Discussion
smeggy said:
Andrew D said:
All things being equal, a vehicle travelling at a lower speed has more time to avoid an accident, and should an accident occur, requires less energy discharge to come to a stop. Less energy mean less violence, less smashed glass and metal, fewer Basal Skull Fractures, less injury.
While that is true it's not the whole story. It is known that free travelling speed does not equal impact speed otherwise there would be many, many more deaths; driver awareness is the key difference. This awareness is being eroded and lost with our increasing dependence in relying upon numbers to guide us.
Andrew D said:
Unfortunately the law does not yet allow us to ban all those drivers that lack the requisite level of skill to allow speed limits to be raised for the rest of us.
It has done for many years, they are called:
a) The driving test
b) Driving without due care and attention.
and perhaps
c) Evolution by Natural Selection
There is also the subtle fact that motorway tuition/testing is, for reasons that escape me, banned for learner drivers. This would go a long way to nipping said problem in the bud.
Andrew D said:
I'm afraid that's misrepresenting what I said. I actually said that people who indulge themselves by driving above the limit, restrained only by the degree of risk of getting caught that they are willing to accept, are retards. I very much doubt that most drivers make a reasoned judgement about what is a safe speed, and drive to that speed. This is because most drivers will not be in possession of sufficient information to make that judgement, even though they might think that they are.
Well if I've misrepresented what you said I apologise, but it isn't clear to me that I have. In fact you same to be saying much the same thing now. I do not accept that most drivers who exceed the speed limit do so only on the basis that they are willing to risk punishment, with no regard to whether it is safe and whether they are competant to judge whether it is safe. If this is not what you're saying, please clarify.
Your other comments about speed and safety seem to be considering impact speed and hence severity as the primary factor of interest. You are conveniently ignoring many other factors that determine the probability and likely severity of a collision, some of which increase and some of which decrease the risk with speed. In my opinion compliance with the speed limit is almost irrelevent to road safety compared to the other far more important factors.
If exceeding the speed limit actually is unacceptably dangerous most of the time, then I could sympathise with your position that the people who choose to do it are acting foolishly. Personally I don't think that this is the case, and I haven't seen any figures that suggest it is the case. If you think that it is, please explain why you think this.
I think that the vast majority of people are perfectly competent to judge what is safe and what is not, in the environments they're used to. They have a strong vested interest in getting the judgement right, and the accident statistics suggest that most people get it right most of the time. This includes deciding what speed is safe and appropriate for the circumstances. Your suggestion that most people are not competant to make this judgement strikes me as arrogant, and unjustified. If you can justify it, please do.
GreenV8S said:
Your other comments about speed and safety seem to be considering impact speed and hence severity as the primary factor of interest. You are conveniently ignoring many other factors that determine the probability and likely severity of a collision, some of which increase and some of which decrease the risk with speed. In my opinion compliance with the speed limit is almost irrelevent to road safety compared to the other far more important factors.
The crux of my argument is that, all things being equal, driving at a lower speed is inherently safer. The bulk of counter-arguments, however, have been based on the premise that a driver with a high level of skill can drive safely at higher speeds, but in that case all things aren't equal. Considering the same driver in the same vehicle at the same time on the same road, is a lower speed not safer in the vast majority of instances? And whilst it may be able to contrive a scenario by which it would be beneficial to be travelling at a higher speed, to avoid an object approaching from the rear for example, it would constitute only a minority of instances. I haven't focused purely on impact speed, hence my comments regarding controlling blowouts or avoiding an incident incured by the car in front, both of which would be easier at a lower speed (considering the same driver in the same vehicle at the same time on the same road).
GreenV8S said:
I think that the vast majority of people are perfectly competent to judge what is safe and what is not, in the environments they're used to. They have a strong vested interest in getting the judgement right, and the accident statistics suggest that most people get it right most of the time. This includes deciding what speed is safe and appropriate for the circumstances. Your suggestion that most people are not competant to make this judgement strikes me as arrogant, and unjustified. If you can justify it, please do.
