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PostPosted: Fri Oct 15, 2004 11:43 
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Rigpig wrote:
BTW - I give up. Can't discuss the issue through this disjointed media.


Give me a ring if you like. I'm on 01862 832000.

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PostPosted: Tue Oct 26, 2004 13:32 
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Thank you for our chat. I think it makes sense now.


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PostPosted: Tue Oct 26, 2004 15:59 
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Rigpig wrote:
Thank you for our chat. I think it makes sense now.


It was a pleasure to talk to you!

For the benefit of others, I'd like to set out what we discussed.

Rigpig said that picking one group for the 12mph page was unfair. I said it was a fair sample because it reflects similar casualty severity ratios in EVERY group of road users. I referred to the data on page:

http://www.safespeed.org.uk/ten.html

I explained that no physics based model of crash causation could possibly lead to these casualty ratios. Instead we need a human factors or psychological model. A physics model would lead to a far greater proportion of fatalities given typical vehicle speeds. For example a 60mph impact is ususally fatal, yet only about 1 crash in a thousand is fatal.

Then we talked about the risk triangle present in modern health and safety thinking. This is where the figures on the "ten" page are placed in a heirachy, and it is usually considered that if we want fewer deaths we need to concentrate on the huge number of incidents at the base of the risk triangle. Yet road safety policy is concentrating on the deaths atthe top of the risk triangle. In so doing it risks provoking more incidents at the bottom of the triangle and making the situation worse.

Ultimately I explained that the purpose of the 12mph page was to highlight the comparative rarity of fatal accidents and point out that the physics model used by the modern road safety establishment is misleading us to look at high severity accidents and not the huge number of low severity incidents where the problems truly lie.

Is that a fair summary Rigpig?

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PostPosted: Tue Oct 26, 2004 23:32 
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SafeSpeed wrote:
Rigpig wrote:
Thank you for our chat. I think it makes sense now.


It was a pleasure to talk to you!

For the benefit of others, I'd like to set out what we discussed.

Rigpig said that picking one group for the 12mph page was unfair. I said it was a fair sample because it reflects similar casualty severity ratios in EVERY group of road users. I referred to the data on page:

http://www.safespeed.org.uk/ten.html

I explained that no physics based model of crash causation could possibly lead to these casualty ratios. Instead we need a human factors or psychological model. A physics model would lead to a far greater proportion of fatalities given typical vehicle speeds. For example a 60mph impact is ususally fatal, yet only about 1 crash in a thousand is fatal.

Then we talked about the risk triangle present in modern health and safety thinking. This is where the figures on the "ten" page are placed in a heirachy, and it is usually considered that if we want fewer deaths we need to concentrate on the huge number of incidents at the base of the risk triangle. Yet road safety policy is concentrating on the deaths atthe top of the risk triangle. In so doing it risks provoking more incidents at the bottom of the triangle and making the situation worse.

Ultimately I explained that the purpose of the 12mph page was to highlight the comparative rarity of fatal accidents and point out that the physics model used by the modern road safety establishment is misleading us to look at high severity accidents and not the huge number of low severity incidents where the problems truly lie.

Is that a fair summary Rigpig?



Absolutely. Insofar as the 12mph page goes, I think you have to look beyond the message that appears to hit you straight in the face.
The authorities are trying to tackle the issue from the top down rather than eating away at it from the bottom up - where it will have more effect. Furthermore, by isolating one factor (because it's readily measurable against a fixed datum?) we are ignoring all the other factors - almost as if they are incidental.
So lets beat this thing :!:


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PostPosted: Fri Aug 19, 2005 14:32 
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I've just had a serious discussion with an intelligent and educated person who insists that you're actually saying that if we all drove at 12mph then there would be no reduction in fatalities.

*I* don't have any trouble understanding the point that you're making (and I've explained it myself even though you do so twice), but I would suggest that there's a slight danger of a casual or hostile reader using the headline as evidence that you're a kook.


