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 Post subject: 12mph
PostPosted: Sun Mar 07, 2004 11:38 
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Comments on Safe Speed page:

http://www.safespeed.org.uk/12mph.html

Old web based comments page:

http://www.safespeed.org.uk/12mphcomment.html

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 Post subject: 12mph Page - pointless ?
PostPosted: Mon Mar 08, 2004 17:11 
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The 12mph can be summed in a single sentence:

"We'd have more accidents if drivers drove with their eyes shut"

Why does Paul find this rather obvious observation so revolutionary ?


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PostPosted: Mon Mar 08, 2004 17:25 
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EvilInky wrote:
The 12mph can be summed in a single sentence:

"We'd have more accidents if drivers drove with their eyes shut"

Why does Paul find this rather obvious observation so revolutionary ?


The "12mph" page provides actual evidence that driver response is far more important as a contributor to average accident severity than free travelling speed.

The authorities are busy claiming that reducing free travelling speed will reduce accident severity. After a whole decade there is no sign of this actually happening - very much the opposite in fact.

If there's any comparable evidence anywhere else, I'd very much like to see it.

If we make drivers a tiny little bit less attentive to a risk ahead with our national "speed kills" policy, it should be easy to see that average accident severity might well increase because the attention effect swamps the speed effect.

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 Post subject:
PostPosted: Mon Mar 08, 2004 19:53 
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Evil Inky made a reply that changed the topic. It was moved to:

http://www.safespeed.org.uk/forum/viewtopic.php?t=57

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 Post subject: Driver Response
PostPosted: Thu Apr 08, 2004 14:15 
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On the 12mph page, you make reference to "driver response", as in "driver response is more than 42 times more important then pre accident speed".

Two questions:

1) How is driver response quantified and/or measured ?

2) Does the 42 times relationship only hold for appropriate pre-accident speeds ? I'm willling to assume that the best drivers on the roads are at least five times "better" than a just-passed 17 year old, but the former driver surely can't pose the same risk at 150mph as the latter driver at 30mph ( assuming an urban enviroment ).


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 Post subject: Re: Driver Response
PostPosted: Tue Apr 13, 2004 15:41 
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Jolly Roger wrote:
On the 12mph page, you make reference to "driver response", as in "driver response is more than 42 times more important then pre accident speed".

Two questions:

1) How is driver response quantified and/or measured ?


In this case we're talking about measuring average driver response in terms of the percentage of fatal outcomes. There must be zillions of parameters that go together to make up driver response, however the arithmetic mean response is what interests us here.

Jolly Roger wrote:
2) Does the 42 times relationship only hold for appropriate pre-accident speeds ?


No. It holds for the mean of all behaviours. The good objective here is to move the mean in the right direction.

Jolly Roger wrote:
I'm willling to assume that the best drivers on the roads are at least five times "better" than a just-passed 17 year old, but the former driver surely can't pose the same risk at 150mph as the latter driver at 30mph ( assuming an urban enviroment ).


Yes. The arithmetic mean behaviour is the one that needs adjusting. If you imagine a standard distribution of "driver quality" against frequency (where "driver quality" indicates success at avoiding accidents) we need to apply different strategies to different parts of the curve.

The bottom 15%, for example, are the ones in most urgent need of the attentions of the law, or retraining.

The large middle ground needs motivation, and the top 15% need encoragement and reward.

edited for typos - thrice :(

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Last edited by SafeSpeed on Tue Apr 13, 2004 15:52, edited 3 times in total.

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 Post subject:
PostPosted: Tue Apr 13, 2004 15:43 
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Note to readers...

"Evil Inky" and "Jolly Roger" are one and the same person.

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 Post subject: Re: Driver Response
PostPosted: Wed Apr 14, 2004 09:29 
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SafeSpeed wrote:
Jolly Roger wrote:
On the 12mph page, you make reference to "driver response", as in "driver response is more than 42 times more important then pre accident speed".

Two questions:

1) How is driver response quantified and/or measured ?


In this case we're talking about measuring average driver response in terms of the percentage of fatal outcomes. There must be zillions of parameters that go together to make up driver response, however the arithmetic mean response is what interests us here.



And how is the arithmetic mean response measured ? In other words,
how do you come up with the number that is 42 times more important than pre-incident speed in determining the crash speed ?
Quote:
Jolly Roger wrote:
2) Does the 42 times relationship only hold for appropriate pre-accident speeds ?


No. It holds for the mean of all behaviours. The good objective here is to move the mean in the right direction.

