantra.
Thanks very much for pointing this out SK425!25NAD90TUL said:
SK425 said:
And the other flaw that's going on in this discussion is that we're working with half a mantra. It's not "always be able to stop in the distance you can see to be clear", it's "always be able to stop in the distance you can see to be clear and can reasonably expect to remain clear". As long as I can see further up the motorway that there is no immovable object for the vehicle ahead to hit (and almost all the time I can see that), it is very reasonable for me to expect that it will not stop instantly, and so I do not need my following distance to be as great as my stopping distance in order to comply with the mantra.
Thanks very much for pointing this out SK425!
Some very good input.
Today I've been conducting some research into this subject of safe following distances and have some quotes which I'm going to add to this thread.
Firstly from 'Pass your Advanced Driving Test' IAM 1998 edition. I felt that anything earlier would be unsuitable for the thread due to changes in car technology such as ABS etc...
Page 65 'Advanced motorway driving'
'...the IAM recommends a 'two second rule', whereby you allow a gap which keeps you a minimum of two seconds behind the vehicle in front'
All well and good for you 2 seconders? BUT it then goes on to say:
'Even this is barely enough: two seconds at 70mph gives you only 62 metres. You are deluding yourself if you think that driving within this safe distance is acceptable because you can see several vehicles ahead. This attitude ignores all kinds of possibilities: the driver ahead might brake unexpectedly if something lying in the road is seen, a vehicle from the opposing carriageway could crash through the central reservation, or the vehicle a little way ahead might even suffer a tyre blow-out'.
From 'The Official Highway code' DoT 2007:
Page 41. Bullet point 126:
'Drive at a speed that will allow you to stop well within the distance you can see to be clear.
You should:
Leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance'
As we know the stopping distance at 70mph is stated as 96 metres.
So, my interpretation of this is that you should be at least the FULL stopping distance behind at 70mph. The two seconds rule is considered a BARE MINIMUM and you're deluded if you think it's enough.
I also spoke to a Police Advanced friend who said he considered a minimum of '30 feet at 30mph and an extra yard for every extra mile per hour. At 70mph 30 feet plus forty yards, giving a safe minimum following distance of 150 feet'.
So while I accept, and never disputed, that my 7 seconds is more than double what is considered the minimum, this information also points to your 2 seconds at 70 as INADEQUATE which, if you remember was what I said originally.
Now if anyone wants to dispute that then good luck to 'em.
Today I've been conducting some research into this subject of safe following distances and have some quotes which I'm going to add to this thread.
Firstly from 'Pass your Advanced Driving Test' IAM 1998 edition. I felt that anything earlier would be unsuitable for the thread due to changes in car technology such as ABS etc...
Page 65 'Advanced motorway driving'
'...the IAM recommends a 'two second rule', whereby you allow a gap which keeps you a minimum of two seconds behind the vehicle in front'
All well and good for you 2 seconders? BUT it then goes on to say:
'Even this is barely enough: two seconds at 70mph gives you only 62 metres. You are deluding yourself if you think that driving within this safe distance is acceptable because you can see several vehicles ahead. This attitude ignores all kinds of possibilities: the driver ahead might brake unexpectedly if something lying in the road is seen, a vehicle from the opposing carriageway could crash through the central reservation, or the vehicle a little way ahead might even suffer a tyre blow-out'.
From 'The Official Highway code' DoT 2007:
Page 41. Bullet point 126:
'Drive at a speed that will allow you to stop well within the distance you can see to be clear.
You should:
Leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance'
As we know the stopping distance at 70mph is stated as 96 metres.
So, my interpretation of this is that you should be at least the FULL stopping distance behind at 70mph. The two seconds rule is considered a BARE MINIMUM and you're deluded if you think it's enough.
I also spoke to a Police Advanced friend who said he considered a minimum of '30 feet at 30mph and an extra yard for every extra mile per hour. At 70mph 30 feet plus forty yards, giving a safe minimum following distance of 150 feet'.
So while I accept, and never disputed, that my 7 seconds is more than double what is considered the minimum, this information also points to your 2 seconds at 70 as INADEQUATE which, if you remember was what I said originally.
Now if anyone wants to dispute that then good luck to 'em.
Edited by 25NAD90TUL on Sunday 19th January 05:07
Vipers said:
Just found this thread, very interesting.
I wonder though why these chevrons are only one or two roads, unless I have missed them.