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PostPosted: Thu Sep 01, 2005 14:34 
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let me try and explain in a diferent way

If we all drove at 12MPH:
kids wouldn't look before they crossed the road in a false sense of security

you wouldn't hear cars coming

people would fall asleep and hit items that might fall on them and kill them (or others) such as lamp posts

tractor drivers and construction workers die in low speed accidents because they are traveling slowly they attempt slopes or obsticles they would not attempt at higher speeds.

accidents due to crosssing train tracks would take longer

traveling from Newcastle to Cornwall would take days and there would be a temptation to travel 14 hours a day or more to cut the no of days down

there would be less need for driver input so that the driver would be tempted to do other things like texting, or even read a newspaper!

There would be a temptation to take the shortest route even if it were less safe.

cyclists travel at 12mph average... look how many of them die just falling over a kerb with no other involvement.

even at 12mph a small car can be crushed by a truck (especialy if he is texting)


however a driver allowed to choose his own speed will choose a speed at around 90% of his concertation limit. He will watch the road and do nothing else. (I have not seen any motor racing drivers texting)

What happens when a human is asked to watch toast under the grill?
for 30 seconds you watch it, then you decide to make a cup of coffee , forget the toast. before you know it it is black

drive at 12 mph and people will find other things to do on the trip like watch the scenery

I think Pauls mathmatical approach is stumping a few people.


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PostPosted: Sun Nov 20, 2005 01:03 
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Hi Guys,

Sorry to bring you bad news but you've made a basic error in your statistics on the 12mph page, which is why the otherwise reasonable (if out of date) data is leading you to such a crazy conclusion. It's also the reason why some of the people posting to this thread have this gut feeling that "something's wrong"

What you've done is taken a mean result across all speeds (i.e. that 1 out of 113 accidents with injuries result in a fatality), and tried to apply that at a specific speed. I.e. the following paragraph is incorrect:

Quote:
Putting the two together

We realised that these two pieces of information could be combined. We can calculate the average crash delta v from the proportion of drivers killed. We know the proportion of drivers who are killed, and we can use the equation from fig 2 to calculate an average impact speed.


These two pieces of information can't be combined.

The point is, that whilst 1/113 is the mean probability of death across all speeds, it is not that probability at all speeds. It might not be that probability at any speed at all, it's just the mean. Certainly you can't assume that the mean probability at all speeds is the same as the probability at the mean speed. They're just not the same thing.

To illustrate: the mean number you can throw on a six sided dice is 3.5 (that's 1 + 2 +3 + 4 + 5 + 6 divided by 6). You're argument basically relies on taking the mean and applying it everywhere. Assuming that the 1:113 ratio applies at all speeds is like taking the mean of our dice and assuming that all six faces give you 3.5. As with the mean value on a dice, there might not be *any* speed at which 1:113 is the actual ratio, that's just the average across all speeds. It makes no more sense to try to work out what speed that the average ratio occurs at than it does to work out which face of the dice the mean score occurs on. At least with a dice we could say something, as we know that each number occurs with equal probability, but there is no indication given on this page of how likely an accident is to happen at any given speed - this really is fishing in the dark, which is why you end up with a crazy figure like 12mph/

PM me if you have any questions as I don't read this forum normally (PMs will notify me by e-mail), I just popped in to help.

I hope this is useful (if a little hard to hear)

Cheers

Munch


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PostPosted: Sun Nov 20, 2005 02:01 
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Munchgreeble wrote:
Hi Guys,

Sorry to bring you bad news but you've made a basic error in your statistics on the 12mph page, which is why the otherwise reasonable (if out of date) data is leading you to such a crazy conclusion. It's also the reason why some of the people posting to this thread have this gut feeling that "something's wrong"

What you've done is taken a mean result across all speeds (i.e. that 1 out of 113 accidents with injuries result in a fatality), and tried to apply that at a specific speed. I.e. the following paragraph is incorrect:

Quote:
Putting the two together

We realised that these two pieces of information could be combined. We can calculate the average crash delta v from the proportion of drivers killed. We know the proportion of drivers who are killed, and we can use the equation from fig 2 to calculate an average impact speed.


These two pieces of information can't be combined.

The point is, that whilst 1/113 is the mean probability of death across all speeds, it is not that probability at all speeds. It might not be that probability at any speed at all, it's just the mean. Certainly you can't assume that the mean probability at all speeds is the same as the probability at the mean speed. They're just not the same thing.