There's a problem here. Let's say in a particular situation, I am just about able to come to a safe halt with a pre-incident speed of 20mph ( I'm not a very good driver.) A driver who is twice as skilled as I am ( with a driver response twice that of mine ), in exactly the same situation, would be able to come to a safe halt from a pre-incident speed of 840mph ( 20mph x 42 x( 2 - 1 ) ). This surely can't be right ?
Quote:
Jolly Roger wrote:
I'm willling to assume that the best drivers on the roads are at least five times "better" than a just-passed 17 year old, but the former driver surely can't pose the same risk at 150mph as the latter driver at 30mph ( assuming an urban enviroment ).


Yes.

Yes, they do pose the same risk ?
Quote:
The arithmetic mean behaviour is the one that needs adjusting. If you imagine a standard distribution of "driver quality" against frequency (where "driver quality" indicates success at avoiding accidents) we need to apply different strategies to different parts of the curve.

Is the "driver quality" here the same thing as "driver response" ?


[ Edited to correct maths mistake ]


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 Post subject: Re: Driver Response
PostPosted: Thu Apr 15, 2004 19:06 
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Sorry - I see this was posted yesterday. I missed it until now.

Jolly Roger wrote:
SafeSpeed wrote:
Jolly Roger wrote:
On the 12mph page, you make reference to "driver response", as in "driver response is more than 42 times more important then pre accident speed".

Two questions:

1) How is driver response quantified and/or measured ?


In this case we're talking about measuring average driver response in terms of the percentage of fatal outcomes. There must be zillions of parameters that go together to make up driver response, however the arithmetic mean response is what interests us here.



And how is the arithmetic mean response measured ? In other words,
how do you come up with the number that is 42 times more important than pre-incident speed in determining the crash speed ?


The arithmetic mean response is measured in outcomes.

The page says "Taking a simple case we saw that making existing accidents into impacts at just 30 mph would kill 42 times more car drivers than die at present. This is a way of illustrating that driver response is more than 42 times more important than pre accident speed in the real world." Since the lowest free travelling speeds in general use are in about or beyond 30mph, it is reasonable to compare the outcomes of idealised 30mph crashes with the population of real crashes.

Jolly Roger wrote:
Quote:
Jolly Roger wrote:
2) Does the 42 times relationship only hold for appropriate pre-accident speeds ?


No. It holds for the mean of all behaviours. The good objective here is to move the mean in the right direction.

There's a problem here. Let's say in a particular situation, I am just about able to come to a safe halt with a pre-incident speed of 20mph ( I'm not a very good driver.) A driver who is twice as skilled as I am ( with a driver response twice that of mine ), in exactly the same situation, would be able to come to a safe halt from a pre-incident speed of 840mph ( 20mph x 42 x( 2 - 1 ) ). This surely can't be right ?


That's not a valid sum. You're working on the "wrong side" of the speed / fatality risk equation.


Jolly Roger wrote:
Quote:
Jolly Roger wrote:
I'm willling to assume that the best drivers on the roads are at least five times "better" than a just-passed 17 year old, but the former driver surely can't pose the same risk at 150mph as the latter driver at 30mph ( assuming an urban enviroment ).

Yes.

Yes, they do pose the same risk ?
Quote:
The arithmetic mean behaviour is the one that needs adjusting. If you imagine a standard distribution of "driver quality" against frequency (where "driver quality" indicates success at avoiding accidents) we need to apply different strategies to different parts of the curve.

Is the "driver quality" here the same thing as "driver response" ?


We're only looking at average risk. There's clearly a massive spread from the most dangerous driver in the country to the safest. I would agree that a "better quality" driver would deliver "better responses" - the terms are both used in relation to accident risk.

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 Post subject: Re: Driver Response
PostPosted: Fri Apr 16, 2004 10:52 
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SafeSpeed wrote:

The page says "Taking a simple case we saw that making existing accidents into impacts at just 30 mph would kill 42 times more car drivers than die at present. This is a way of illustrating that driver response is more than 42 times more important than pre accident speed in the real world." Since the lowest free travelling speeds in general use are in about or beyond 30mph, it is reasonable to compare the outcomes of idealised 30mph crashes with the population of real crashes.


But comparing the outcomes of idealised 30mph crashes with the population of real crashes only demonstrates the relationship between driver response and crash frequency, as the average speeds are the same in each case. It is valid to say that we would kill 42 more car drivers if driver response was reduced to zero, but this doesn't tell us anything about how pre-incident speed affects the outcome.