In my experience a good proportion of drivers are unaware that it requires, typically, 295m of forward visibility at 70mph to be able to stop safely and prevent shunt accidents in the traffic behind. Moreover, many are unaware that relaxations in this standard are commonplace throughout the highway network, and are granted in conjunction with tightened horizontal curvature with the intention that drivers would perceive the tightened radius and reduce speed accordingly. However I often witness vehicles maintaining their speed through such corners and relying (although not consciously) on providence that, for example, a concrete block hasn't been dropped from an overbridge (as was recently the case on the M1 south of Junction 37) into their path.Sorry to cherry pick your arguments, but a) I'm a bit busy and b) there are plenty of others here disagreeing with you
I realise this is a throw away line, and it's too late for the unfortunate 11 but for the sake of their colleagues do you not think people should know this? Particularly you as a highways engineer sticking up for lower speed limits.
Unfortunately all things aren't equal. At lower speeds drivers pay less attention, tailgate more and get more frustrated. While the results of any accidents might be less (and you've not shown us any evidence to suggest this) they are IMO more likely to happen.
Andrew D said:
And I'm not sure about the frequency of injuries per mile, but I doubt the 11 dead workers really care about that statistic.
I realise this is a throw away line, and it's too late for the unfortunate 11 but for the sake of their colleagues do you not think people should know this? Particularly you as a highways engineer sticking up for lower speed limits.
Andrew D said:
The crux of my argument is that, all things being equal, driving at a lower speed is inherently safer.
Unfortunately all things aren't equal. At lower speeds drivers pay less attention, tailgate more and get more frustrated. While the results of any accidents might be less (and you've not shown us any evidence to suggest this) they are IMO more likely to happen.
Andrew D said:
The crux of my argument is that, all things being equal, driving at a lower speed is inherently safer.
The crux of my argument is that you haven't demonstrated that exceeding the speed limit under the circumstances in question increases the risk to an unacceptable level, or even that it increases the risk at all. That being the case, I think that your personal insults regarding people who do it are unjustified and offensive.
mechsympathy said:
I realise this is a throw away line, and it's too late for the unfortunate 11 but for the sake of their colleagues do you not think people should know this? Particularly you as a highways engineer sticking up for lower speed limits.
The comparison of injuries per mile suggested would serve only as an attempt to write off the 11 deaths as "not disproportionate normal road conditions", and I'm not sure how that would help their colleagues. And I'm not arguing in favour of lower speed limits, I'm arguing against the perception that it's acceptable for a driver to speed simply because they disagree with the speed limit. mechsympathy said:
Andrew D said:
The crux of my argument is that, all things being equal, driving at a lower speed is inherently safer.
Unfortunately all things aren't equal. At lower speeds drivers pay less attention, tailgate more and get more frustrated. While the results of any accidents might be less (and you've not shown us any evidence to suggest this) they are IMO more likely to happen.
GreenV8S said:
The crux of my argument is that you haven't demonstrated that exceeding the speed limit under the circumstances in question increases the risk to an unacceptable level, or even that it increases the risk at all. That being the case, I think that your personal insults regarding people who do it are unjustified and offensive.
I'm afraid that again misrepresents my position, I didn't refer to all who exceed the limit, simply those that do so as a result of inability to control their temptation. Accidents are occuring resulting in roadworker injuries, 88% of roadworkers testify that they are due to excess speed (from the Highways Agency Roadworker Safety Plan). A reduction in speeds would reduce the incidence of accidents. Whether you think that level of accidents is unacceptable or not is upto you.
Edited by Andrew D on Tuesday 8th August 12:33
Andrew D said:
The comparison of injuries per mile suggested would serve only as an attempt to write off the 11 deaths as "not disproportionate normal road conditions", and I'm not sure how that would help their colleagues.
This is what I actually said:
mechsympathy said:
But how many of these accidents were due to speed, rather than inattention, tailgating, rummaging around in the glovebox for a mars bar etc etc? And how do these stats work out by mile? (And compare to non-roadwork stats?)
And I'd be interested to know how those roadworker accidents occurred.
And I'd be interested to know how those roadworker accidents occurred.
mechsympathy said:
Unfortunately all things aren't equal. At lower speeds drivers pay less attention, tailgate more and get more frustrated. While the results of any accidents might be less (and you've not shown us any evidence to suggest this) they are IMO more likely to happen.