To illustrate: the mean number you can throw on a six sided dice is 3.5 (that's 1 + 2 +3 + 4 + 5 + 6 divided by 6). You're argument basically relies on taking the mean and applying it everywhere. Assuming that the 1:113 ratio applies at all speeds is like taking the mean of our dice and assuming that all six faces give you 3.5. As with the mean value on a dice, there might not be *any* speed at which 1:113 is the actual ratio, that's just the average across all speeds. It makes no more sense to try to work out what speed that the average ratio occurs at than it does to work out which face of the dice the mean score occurs on. At least with a dice we could say something, as we know that each number occurs with equal probability, but there is no indication given on this page of how likely an accident is to happen at any given speed - this really is fishing in the dark, which is why you end up with a crazy figure like 12mph/

PM me if you have any questions as I don't read this forum normally (PMs will notify me by e-mail), I just popped in to help.

I hope this is useful (if a little hard to hear)

Cheers

Munch


Nice theory, but no cigar I'm afraid.

Clearly there are 'populations' on both sides of the equation. You're right in as much as skews in the populations would skew the results (although in the real world I don't think there is much skew). On the other hand if you look at the directions of any likely real world skews you find that the adjustments tend to suggest even lower average impact speeds.

We do know that average impact speeds are tiny in relation to free travelling speeds, and the estimation method on this page is just one more confirmation of the fact.

I'll send a PM notification of this reply.

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PostPosted: Sun Nov 20, 2005 17:27 
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Munchgreeble wrote:
To illustrate: the mean number you can throw on a six sided dice is 3.5 (that's 1 + 2 +3 + 4 + 5 + 6 divided by 6).


But if you threw the dice a few thousand times and came up with a mean figure of, say, 2.5, you would (correctly) assume that the dice was heavily loaded.
When it comes to collision speeds, it's equally clear that there is a heavy loading towards the lower end. Paul's whole point is that there must be something which creates this loading.

Cheers
Peter

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PostPosted: Mon Nov 21, 2005 23:49 
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Hi,

Thanks for the PM. Look, you're banding around statistical language, but what you're saying doesn't really hold water. That's not to say your end point is wrong, just that you're not doing yourself any favours presenting an argument which is just plain wrong.

I stopped at the very start of your 12mph page because the whole foundation of the argument is unsound - there seemed no point in continue. However, the argument continues to become more illogical so that the end conclusion is just plain ridiculous.

I'm not going to spend any more time illustrating that the foundation is bogus, but let me just tackle a couple of points that the argument goes to next.

1. You are keen to separate free travel speed and delta V as two separate concerns, yet when you reach your conclusion you suddenly forget this distinction and declare that a calculated delta v of 12mph indicates that the speed limit should be 12mph! As you yourself said in your last post, "average impact speeds are tiny in relation to free travelling speeds". In fact, since even the worst drivers brake before a collision this really is common sense - who in their right mind would set the speed limit based on the assumption that nobody uses the brakes??!! Do you see how ludicrous the argument sounds?

2. You again mix up the population mean with the population distribution. Supposing all the argumentation so far were in fact correct - the tying up of the population mean with a single point on the graph, the assumption that drivers never use breaks (and that that's how speed limits are set) etc. we now come to a startling revalation - dispite the fact that your graph in figure 2 shows an extreme bias in ratio of fatalities towards the top end of the graph, you seem to think that limiting the speed to the mean will have no effect. Let's go back to our example of the dice. What you're saying is, if we dissallow any rolls of the dice higher than the mean (3.5) that it will have no effect on the mean. That is, you reckon if we disallow 4, 5 and 6 to be thrown, then it won't lower the average from 3.5. Of course it will! If you had an average of 12mph that means some higher speed impacts and some lower speed impacts balance out at an average of 12mph. Getting rid of the stuff above 12mph (or 30 or whatever speed you want to pick) only leaves the stuff below 12mph - which certainly doesn't have an average of 12mph - or do you think that these imaginary people who never use their brakes always travel at a fixed speed of 12mph?

If you present arguments like this to back up your case, you could easily be written off as "not serious" and trying to "fabricate" reasoning to back up your point of view. If you really do have hard scientific evidence that enforcing speed limits doesn't reduce road deaths then focus on that (I suspect you have other pages which cover this like I say I'm not a regular reader). Don't undermine your case by peddling arguments that sound persuasive to Joe public, but actually make no sense.