My driving south once a year consists of Aberdeen to London via M74, M6, M1, and I can only recall seeing them on one stretch of the road.
They're very frequent on the M5, especially in areas near damp ground where sudden patches of thick fog may be a possibility or in other similar accident blackspot areas. I wonder though why these chevrons are only one or two roads, unless I have missed them.
My driving south once a year consists of Aberdeen to London via M74, M6, M1, and I can only recall seeing them on one stretch of the road.
25NAD90TUL said:
Some very good input.
Today I've been conducting some research into this subject of safe following distances and have some quotes which I'm going to add to this thread.
Firstly from 'Pass your Advanced Driving Test' IAM 1998 edition. I felt that anything earlier would be unsuitable for the thread due to changes in car technology such as ABS etc...
Page 65 'Advanced motorway driving'
'...the IAM recommends a 'two second rule', whereby you allow a gap which keeps you a minimum of two seconds behind the vehicle in front'
All well and good for you 2 seconders? BUT it then goes on to say:
'Even this is barely enough: two seconds at 70mph gives you only 62 metres. You are deluding yourself if you think that driving within this safe distance is acceptable because you can see several vehicles ahead. This attitude ignores all kinds of possibilities: the driver ahead might brake unexpectedly if something lying in the road is seen, a vehicle from the opposing carriageway could crash through the central reservation, or the vehicle a little way ahead might even suffer a tyre blow-out'.
From 'The Official Highway code' DoT 2007:
Page 41. Bullet point 126:
'Drive at a speed that will allow you to stop well within the distance you can see to be clear.
You should:
Leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance'
As we know the stopping distance at 70mph is stated as 96 metres.
So, my interpretation of this is that you should be at least the FULL stopping distance behind at 70mph. The two seconds rule is considered a BARE MINIMUM and you're deluded if you think it's enough.
I also spoke to a Police Advanced friend who said he considered a minimum of '30 feet at 30mph and an extra yard for every extra mile per hour. At 70mph 30 feet plus forty yards, giving a safe minimum following distance of 150 feet'.
So while I accept, and never disputed, that my 7 seconds is more than double what is considered the minimum, this information also points to your 2 seconds at 70 as WOEFULLY INADEQUATE which, if you remember was what I said originally.
Now if anyone wants to dispute that then good luck to 'em.
If it is woefully inadequate why does a H&S obsessed government advise it?Today I've been conducting some research into this subject of safe following distances and have some quotes which I'm going to add to this thread.
Firstly from 'Pass your Advanced Driving Test' IAM 1998 edition. I felt that anything earlier would be unsuitable for the thread due to changes in car technology such as ABS etc...
Page 65 'Advanced motorway driving'
'...the IAM recommends a 'two second rule', whereby you allow a gap which keeps you a minimum of two seconds behind the vehicle in front'
All well and good for you 2 seconders? BUT it then goes on to say:
'Even this is barely enough: two seconds at 70mph gives you only 62 metres. You are deluding yourself if you think that driving within this safe distance is acceptable because you can see several vehicles ahead. This attitude ignores all kinds of possibilities: the driver ahead might brake unexpectedly if something lying in the road is seen, a vehicle from the opposing carriageway could crash through the central reservation, or the vehicle a little way ahead might even suffer a tyre blow-out'.
From 'The Official Highway code' DoT 2007:
Page 41. Bullet point 126:
'Drive at a speed that will allow you to stop well within the distance you can see to be clear.
You should:
Leave enough space between you and the vehicle in front so that you can pull up safely if it suddenly slows down or stops. The safe rule is never to get closer than the overall stopping distance'
As we know the stopping distance at 70mph is stated as 96 metres.
So, my interpretation of this is that you should be at least the FULL stopping distance behind at 70mph. The two seconds rule is considered a BARE MINIMUM and you're deluded if you think it's enough.
I also spoke to a Police Advanced friend who said he considered a minimum of '30 feet at 30mph and an extra yard for every extra mile per hour. At 70mph 30 feet plus forty yards, giving a safe minimum following distance of 150 feet'.
So while I accept, and never disputed, that my 7 seconds is more than double what is considered the minimum, this information also points to your 2 seconds at 70 as WOEFULLY INADEQUATE which, if you remember was what I said originally.
Now if anyone wants to dispute that then good luck to 'em.
Gassing Station | Advanced Driving | Top of Page | What's New | My Stuff