Andrew D said:
The term "all things being equal" means that only one variable is modified (i.e. speed) and the others maintained. Tailgating, et al, are not variables but the result of the modification. Whilst I'm sure that you're correct in suggesting that those factors would occur, I suspect that they are outweighed by the benefits of reduced severity and increased avoidance.
But we're talking about the real world.
Andrew D said:
But if you have an evidence to the contrary, please let me know.
Likewise, old fruit. It's all opinions, which is hardly a sound basis for an arbitrary speed limit.
And if I can remind you, earlier...
Andrew D said:
It's true. In the manner in which most people do 80mph on a 70mph motorway, they do 50mph through 40mph roadworks. It's because they realise that they should be going slower, but they still can't restrain themselves from going just a little faster than the limit... because most drivers are retards.
If there was a decent basis for speed limits then you might have a point about calling drivers retards, but until then it seems a little condescending.
AndrewD "Accidents are occuring resulting in roadworker injuries, 88% of roadworkers testify that they are due to excess speed (from the Highways Agency Roadworker Safety Plan)."
How do roadworkers measure the speed of the traffic? Do they stand watching the traffic all the time? Would explain why roadworks take so long!
How do roadworkers measure the speed of the traffic? Do they stand watching the traffic all the time? Would explain why roadworks take so long!
Andrew D said:
The crux of my argument is that, all things being equal, driving at a lower speed is inherently safer. The bulk of counter-arguments, however, have been based on the premise that a driver with a high level of skill can drive safely at higher speeds, but in that case all things aren't equal.
I think we can accept that, in real world driving conditions, all things are not equal.
Andrew D said:
Considering the same driver in the same vehicle at the same time on the same road, is a lower speed not safer in the vast majority of instances?
So where do we draw the line? 30mph on motorways? Re-introduction of Red Flag Man? I’m sure we can agree it’s all about achieving a reasonable balance.
Andrew D said:
I haven't focused purely on impact speed, hence my comments regarding controlling blowouts……..
The relative share of consequence from mechanical failure (defective brakes + tyre deflation before impact + other vehicle defects) is tiny when weighed up against the other contributing factors (less than 1%), so this is insignificant and unreasonable to give significant focus upon it.
Andrew D said:
In my experience a good proportion of drivers are unaware that it requires, typically, 295m of forward visibility at 70mph to be able to stop safely and prevent shunt accidents in the traffic behind.
At a constant 70mph that would equate to 9.4 seconds of travel time (and that’s disregarding the beneficial effect of reduced speeds during braking); I’m curious as to how you came to that figure.
Andrew D said:
Moreover, many are unaware that relaxations in this standard are commonplace throughout the highway network, and are granted in conjunction with tightened horizontal curvature with the intention that drivers would perceive the tightened radius and reduce speed accordingly. However I often witness vehicles maintaining their speed through such corners and relying (although not consciously) on providence that, for example, a concrete block hasn't been dropped from an overbridge (as was recently the case on the M1 south of Junction 37) into their path.
So what do you think is the best solution, dropping the speed limit for all corners such that all drivers can be guaranteed reasonable time to be able to react accordingly? To do so would be to encourage dependence on the speed limit and would be the start of a very downwards, very slippery slope……
Andrew D said:
"In my experience a good proportion of drivers are unaware that it requires, typically, 295m of forward visibility at 70mph to be able to stop safely and prevent shunt accidents in the traffic behind."
That's well over 3 times the emergency stopping distance. So rather a large allowance for following drivers. Then again, seeing how close together many cars drive on the motorway you may well be right.
"In my experience a good proportion of drivers are unaware that it requires, typically, 295m of forward visibility at 70mph to be able to stop safely and prevent shunt accidents in the traffic behind."
That's well over 3 times the emergency stopping distance. So rather a large allowance for following drivers. Then again, seeing how close together many cars drive on the motorway you may well be right.
Andrew D said:
Some stuff
Lots of other people said:
Some other stuff
Lots of questions there, too many to answer individually.