In fact, your own figures show the "driver response is 42 times more important than pre-incident speed" claim to be bogus: at an average pre-incident speed of 30mph we'd kill 42 times as many as we do at present; but at an average pre-incident speed of 12mph, we'd only kill about the same as we do at present. So we've achieved a 42-fold drop in deaths by reducing pre-incident speed by around two-thirds.

Quote:
Jolly Roger wrote:

There's a problem here. Let's say in a particular situation, I am just about able to come to a safe halt with a pre-incident speed of 20mph ( I'm not a very good driver.) A driver who is twice as skilled as I am ( with a driver response twice that of mine ), in exactly the same situation, would be able to come to a safe halt from a pre-incident speed of 840mph ( 20mph x 42 x( 2 - 1 ) ). This surely can't be right ?


That's not a valid sum. You're working on the "wrong side" of the speed / fatality risk equation.



I'm afraid you've completely lost me here.

I'm assuming that the risk is directly proportional to the pre-incident speed, and inversely proportional to the driver response. If both these elements had similar effects, then doubling the pre-incident speed would have the same effect on the risk as halving the driver response. However, as you are stating that driver reponse is 42 times more important than pre-incident speed, I take it that halving the driver reponse has the same effect on the risk as multiplying the pre-incident speed by 42.

( Edited by adding last paragraph )


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 Post subject:
PostPosted: Tue Apr 20, 2004 22:39 
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Hi JR

I had to read the 12mph page myslef 3 or 4 times before I got what I think is the point.

Quote:
It is valid to say that we would kill 42 more car drivers if driver response was reduced to zero


That I believe is the point, if I'm wrong them I'm sure someone will leap in and put me right.

The 12mph crash is a theoretical model which the driver does nothing to prevent..he/she crashes into a concrete block at 12mph without taking any avoiding action. If that were how crashes actually happened, 42 times more people would die as a result than do now.
But of course that isn't how it happens, crashes occur at higher speeds and under vaying circumstances; it is the very fact that the drivers involved take some sort of avoiding action (albeit too late in some cases) that lessens the liklihood of them being killed, 42 times less likely than if they'd had the theoretical crash at 12 mph without doing anything.

Well that's what I think is the point, course I could be wrong. Even if I'm right, I'm not sure if its a red hering or not.[/i][/b]


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 Post subject:
PostPosted: Wed Apr 21, 2004 09:47 
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Rigpig wrote:
Hi JR

I had to read the 12mph page myslef 3 or 4 times before I got what I think is the point.

Hi Rigpig
I and some of the guys from uk.rec.cycling have gone over it line-by-line, discussed it with Paul, and it's still difficult to pin down the point he's trying to make :)
Quote:
Quote:
It is valid to say that we would kill 42 more car drivers if driver response was reduced to zero


That I believe is the point, if I'm wrong them I'm sure someone will leap in and put me right.

The 12mph crash is a theoretical model which the driver does nothing to prevent..he/she crashes into a concrete block at 12mph without taking any avoiding action. If that were how crashes actually happened, 42 times more people would die as a result than do now.
But of course that isn't how it happens, crashes occur at higher speeds and under vaying circumstances; it is the very fact that the drivers involved take some sort of avoiding action (albeit too late in some cases) that lessens the liklihood of them being killed, 42 times less likely than if they'd had the theoretical crash at 12 mph without doing anything.

Slight correction : Paul says that 42 times more people would be killed if speeds remained the same, and driver response was reduced to zero. At 12mph, he claims we'd kill around the same number of people ( again with zero driver response ).

Quote:
Well that's what I think is the point, course I could be wrong. Even if I'm right, I'm not sure if its a red hering or not.[/i][/b]


I think it's the point as well. However, when I reworded this as "lots more people would die in road accidents if every driver stopped taking any kind of avoiding action", I was roundly rebuked!


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PostPosted: Fri Apr 30, 2004 14:21 
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Paul, it's been over two weeks since your last post on the subject of the 12mph page. Are you ignoring me, and hoping I'll go away, or would you like to continue the discussion ? I'm particularly interested in

a) the equation you use to derive crash risk from a combination of driver response and pre-incident speed,

b) how you determine driver response for a given driver. We've established that a driver response of 0.0 corresponds to a driver who takes no avoiding action, however, you haven't provided values for eg a novice driver, an average driver, or an experienced driver. ( And do negative values correspond to homicidal drivers ? )


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PostPosted: Fri Apr 30, 2004 15:08 
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Jolly Roger wrote:
Are you ignoring me, and hoping I'll go away, or would you like to continue the discussion ?