Now for some hard physics. As I suspect you know, the amount of energy in a moving vehicle is half of it's weight (mass) mutiplied by it's speed and multiplied by it's speed again (0.5mv^2). That is to say, if you're going say 50% faster (e.g. 30mph instead of 20mph) there is more than double the energy in your car. The amount of time you have to react to a given situation is cut by 50% and therefore the amount of time you have to take preventative action is cut still further. When you do react, you are less able to turn to avoid impact and have a much longer stopping distance. Finally, if and when you do have an impact, all these factors add up to making an impact speed way higher than the 50% difference you started ot with, causing a much worse accident than at the slower speed. In fact in a lot of cases, if you're just travelling slower your reactions, evasive action etc. will be enough to avoid an accident completely, that, at a higher speed would have been unavoidable. Clearly 50% is fairly extreme, but the same is true to a lesser extent with any speed difference.

No matter how good your road sense, reactions, vehicle brakes etc. they don't get better the faster you go. You will almost always be able to handle a potential collision less easily than if you are going slower. The only time speed is an advantage is if you're facing a side impact or some similar where you need speed to get you out of the way in time.

These are the kinds of basic issues you're going to have to tackle to convince people like me. If you have a good argument against such basic laws of physics then I'd be glad to hear it. Personally, since driver reactions don't increase with vehicle speed, and since ability to correct the vehicle's trajectory goes down as speed increases, I'll be very surprised if there is such an argument, but I'm willing to hear it if there is one. Just one plea, if you don't have a real argument to present, please don't try to dress it up with scientific sounding words when the basic argument doesn't hold water!

I guess this must be hard to hear, but hopefully you'll end up with a more solid case to present at the end of the day. And for what it's worth, whilst that particular argument leaves me rather nonplussed, I like the approach you're taking of trying to get to the heart of what causes accidents and encouraging safe driving.

All the best with your endeavours.

Cheers

Munch


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PostPosted: Tue Nov 22, 2005 03:10 
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Now I think you are trolling rather than making a serious contribution to the debate. Trolling is unwelcome here and can result in the suspension of your user account.

Munchgreeble wrote:
I stopped at the very start of your 12mph page because the whole foundation of the argument is unsound - there seemed no point in continue. However, the argument continues to become more illogical so that the end conclusion is just plain ridiculous.


This is how I know you are trolling. Clearly you didn't 'stop at the start', and it's utterly bizarre to dismiss an argument without considering it as a whole.

Munchgreeble wrote:
I'm not going to spend any more time illustrating that the foundation is bogus, but let me just tackle a couple of points that the argument goes to next.


Munchgreeble wrote:
1. You are keen to separate free travel speed and delta V as two separate concerns, yet when you reach your conclusion you suddenly forget this distinction and declare that a calculated delta v of 12mph indicates that the speed limit should be 12mph! As you yourself said in your last post, "average impact speeds are tiny in relation to free travelling speeds". In fact, since even the worst drivers brake before a collision this really is common sense - who in their right mind would set the speed limit based on the assumption that nobody uses the brakes??!! Do you see how ludicrous the argument sounds?


The 'ludicrous' argument is EXACTLY the argument that the authorities are using. The purpose of the page is to illustrate and quantify it's absurdity.

Munchgreeble wrote:
2. You again mix up the population mean with the population distribution. Supposing all the argumentation so far were in fact correct - the tying up of the population mean with a single point on the graph, the assumption that drivers never use breaks (and that that's how speed limits are set) etc. we now come to a startling revalation - dispite the fact that your graph in figure 2 shows an extreme bias in ratio of fatalities towards the top end of the graph, you seem to think that limiting the speed to the mean will have no effect. Let's go back to our example of the dice. What you're saying is, if we dissallow any rolls of the dice higher than the mean (3.5) that it will have no effect on the mean. That is, you reckon if we disallow 4, 5 and 6 to be thrown, then it won't lower the average from 3.5. Of course it will! If you had an average of 12mph that means some higher speed impacts and some lower speed impacts balance out at an average of 12mph. Getting rid of the stuff above 12mph (or 30 or whatever speed you want to pick) only leaves the stuff below 12mph - which certainly doesn't have an average of 12mph - or do you think that these imaginary people who never use their brakes always travel at a fixed speed of 12mph?