The 295m isn't something that I came up with, it's quoted from TD 9/93, which is the standard for the design of highway alignments. It's used for the design of every major road. It's based upon the experience of many hundreds of engineers and scientists and empirical data gathered over several decades. So I think that it's quite a useful source.
To clarify my point; safety is dependent on a large number of factors, one of which is speed. Whilst reducing speed has a number of effects, both positive and negative, it is the finding of my training and experience that the net result is beneficial for safety.
Andrew D said:
The 295m isn't something that I came up with, it's quoted from TD 9/93, which is the standard for the design of highway alignments.
I'm not familiar with TD 9/93, but from the context it appears to define how far drivers should be able to see i.e. how much visibility will be provided to them. That is completely different to saying that cars need 295m in order to stop, or that drivers need to be looking at least 295m ahead in order to be able to stop safely. In fact I don't see any significance to the value of 295m, other than that it is presumably chosen to be well in excess of the distance that drivers need to be able to see under any forseeable conditions. I'm sure you're right that most drivers don't know that the roads are designed so that they can see at least 295m. But this seems utterly irrelevent to the question of whether most drivers are competent to judge whether it is safe to drive through a 40 limit in roadworks at 50 mph. Nothing you've said backs up your opinion that most people are NOT competent to judge this, or that people who do so are retards.
The 295m that I cited is the "Stopping Sight Distance" for the design speed of a 70mph road (although the design speed is actually 120kph, or 75mph, to allow for a degree of exceeding the limit, funnily enough). The value for a 40mph is 120m. I imagine that by nailing the brakes in a Carrera GT you could stop in a lot less distance, but the line of four rusty Renault 5 Campus's behind you might have trouble matching it, which is why it is set at what seems a long distance.
Most people don't consider that each person in the outside lane queue will add their own thinking and reaction time to the distance required for everyone to stop. They don't all see the object at the same time and stop at the same rate.
And there's a lot more intricacies where that came from.
So I suppose what I'm getting at is that a lot of people who think that they can second guess the limit setting authority might not be in possession of all the facts.
But once again, I didn't say that everyone who speeds in roadworks is a retard (no matter how often you might assert that I did). I imagine the TrafPol occasionally do, but they're quite capable of doing so safely.
Most people don't consider that each person in the outside lane queue will add their own thinking and reaction time to the distance required for everyone to stop. They don't all see the object at the same time and stop at the same rate.
And there's a lot more intricacies where that came from.
So I suppose what I'm getting at is that a lot of people who think that they can second guess the limit setting authority might not be in possession of all the facts.
But once again, I didn't say that everyone who speeds in roadworks is a retard (no matter how often you might assert that I did). I imagine the TrafPol occasionally do, but they're quite capable of doing so safely.
Andrew D said:
The 295m that I cited is the "Stopping Sight Distance" for the design speed of a 70mph road (although the design speed is actually 120kph, or 75mph, to allow for a degree of exceeding the limit, funnily enough). The value for a 40mph is 120m. I imagine that by nailing the brakes in a Carrera GT you could stop in a lot less distance, but the line of four rusty Renault 5 Campus's behind you might have trouble matching it, which is why it is set at what seems a long distance.
Most people don't consider that each person in the outside lane queue will add their own thinking and reaction time to the distance required for everyone to stop. They don't all see the object at the same time and stop at the same rate.
Most people don't consider that each person in the outside lane queue will add their own thinking and reaction time to the distance required for everyone to stop. They don't all see the object at the same time and stop at the same rate.
Googled:
AASHTO said:
Stopping sight distance is the sum of two distances: the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied and the distance required to stop the vehicle from the instant brake application begins. These are referred to as brake reaction distance and braking distance, respectively.
link
They seem like bog standard braking distances to me; the HC states <100m for what is nowadays considered to be a below average car. I would go further and give my interpretation: 70mph roads have been designed to give a minimum of 295 meters sight distance, which is good for over 120mph (extrapolating the HC).
Andrew D said:
The 295m that I cited is the "Stopping Sight Distance" for the design speed of a 70mph road (although the design speed is actually 120kph, or 75mph, to allow for a degree of exceeding the limit, funnily enough). The value for a 40mph is 120m. I imagine that by nailing the brakes in a Carrera GT you could stop in a lot less distance, but the line of four rusty Renault 5 Campus's behind you might have trouble matching it, which is why it is set at what seems a long distance.
