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 Post subject: Rules
PostPosted: Sun Mar 07, 2004 19:03 
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This topic is for comments on the Safe Speed page:

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

The page reveals a blunder in the rules for speed camera placement.

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PostPosted: Wed Feb 09, 2005 19:56 
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So... Darling's had a year to consider Paul's reasoned, if somewhat forceful missive.

Is it time to send a chase with a copy of the one of a year last week for ease of reference? :twisted:


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PostPosted: Thu Apr 14, 2005 23:46 
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Well.. Colour me utterly gobsmacked! I know it's old news to you lot, but I'm relatively new to your site - and that of the ABD - between which I've been hopping and reading all day (if I get sacked, I shall blame it on you lot - or HMG)...

I know that politicians can be duplicitous or downright thick, but it would appear from your studies that the rules for camera placement are exactly 180 degrees out of phase... I don't feel so now bad about having been nicked by a not-exactly-obvious Gatso somewhere in mid-Wales a year or two ago.. I guess I was travelling at almost exactly the 85th percentile speed.. :)

But... As we all know, "Nanny knows best".


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PostPosted: Fri Apr 29, 2005 13:25 
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It's a nice theory, but I don't think it works. Cameras don't need to go where speeding is safe, nor where speeding is obviously dangerous, but where speeding appears to be safe but is actually dangerous.

That is exactly the situation where a road has a history of serious speed-related accidents but the 85th percentile drivers are going too fast, indicating that their normally-good judgement is letting them down at the location concerned.

In the sort of scene shown in your first photograph there may be a hidden hazard, which even the best drivers cannot react to, because they don't know it's there. However, good drivers are likely to slow down where there are cameras.

In the nice little village in the second picture, only a lunatic would speed, because the almost certain result would be a nasty accident. If that lunatic is not deterred by the prospect of killing someone, paying their policy excess, losing their no-claims bonus, and landing in court for dangerous driving, they're hardly likely to worry about a ticket from a camera. The most a camera could do in that location is add another £40 to the punishment, and that's not what cameras are supposed to be for.

So I think the rule might be right after all.

(And no, I don't work for the ministry...)


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PostPosted: Fri Apr 29, 2005 13:38 
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Quote:
In the sort of scene shown in your first photograph there may be a hidden hazard, which even the best drivers cannot react to, because they don't know it's there. However, good drivers are likely to slow down where there are cameras.


BUT - if there is a hidden hazard, the best solution by far is to abate the hazard with road engineering, or as a close second best, to draw attention to it, not with a camera, but with, eg, a "SLOW" sign painted in the road. This heightens awareness to good and less good drivers, without the attendant distracts of the cameras.

Welcome along, by the way.


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PostPosted: Fri Apr 29, 2005 17:24 
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KJP wrote:
That is exactly the situation where a road has a history of serious speed-related accidents but the 85th percentile drivers are going too fast, indicating that their normally-good judgement is letting them down at the location concerned.


The rules state that there have had to be x number of accidents in x number of years, and say nothing about any of them having to be speed-related. So they have a spate of accidents on an otherwise safe road, caused by freak weather conditions or something, and that justifies them putting up a camera.

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In the sort of scene shown in your first photograph there may be a hidden hazard, which even the best drivers cannot react to, because they don't know it's there. However, good drivers are likely to slow down where there are cameras.


Speed limits used to be a useful indicator of hidden hazards and the like. But with the wholesale misuse of speed limits in recent years, that's sadly no longer the case. What is the point, for example, of lowering speed limits on safe, rural dual-carriageways with no accident history to 50mph, or even 40mph? There are plenty examples of this.
In any case, if there are hidden hazards which can't be engineered out then surely the right thing to do is slow people down - by way of warning signs or whatever - rather than simply fining them for going too fast.

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The most a camera could do in that location is add another £40 to the punishment, and that's not what cameras are supposed to be for.


But that's the most any camera ever does - just doles out £60 (not £40 btw) fines plus points.

Cheers
Peter


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PostPosted: Sun May 15, 2005 12:55 
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Pete317 wrote:
The rules state that there have had to be x number of accidents in x number of years, and say nothing about any of them having to be speed-related. So they have a spate of accidents on an otherwise safe road, caused by freak weather conditions or something, and that justifies them putting up a camera.


The accidents also don't have to be particularly near the camera spot; they can be 3 miles away on a different road.


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 Post subject:
PostPosted: Wed Apr 02, 2008 02:52 
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I believe there is a mistake on the images on the page. The "safest drivers" should be a line or narrow bar on the 85th percentile, not a bar between the 85th percentile and "too fast". Given that a normal distribution is being modelled the "most drivers", ie about two-thirds bar should only be one standard deviation from the 50th percentile both ways. Consider "safe drivers", "safest drivers", "too fast" and "too slow" to help elaborate the point.



Image


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 Post subject: Re: Rules
PostPosted: Sun Aug 24, 2008 14:32 
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Humm OK well I will look into this and check - I kinda doubt that you are right as Paul was extremely careful and thought about things to such a great extent that it would be very unusual to find an error of this kind, but I will get it double checked.
If it is wrong it will be changed.

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 Post subject: Re: Rules
PostPosted: Sun Aug 24, 2008 15:36 
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As far as I can see, Pauls' analysis that the safest speeds are those at the 85th to 90th percentile is correct as per:

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

I am happy to be proved wrong by stats. boffins.

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 Post subject: Re: Rules
PostPosted: Sun Aug 24, 2008 17:50 
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Nicycle.. Different safest drivers have, in a given situation, a different optimum speed. That optimum speed is usually (but not exclusively) between the 85th and 90th percentile. There are some very safe drivers a tad faster (but only a tad) and some very safe drivers who are slower. However, those who modulate according to the speed limit (when it exceeds the 85th or 90th percentile are very dangerous, and those who do so when the speed limit is below the 85th percentile can fall into either safe or dangerous.

I am convinced that those who would, were it not for the speed limit, be driving at or around 85th to 90th perc entile are not as safe being confined to lolly speed - but that is of course a difficult one to prove.


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