We're properly looking at real world average results. The facts are there.

Munchgreeble wrote:
If you present arguments like this to back up your case, you could easily be written off as "not serious" and trying to "fabricate" reasoning to back up your point of view. If you really do have hard scientific evidence that enforcing speed limits doesn't reduce road deaths then focus on that (I suspect you have other pages which cover this like I say I'm not a regular reader). Don't undermine your case by peddling arguments that sound persuasive to Joe public, but actually make no sense.


You're wrong in your claim that the argument makes no sense. There are limited opportunities to characterise the operation of our road safety systems in the real world. This page takes a useful view of real world data that illustrates the vital contribution of driver behaviour.

Munchgreeble wrote:
Now for some hard physics. As I suspect you know, the amount of energy in a moving vehicle is half of it's weight (mass) mutiplied by it's speed and multiplied by it's speed again (0.5mv^2). That is to say, if you're going say 50% faster (e.g. 30mph instead of 20mph) there is more than double the energy in your car. The amount of time you have to react to a given situation is cut by 50% and therefore the amount of time you have to take preventative action is cut still further. When you do react, you are less able to turn to avoid impact and have a much longer stopping distance. Finally, if and when you do have an impact, all these factors add up to making an impact speed way higher than the 50% difference you started ot with, causing a much worse accident than at the slower speed. In fact in a lot of cases, if you're just travelling slower your reactions, evasive action etc. will be enough to avoid an accident completely, that, at a higher speed would have been unavoidable. Clearly 50% is fairly extreme, but the same is true to a lesser extent with any speed difference.


And as this page shows, it's clearly not the speed limit that enables us to mitigate the kinetic energy of vehicle movement. It's drivers responding to hazards.

Munchgreeble wrote:
No matter how good your road sense, reactions, vehicle brakes etc. they don't get better the faster you go. You will almost always be able to handle a potential collision less easily than if you are going slower. The only time speed is an advantage is if you're facing a side impact or some similar where you need speed to get you out of the way in time.


That claim is absolute rubbish. Dsrivers must slow in response to hazards. Time to react is not a function of speed, but something that drivers routinely create by appropriate early response to hazards.

Munchgreeble wrote:
These are the kinds of basic issues you're going to have to tackle to convince people like me. If you have a good argument against such basic laws of physics then I'd be glad to hear it. Personally, since driver reactions don't increase with vehicle speed, and since ability to correct the vehicle's trajectory goes down as speed increases, I'll be very surprised if there is such an argument, but I'm willing to hear it if there is one. Just one plea, if you don't have a real argument to present, please don't try to dress it up with scientific sounding words when the basic argument doesn't hold water!


It holds excellent water, thanks.

Munchgreeble wrote:
I guess this must be hard to hear, but hopefully you'll end up with a more solid case to present at the end of the day. And for what it's worth, whilst that particular argument leaves me rather nonplussed, I like the approach you're taking of trying to get to the heart of what causes accidents and encouraging safe driving.


Spoken like a true troll.

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PostPosted: Tue Nov 22, 2005 23:13 
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Munchgreeble wrote:
Now for some hard physics. As I suspect you know, the amount of energy in a moving vehicle is half of it's weight (mass) mutiplied by it's speed and multiplied by it's speed again (0.5mv^2). That is to say, if you're going say 50% faster (e.g. 30mph instead of 20mph) there is more than double the energy in your car. The amount of time you have to react to a given situation is cut by 50% and therefore the amount of time you have to take preventative action is cut still further. When you do react, you are less able to turn to avoid impact and have a much longer stopping distance. Finally, if and when you do have an impact, all these factors add up to making an impact speed way higher than the 50% difference you started ot with, causing a much worse accident than at the slower speed. In fact in a lot of cases, if you're just travelling slower your reactions, evasive action etc. will be enough to avoid an accident completely, that, at a higher speed would have been unavoidable. Clearly 50% is fairly extreme, but the same is true to a lesser extent with any speed difference.