I'm no expert, but I think you may find that the "Stopping Sight Distance" you refer to means something very different to the stopping distances given in the highway code. For example, a realistic stopping distance in good conditions at 40 mph is around 40m. This assumes that you are caught by surprise and take a full second to react but then manage to sustain 1G braking. I imagine you could get to a figure of 120m if you assume that the driver is drunk and half asleep, and driving a car with three bald tyres and defective brakes in heavy rain. You shouldn't confuse the "Stopping Sight Distance" with the actual stopping distance under typical conditions, or assume that the Stopping Sight Distance has any relevence to every day driving. In any case I'm not sure what stopping distances has to do with speeding drivers being retards.
Andrew D said:
Most people don't consider that each person in the outside lane queue will add their own thinking and reaction time to the distance required for everyone to stop. They don't all see the object at the same time and stop at the same rate.
And there's a lot more intricacies where that came from.
So I suppose what I'm getting at is that a lot of people who think that they can second guess the limit setting authority might not be in possession of all the facts.
Don't most people? It seems bl**dy obvious to me and fundamental to choosing a safe speed and vehcle separation, among other things. Why do you assume that most people don't understand these 'intricacies'? The fact is that the vast majority of drivers are highly skilled at judging a safe speed for the conditions, as demonstrated by the extremely low accident rate. I would say that a competent and alert driver is far better placed to judge what speed is actually appropriate for the actual circumstances, than somebody who is choosing what number to put on a stick. The speed limit should provide a guide to a reasonable safe speed under typical circumstances. That is all it is good for. You absolutely can't draw any general conclusions about whether it is safe or dangerous to exceed the speed limit.
Andrew D said:
But once again, I didn't say that everyone who speeds in roadworks is a retard (no matter how often you might assert that I did). I imagine the TrafPol occasionally do, but they're quite capable of doing so safely.
Again if I misquoted you I apologise. Do you want to retract the 'retard' insult completely, or just clarify which drivers you are accusing of being retards? It certainly isn't 'all drivers', which I think is what you actually said in the first place.
Stopping sight distances are something distinct from the stopping distances given in the HC. As mentioned in my previous post they take into account the presence of other traffic, and the knock-on effects of heavy braking within a group of vehicles. The british standard (which was sat open on my desk at the time of my last post) goes into great detail about the relationship between SSD, curvature and imparting restraint on drivers to reduce speeds at locations of SSD relaxations. The example in my last post (the Carrera GT and Renault 5's) clearly illustrates the difference between SSD and vehicle stopping distances, and also the relevence to the "real world".
Firstly, I did not refer to "all drivers" as you claim. The term I used was "most", although I was in fact refering to the driver that I had described it the passage before. And my reasoning for that comment is as follows.
Whether or not a driver agrees with the limit imposed, so exceed it is to flirt with prosecution. I can't really see how exceeding the limit would be an effective form of protest if the driver did disagree. Thus I can only see two reasons why a driver would exceed the limit. Either the time saved (20% in the example) is critical and worth taking the risk, which I suppose is valid but I find unlikely. Or else the driver is too impatient, excitable, or afraid to loose face to drive within the limit imposed, which risks prosecution for no return, and is the behaviour that I termed that of a "retard".
GreenV8S said:
Again if I misquoted you I apologise. Do you want to retract the 'retard' insult completely, or just clarify which drivers you are accusing of being retards? It certainly isn't 'all drivers', which I think is what you actually said in the first place.
You seem to have taken my original comment rather personally, which is a pity because it certainly wasn't meant that way. However, you don't seem to have understood what I actually said, so I'll once again attempt to clarify it. Firstly, I did not refer to "all drivers" as you claim. The term I used was "most", although I was in fact refering to the driver that I had described it the passage before. And my reasoning for that comment is as follows.