Sorry - no - I just hadn't noticed these posts.

I'll give you a reply as soon as poss - probably later today. I'm absolutely up to my eyes just now.

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 Post subject: Re: Driver Response
PostPosted: Sat May 01, 2004 09:29 
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Jolly Roger wrote:
SafeSpeed wrote:

The page says "Taking a simple case we saw that making existing accidents into impacts at just 30 mph would kill 42 times more car drivers than die at present. This is a way of illustrating that driver response is more than 42 times more important than pre accident speed in the real world." Since the lowest free travelling speeds in general use are in about or beyond 30mph, it is reasonable to compare the outcomes of idealised 30mph crashes with the population of real crashes.


But comparing the outcomes of idealised 30mph crashes with the population of real crashes only demonstrates the relationship between driver response and crash frequency, as the average speeds are the same in each case. It is valid to say that we would kill 42 more car drivers if driver response was reduced to zero, but this doesn't tell us anything about how pre-incident speed affects the outcome.


In the real world the wildest dreams of crash reduction through speed controls are about 50%. (Oliver Carsten). In the context of the gross estimates we're working with here any achievable real world reduction is close enough to zero that I've felt comfortable neglected it.

Suppose a non-wild estimate is 20%. One the one hand we have speed controls delivering a reduction from 1,000 to 800, and on the other we have driver response delivering a reduction from 42,000 to 1,000.

Jolly Roger wrote:
In fact, your own figures show the "driver response is 42 times more important than pre-incident speed" claim to be bogus: at an average pre-incident speed of 30mph we'd kill 42 times as many as we do at present; but at an average pre-incident speed of 12mph, we'd only kill about the same as we do at present. So we've achieved a 42-fold drop in deaths by reducing pre-incident speed by around two-thirds.


I think you're forgetting the purpose here - we're working on estimating the contribution to safety of driver response. We're doing that by looking at real world outcomes compared with estimated rates of death. You can't seriously be suggesting that we have the option of reducing free travelling speeds on our road network into the 12mph range - if we did we'd probably have many thousands dying in delayed ambulances!

You last sentence there is simply restating the Joksch equation - it does nothing to advance the debate.

Jolly Roger wrote:
SafeSpeed wrote:
Jolly Roger wrote:

There's a problem here. Let's say in a particular situation, I am just about able to come to a safe halt with a pre-incident speed of 20mph ( I'm not a very good driver.) A driver who is twice as skilled as I am ( with a driver response twice that of mine ), in exactly the same situation, would be able to come to a safe halt from a pre-incident speed of 840mph ( 20mph x 42 x( 2 - 1 ) ). This surely can't be right ?


That's not a valid sum. You're working on the "wrong side" of the speed / fatality risk equation.


I'm afraid you've completely lost me here.


The driver who was twice as skilled (at avoiding a fatality) would be twice as skilled at avoiding a fatality - we can't infer from that what the speed difference would be unless we predefined a particular speed. So, look at figure 2...

Jolly Roger wrote:
I'm assuming that the risk is directly proportional to the pre-incident speed, and inversely proportional to the driver response. If both these elements had similar effects, then doubling the pre-incident speed would have the same effect on the risk as halving the driver response. However, as you are stating that driver reponse is 42 times more important than pre-incident speed, I take it that halving the driver reponse has the same effect on the risk as multiplying the pre-incident speed by 42.


I think there's an interesting point lurking in there, but I can't quite put my finger on it. Obviously 42 times the speed would have a massive effect on risk, and that's absolutely not anything that I have tried to suggest. 42 times fewer fatalities do not arise from 1/42 of the speed. There's a fourth power in the equation.

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 Post subject: Re: Driver Response
PostPosted: Tue May 04, 2004 09:06 
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SafeSpeed wrote:
Jolly Roger wrote:
SafeSpeed wrote:

The page says "Taking a simple case we saw that making existing accidents into impacts at just 30 mph would kill 42 times more car drivers than die at present. This is a way of illustrating that driver response is more than 42 times more important than pre accident speed in the real world." Since the lowest free travelling speeds in general use are in about or beyond 30mph, it is reasonable to compare the outcomes of idealised 30mph crashes with the population of real crashes.