That old argument is invalid - for the simple reason that it completely ignores another bit of hard physics.
A moving body has a particular position at any particular point in time, and you cannot change its speed without also either changing either its position at the particular time, the time at which its at a particular position, or both.
Putting it bluntly, if you collide with a hazard which invades your roadspace, the collision only occurs because you happened to be at a particular place (within your stopping distance of the hazard) at a particular time. (the time the hazard appears in the road).
At that particular time, you would be somewhere else entirely if your speed was different - either higher or lower.
Whether or not you collide with the hazard, or how hard you hit should you collide, depends on a large number of mostly random factors.

The risk you have of colliding with such a hazard is proportional to exposure. It can be shown that the risk at 30mph is only about 20% higher than at 20mph. I can provide proof of this if you like, but I thought you might like to work it out for yourself. You might start to see things a bit differently.

Besides all that, drivers usually slow down for hazardous situations anyway, so their free travelling speed away from such situations is almost completely immaterial.

BTW Try viewing Paul's web page from a different angle - it may just start making sense to you.

Cheers
Peter

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 Post subject: Troll?
PostPosted: Wed Nov 23, 2005 22:41 
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Hi There,

I'm sorry you felt I was trolling. Really - I'm just trying to help you. Thanks Pete317 for the PM =)

I'm honestly really busy at home and work and don't spend time going through this stuff lightly - if it hasn't helped well never mind!

Feel free to revoke my account, I don't think there's any point in me visiting here again.

Like I said before, all the best with your endeavours

Regards

Munch


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PostPosted: Sat Feb 13, 2010 18:05 
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SafeSpeed wrote:
Munchgreeble wrote:
1. You are keen to separate free travel speed and delta V as two separate concerns, yet when you reach your conclusion you suddenly forget this distinction and declare that a calculated delta v of 12mph indicates that the speed limit should be 12mph! As you yourself said in your last post, "average impact speeds are tiny in relation to free travelling speeds". In fact, since even the worst drivers brake before a collision this really is common sense - who in their right mind would set the speed limit based on the assumption that nobody uses the brakes??!! Do you see how ludicrous the argument sounds?


The 'ludicrous' argument is EXACTLY the argument that the authorities are using. The purpose of the page is to illustrate and quantify it's absurdity.


Munchgreeble is perfectly correct. There is a glaring error on this 12mph page which Safespeed has not acknowledged. Safespeed says, on the 12mph page, that the average change in speed of an injury accident is 21.8mph (i.e. not equal to the speed limit, but a fraction of it). He then goes on to say that if this average change in speed of injury accidents was actually 30mph, rather than 21.8mph, there would be more deaths. He then somehow assumes that if the speed limit was 30mph, this would mean that the average change in speed of injury accidents would somehow also become 30mph, forgetting that it is not the same as the speed limit, but is a fraction of it. Safespeed seems not to have understood this point which Munchgreeble was making, instead thinking Munchgreeble was saying that the authorities assume people do not use their brakes. This is not the point Munchgreeble was making.

To reiterate, there is a mistake on the 12mph page, a gap in the argument, which arises here:

Quote:
We've calculated that at an average impact speed of 30 mph, 4,113 drivers in injury accidents would die.

This is about 4 times the number who do die, so we can see clearly that the average impact speed is significantly less than 30 mph.

So we could reduce the speed limit to 30 mph over the entire country, enforce it rigidly and still kill 4 times more car drivers than we do at present.


Safespeed seems to think that an average impact speed of 30mph would result from a speed limit of 30mph, even though he has explained on the same page that the average impact speed is smaller than the speed limit, not the same as it. He talks about the number of people who would die if the average impact speed was 30mph, and then tries to claim that is the number of people would die if the speed limit was 30mph, forgetting that, as he has pointed out, the average impact speed is lower than the speed limit. If you have forgotten where he said that the average impact speed is lower than the speed limit, here is one of the places where he says it, on the 12mph page:

Quote:
in the vast majority of crash situations the "delta V" is significantly less than the pre accident speed of the vehicles involved.


The pre-accident speed, presumably, would be closer to the speed limit. The delta V, or change in speed, is a lot lower. So the delta V of a 30mph speed limit would NOT be 30mph, which Safespeed's argument relies on it being - that is what he is basing his claim on that a 30mph speed limit would cause more deaths than we currently see with a higher speed limit. His argument is based on a mistake. In light of this I would request the removal of the 12mph from the site, since its headline claims are based on a mistake.


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