Whether or not a driver agrees with the limit imposed, so exceed it is to flirt with prosecution. I can't really see how exceeding the limit would be an effective form of protest if the driver did disagree. Thus I can only see two reasons why a driver would exceed the limit. Either the time saved (20% in the example) is critical and worth taking the risk, which I suppose is valid but I find unlikely. Or else the driver is too impatient, excitable, or afraid to loose face to drive within the limit imposed, which risks prosecution for no return, and is the behaviour that I termed that of a "retard".
Edited by Andrew D on Tuesday 8th August 22:17
Andrew D said:
Stopping sight distances are something distinct from the stopping distances given in the HC. As mentioned in my previous post they take into account the presence of other traffic, and the knock-on effects of heavy braking within a group of vehicles.
Sorry mate but I reckon you’re plain wrong!
I’ve done some digging; SSD is related to the HC stopping distances (strictly speaking: in very adverse conditions) and has nothing to do with other drivers behind.
See this for an explanation (Scroll to page 30)
I acknowledge this is not my area of expertise so I welcome any correction of my assessment.
EDIT:
I just saw your above post, it looks like the data in my link is the same as that given in the BS on your desk
Edited by smeggy on Tuesday 8th August 22:33
Thanks for the detail.
No doubt, but…..
Could you link to a better document, the more I think about it the more it seems nonsensical. You certainly can’t expect drivers to keep a distance of 295m – very few would ever make it onto motorways.
Assuming a following driver is moronic enough to drive like a lemming: if the lead driver brakes for a hazard, the following driver will see the brake lights illuminate. So long as the distance between their cars is greater than the decision/reaction distance of the follower (assuming equal braking distances) he need not have to know about the hazard, yet he will still stop in time – thanks to the brake lights. Of course one could argue that the lead driver may not brake, then the normal HC would apply (factoring for conditions).
If your gonna say ‘the brake lights may not be working’ then that’s why we need real trafpol, not speed cameras.
Andrew D said:
As I said in previous posts, the real reason for the disparity between the ESD and SSD is provision for a multitude of other factors present in practice.
No doubt, but…..
Andrew D said:
Stopping sight distances are something distinct from the stopping distances given in the HC. As mentioned in my previous post they take into account the presence of other traffic, and the knock-on effects of heavy braking within a group of vehicles.
Could you link to a better document, the more I think about it the more it seems nonsensical. You certainly can’t expect drivers to keep a distance of 295m – very few would ever make it onto motorways.
Assuming a following driver is moronic enough to drive like a lemming: if the lead driver brakes for a hazard, the following driver will see the brake lights illuminate. So long as the distance between their cars is greater than the decision/reaction distance of the follower (assuming equal braking distances) he need not have to know about the hazard, yet he will still stop in time – thanks to the brake lights. Of course one could argue that the lead driver may not brake, then the normal HC would apply (factoring for conditions).
If your gonna say ‘the brake lights may not be working’ then that’s why we need real trafpol, not speed cameras.
smeggy said:
Could you link to a better document, the more I think about it the more it seems nonsensical. You certainly can’t expect drivers to keep a distance of 295m – very few would ever make it onto motorways.
Assuming a following driver is moronic enough to drive like a lemming: if the lead driver brakes for a hazard, the following driver will see the brake lights illuminate. So long as the distance between their cars is greater than the decision/reaction distance of the follower (assuming equal braking distances) he need not have to know about the hazard, yet he will still stop in time – thanks to the brake lights. Of course one could argue that the lead driver may not brake, then the normal HC would apply (factoring for conditions).
Ah, sorry about that. The SSD doesn't mean the distance between each vehicle, it's effectively the distance the front vehicle needs to be able to see. Assuming a following driver is moronic enough to drive like a lemming: if the lead driver brakes for a hazard, the following driver will see the brake lights illuminate. So long as the distance between their cars is greater than the decision/reaction distance of the follower (assuming equal braking distances) he need not have to know about the hazard, yet he will still stop in time – thanks to the brake lights. Of course one could argue that the lead driver may not brake, then the normal HC would apply (factoring for conditions).
EDIT: I've had a look around and I can't locate a definition or derivation of SSD. Apparently in older standards there was a detailed description, but the literature currently available only gives values, not a definition.
Edited by Andrew D on Wednesday 9th August 14:00
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