But comparing the outcomes of idealised 30mph crashes with the population of real crashes only demonstrates the relationship between driver response and crash frequency, as the average speeds are the same in each case. It is valid to say that we would kill 42 more car drivers if driver response was reduced to zero, but this doesn't tell us anything about how pre-incident speed affects the outcome.


In the real world the wildest dreams of crash reduction through speed controls are about 50%. (Oliver Carsten). In the context of the gross estimates we're working with here any achievable real world reduction is close enough to zero that I've felt comfortable neglected it.

Suppose a non-wild estimate is 20%. One the one hand we have speed controls delivering a reduction from 1,000 to 800, and on the other we have driver response delivering a reduction from 42,000 to 1,000.

This is an invalid comparison: the speed controls figures represent a reduction from current speeds to slightly slower speeds, while the driver response figures represent in increase in driver reponse from zero to something like the current level of response.
Quote:

Jolly Roger wrote:
In fact, your own figures show the "driver response is 42 times more important than pre-incident speed" claim to be bogus: at an average pre-incident speed of 30mph we'd kill 42 times as many as we do at present; but at an average pre-incident speed of 12mph, we'd only kill about the same as we do at present. So we've achieved a 42-fold drop in deaths by reducing pre-incident speed by around two-thirds.


I think you're forgetting the purpose here - we're working on estimating the contribution to safety of driver response. We're doing that by looking at real world outcomes compared with estimated rates of death. You can't seriously be suggesting that we have the option of reducing free travelling speeds on our road network into the 12mph range - if we did we'd probably have many thousands dying in delayed ambulances!

You last sentence there is simply restating the Joksch equation - it does nothing to advance the debate.

Jolly Roger wrote:
SafeSpeed wrote:
Jolly Roger wrote:

There's a problem here. Let's say in a particular situation, I am just about able to come to a safe halt with a pre-incident speed of 20mph ( I'm not a very good driver.) A driver who is twice as skilled as I am ( with a driver response twice that of mine ), in exactly the same situation, would be able to come to a safe halt from a pre-incident speed of 840mph ( 20mph x 42 x( 2 - 1 ) ). This surely can't be right ?


That's not a valid sum. You're working on the "wrong side" of the speed / fatality risk equation.


I'm afraid you've completely lost me here.


The driver who was twice as skilled (at avoiding a fatality) would be twice as skilled at avoiding a fatality - we can't infer from that what the speed difference would be unless we predefined a particular speed. So, look at figure 2...

What am I getting at is this: how much faster would the twice-as-skilled driver have to be travelling in order for the two drivers to have the same crash risk. Your "42-times" claim suggests he would have to be travelling 42 times faster, which is, of course, silly.
Quote:

Jolly Roger wrote:
I'm assuming that the risk is directly proportional to the pre-incident speed, and inversely proportional to the driver response. If both these elements had similar effects, then doubling the pre-incident speed would have the same effect on the risk as halving the driver response. However, as you are stating that driver reponse is 42 times more important than pre-incident speed, I take it that halving the driver reponse has the same effect on the risk as multiplying the pre-incident speed by 42.


I think there's an interesting point lurking in there, but I can't quite put my finger on it. Obviously 42 times the speed would have a massive effect on risk, and that's absolutely not anything that I have tried to suggest. 42 times fewer fatalities do not arise from 1/42 of the speed. There's a fourth power in the equation.


There's a fourth power in the Joksch equation. However, the Joksch equation has no term for driver response. I'm trying to figure about what a modified Joksch equation ( one with a driver response term, and which includes the 42 factor ) would look like.


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 Post subject: Re: Driver Response
PostPosted: Tue May 04, 2004 11:55 
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Jolly Roger wrote:
I'm trying to figure about what a modified Joksch equation ( one with a driver response term, and which includes the 42 factor ) would look like.


Now that is truly fascinating!

I don't have sufficient data to propose an equation based on Joksch, but I might just have enough data to propose the relationship based an Ashton and Mackay 1979. I've been doing some new and closely related work about "average pedestrian impact speeds" based on accident data and the Ashton Mackay fatality risk against speed curves.

The rate of adult pedestrian fatality is 1:28 of that expected from free travelling speed data in injury accidents. If we account for near misses and minor unreported accidents the proportion falls to something like 1:350.

If we project the rates of death by capping the (known) vehicles speeds to 30mph we get an improvement of about 1:1.3.

1:1.3 has proved attractive to some circles of the road safety community, but I believe the 1:350 to be by far the more important and significant factor. Even if we take the rock bottom mimimum estimate of 1:28 it's still more than an order of magnitude greater than 1:1.3.

