Safe Speed Forums

& The Validity Of Speed Camera Sites.....

As some of you may be aware of the 85th Percentile Regulations I wont go in to too much detail, but as a brief intro - the 85th percentile is the percentage of drivers (85%) driving at or below the set speed limit. This is used to determine whether a proposed speed camera site is legitimate.

Ie.

If a proposed site has an existing speed limit of 30MPH and the 85th percentile is 36MPH then the site is valid (10% +2 MPH Over Limit {Applies To WEST YORKSHIRE, Not Sure About Other Regions}) If, however the 85th Percentile is LOWER than 36MPH the site is invalid.

Furthermore, there must be VALID & RECORDED information made publically available to prove this. This information can be found here:

http://www.safetycameraswestyorkshire.co.uk

On Further investigation you will be led to this link:

http://www.safetycameraswestyorkshire.c ... ations.htm

Once a location is selected, click "Show Statistics" and you will be directed to a page showing all recorded statistics for that road - which in itself proves to be very interesting reading.

As you may notice, there are alot of sites in West Yorkshire with a recorded 85th Percentile LOWER than 36 MPH - Which Renders the site INVALID!!! - Some of these sites are Batley Road, Wakefield Road etc.

Please post your thoughts regarding this post. We may be able to push for some proper consideration of the validity of sites within West Yorkshire.

ReFlexx

Please do. I really would like to find out about them and you seem so knowledgeable.

I dont know if that previous post was sarcastic, but the safespeed site has its own page describing the 85th percentile.

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

The information in my first post was written in light of recent allegations within west yorkshire regarding the placement of our cameras.

reflexx

That should be the other way around surely?

If the 85% is above 36 then the speed limit is too low so a camera set for 30 is invalid.

Hello by the way.

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Driving fast is for a particular time and place, I can do it I just only do it occasionally because I am a gentleman.
- James May

That's the way it works, although as Paul points out on this page it's the wrong way round:

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

i.e., cameras should not be placed where speeding is commonplace and relatively safe, but (if anywhere) where it is rare and relatively dangerous.

By the way, the rules on speed camera placement are merely guidelines so failure to adhere to them on the part of the authorities does not invalidate any prosecutions.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Correct Peter, I wasn't stating that it would invalidate prosecutions, rather that it is very strong ground for a complaint on the placement.

They are guidelines, but if there is insufficient evidence regarding the 85th percentile, there is enough grounds to have the placement re-assessed or even taken down.

There is a case currently under dispute on Horbury road, where the guidelines were not met.

This is one of many problems there is with speed cameras that the police don't want us to know about!

reflexx

Toltec :

The 85th percentile is 85% of people travelling AT or Below the highest permitted speed before prosecution - (Speed Limit + 10% + 2)

Therefore 15% of people are travelling over the Limit.

This justification is stupid because, in my eyes 85% of motorists arent daft, if 85% of people find it safe to drive at 36mph in a 30, that stretch of road must be pretty clear and safe. However, in a very built up area where the 85th percentile might be 27mph in a 30mph where a speed camera would make a great difference, outside a school for example.

reflexx

I need to refresh my brain here.

I thought that the 85th percentile defined simply the value (in this case speed) at which 85 cars in every hundred travelled at or below, with the other 15% travelling above it. So, for a sample of, say, 10,000 cars, the top 1,500 werer ruled out and the 85th percentile was the fastest recorded of the remainder.

This is the speed of travel - and nothing to do with the speed limit per se - although historically this value (85th percentile, or sometimes ninetieth percentile) was then rounded to the nearest multiple of ten and posted as an apprporiate limit.

What does this have to do with enforcement, or has YSCP abused the phrase and done something different?

The 85th percentile is as Roger says 85% travel at or below it while 15% travel at that speed or above it. Statistically those who travel at the 85th %ile have fewest accidents.
Nothing to do with enforcement. I assume the Speedcam Partnership are just publishing stats. If in one case the mean speed is 34 and the 85th %ile is 39 for a 30 limit then at least 50% are both able to and consider it safe to travel above the speed limit. Maybe in this case about 64% break the speed limit. And 39% above 10% +2. Why ? It is clearly appears safe to do so. Many motorists are not daft.

Why is it on the site ? the partnership can see that that represents easy money pinging them.

taking issue over the camera outside the school we want motorists to be looking out of the windscreen here not at the speedo

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Richard Ceen
We live in a time where emotions and feelings count far more than the truth, and there is a vast ignorance of science (James Lovelock 2005)

The 85th percentile of traffic speed has traditionally been used as an excellent starting point for speed limit determination.

Under the now defunct speed camera 'cost recovery' programme, one of the rules for speed camera placement was that the 85th percentile of traffic speed should be above the ACPO prosecution threshold at sites approved for speed cameras. This rule was 'inverted' as Peter mentioned above.

With the end of the 'cost recovery' programme the rulebook has become a weak guideline with no force in law or anything else.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

No "rules" apply to the Lancashire Pratnership for Road Safety. My local GATSO (3/4 mile away) was set up in 2004 - 1 KSI in the previous 3 years, 85th percentile speed 32mph - it's loaded 4 weeks out of 6

EVEN Callaghan admitted on the defunct CSCP forum HE wouldnt have put up that GATSO

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That's how Nazi Germany started. They'll be burning books next. (Brian Noble, Wigan coach - updated 20/4/06!!).

Thanks for all your replies.

As for other regions I cannot comment, however I know 100% fact that the 85th percentile guidlines are grounds for camera dispute and are being disputed as we speak.

I believe on a whole that the 85th percentile guidelines are merely an indicator of potential profitable areas to place speed cameras, however it is good grounds to base a complaint.

These statistics are not publically broadcast and it was only after being directed to the site by a friend I came accross the information. Upon further research I think I may dig up more answers but for now, the information alone is enough to prompt further questions.

reflexx

Sorry if this seems pedantic but the order in which you expressed that correlation could be taken to imply an assertion of a cause and effect relationship (which I hope you didn't intend) that is not valid. In particular, it can lead to the wholly erroneous conclusion that it is somehow 'safer' to drive at the 85%ile speed (than at speeds above or below). I have seen that conclusion stated on many occasions and it is completely false.

Better to say "The drivers who have the fewest accidents generally drive at or around the 85%ile speed."

Expressed much better observer thanks

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Richard Ceen
We live in a time where emotions and feelings count far more than the truth, and there is a vast ignorance of science (James Lovelock 2005)