I will be publishing this stuff in full as soon as it's ready. If I can propse the equation you're suggesting, I certainly will.

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PostPosted: Fri Sep 24, 2004 09:54 
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OK, first of all apologies over the Bullshit remark in the other page – was a bad day all round and I was spoiling for an argument.
Paul, you have nonetheless managed to create an interesting diversity of an assertion, one that is both, as the Americans would say, a no-brainer and a specious argument all wrapped up in one.
It’s a no-brainer because of course driver reaction to developing circumstances is important and plays some role in determining the outcome of an incident. The problem is that you are trying to assess the relative importance of an intangible over a tangible. By manipulating the way you choose to handle the data you can prove just about anything you want – in this instance by including near misses; the more near misses you choose to include the greater the ‘proof’ that you are right. But this is hardly controlled and scientific is it?
Each and every day there are, probably, thousands of incidents involving vehicles and other road users and/or furniture etc. Let’s be generous and assume that everyone is travelling within the system speed….
At the bottom end of the scale will come those incidents in which neither driver reaction nor vehicle speed played any part in the outcome – i.e. the parties involved could actually have done nothing at all and there would have been no collision. E.g when a driver sees what he thinks is a hazard up ahead (pedestrian about to step of the kerb) and momentarily reacts by easing off the gas. The hazard doesn't materialize because the pedestrian remains on the pavement (he/she had no intention of stepping off), but the driver reacted nonetheless.
At the other end of the scale are the incidents that are totally unpredictable and in which neither the speed nor the drivers’ reaction could have prevented it. E.g a vehicle suddenly departs its controlled path and collides with one coming in the opposite direction.
In the middle are a number of incidents which could have been prevented or been less severe had the driver(s) involved been going more slowly or had reacted more quickly and/or positively. Unfortunately, without setting up a controlled experiment involving a number of drivers of varying ability, numerous vehicle types and known environmental conditions it is impossible to tell whether speed or driver response is more important because clearly the relationship will fluctuate.
To conclude with an analogy, it’s rather like trying to ascertain whether personality (intangible) or qualifications (tangible) are more important in determining whether someone will be successful in gaining employment (without the luxury of being able to quiz the interview panel). In some instances an individual will simply walk into a job because of their qualifications (or because none were a pre-requisite) and in others a job will be secured by force of personality above all else. In the middle will come all the other circumstances in which either one or the other or both are important – but who can tell which?


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PostPosted: Fri Sep 24, 2004 10:33 
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Rigpig, thanks for your response. (And please don't worry about the "Bullshit" remark.)

Do you have a specific reason for think there's something "wrong" with comparing crash speed with free travelling speed and terming the gap (largely) road user response? I think the comparison is highly illuminationg and helps to direct us towards "what really matters".

The page in question is a rough estimate that reveals an average at least several orders of magnitude apart from popular opinion. I don't see anything wrong with rough estimates, and THIS rough estimate helps to highlight a major fallacy in popular and official thinking.

It's not the only data examination that carries the same sort of message - also see:

http://www.safespeed.org.uk/ten.html
http://www.safespeed.org.uk/killspeed.html
http://www.safespeed.org.uk/proof.html
http://www.safespeed.org.uk/tiger.html

(And btw, the page in question does not include near misses in its calculations - if it did, and it could, the results would be even more remarkable.)

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PostPosted: Fri Sep 24, 2004 13:56 
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If we were having the discussion face to face rather than through the to-and-fro of this forum, you would probably find my body language and facial expressions more illuminating than these typed words - a "I'm sure something doesn't fit here but I can't articulate it properly" kind of look.

By comparing crash speed with free-travelling speed we are acknowledging that 'something' happened in between (obviously). I'll call it the 'gap' from here on if that's OK. That 'something' includes road user response but is also a function of:

Ability
Vehicle factors
Environmental factors
How the individual was driving before responding.

Indentical circumstances could lead to differening outcomes if response is different, ergo differing circumstances and reactions could lead to the same outcomes. Thus, I feel that is wrong to call the gap 'user response' because, unless we can quantify all of the other extraneous factors we are unaware of just how effective user response was.
Thus, the gap is not 'user response' per-se, but exists as a result of user response and its effectiveness is dependent upon the pot-pourri of other factors one of which is the free travelling speed prior to response.
Therefore if the effectiveness of user response is dependent upon, amongst other things, free travelling speed - how can we elevate this intangible above the tangible in degree of importance?

Owww my head hurts :lol:


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