Safe Speed Forums

Update of Circular Roads 1/93, Setting Local Speed Limits

http://www.dft.gov.uk/stellent/groups/d ... 32880.hcsp

Not yet read, but will need to digest before commenting.

There's also an online questionnaire.

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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

After a quick scan of this, I am worried - there seems to be this new suggested reliance on the "mean" speed of traffic as the guide for speed limit setting, instead of the 85th percentile. This because, apparently,
(my bold).
So we're to move from a situation where 15% of drivers exceed the limit, to one where 50%, yes 50% of drivers exceed the limit - by definition! This is nothing short of utter madness. We must protest, and the sooner the better.

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Keep right on to the end of the road ...

You're spot on there. I'm in the process of preparing info for the press. This is news.

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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

Yes, from a quick read that seemed to be a particularly worrying aspect. It's widely recognised that the drivers who choose speed in the 80-90 decile are the safest, and they are to be criminalised.

Also that decisions on the appropriate speed limit for rural roads should depend not on the road configuration but on the accident rate, even if few or none of those accidents are speed-related.

And it's largely pointless if local authorities can still gleefully disregard even these watered-down recommendations, for example by applying 40 mph limits to totally undeveloped rural dual carriageways

They do acknowledge the oft-made point about the need for consistency in speed limit setting, but unfortunately it seems such consistency is only to be achieved by levelling down.

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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

I don't see how this change will make much difference in practice. If anything it might improve things.

Consider if, on a hypothetical road we measured the speed of 20 vehicles and got the following results:

25,26,27,28,28,28,30,30,30,30,31,33,36,37,38,40,45,49,52,60

We would have a mean speed of 35 (40% exceedance), a median speed of 30.5 (50 % exceedance), and an 85th percentile of 45 (15% exceedance). According to the document (section 38), this would result in a limit of 40 being set (20% exceedance), which would also be the limit set by using the 85th percentile under the existing guidelines.

When speed limits have been set using the 85th percentile they were always adjusted downwards to the next available enforceable limit. E.g. a 58mph 85th percentile would result in a 50mph limit, but under the proposed changes a 52mph mean speed would result in a 60mph limit as the limit should be above the mean speed, not below it (section 38 of the document).

In my opinion more thought and discussion is necessary before jumping to the conclusion that this change may lead to further constraints on the competent motorist. It may well be that this could be used to argue for limits to be reassessed upwards, and argue against limits being reduced.

This Safe Speed PR going out to over 700 journalists and editors right now:

PR149: Backdoor Government plan to ratchet down speed limits

NEWS: for immediate release

A recently published new draft of the Government's primary local speed
limit setting advice document contains some worrying changes.
Astonishingly speed limits will be set such that 50% of motorists will
be exceeding them.

Paul Smith, founder of the Safe Speed road safety campaign explains:
"All around the world and for many decades, speed limits have been set
in accordance with the speed of traffic at such a level that only the
fastest few should be prosecuted. Research evidence shows that this is
the safest and most effective way to set speed limits. There has been
some erosion of this principle over the last decade, but the new
document promises a far more restrictive regime. In future the
Government intends that speed limits should be set at the average
speed of traffic. This immediately means that half of all motorists
will be exceeding the new speed limits."

Paul continues: "The effect of this will be a gradual ratcheting down
of speed limits and far more motorists criminalised despite the fact
that they will be driving at a safe and appropriate speed according to
the conditions."

Paul explains the techincal details. "Speed limits have long been set
at the level not exceeded by 85% of motorists. This is called 'the
85th percentile rule'. The new proposals abandon the 85th percentile
rule and replace it with a 'mean speed rule'. This means that with a
typical speed limit defined under the new scheme, 50% of drivers will
be exceeding the speed limit by definition."

For over ten years now we have speed reducing speed limits and speed
cameras are spreading like a virus. The fact is this policy is
comprehensively failing to reduce road deaths, yet the Government
sticks stubbornly to its guns. They must be saying: 'The medicine
isn't working! Let's increase the dose!'

'Speed kills' road safety policy has become a dogma, encouraged by a
series of vested interests. It isn't working and must be abandoned.

Every rational person knows that 'The competent and careful actions
of a majority of responsible people should obviously be considered
legal.' Yet with speed limit laws, applied over-zealously we are
criminalising the vast majority - and the government proposes to make
it worse.

<ends>

Notes for editors

This is a technical subject, but the effects are enormous and
dangerous. Please try and find a way of explaining the changes to your
readers, listeners or viewers.


The new document:

http://www.dft.gov.uk/stellent/groups/d ... 032880.pdf

Contains:

Quote: "37. Circular 1/93 advised the use of 85th percentile speed to
determine local speed limits. This refers to the speed at, or below,
which 85 per cent of the traffic is travelling. Viewed another way it
is the speed only 15 percent of drivers exceed. Practitioners'
thinking has evolved since then and many have expressed concern that
85th percentile speed can be heavily influenced by excessive speeds
travelled by a minority of drivers. Some Traffic Authorities have
therefore adopted the use of Mean speeds in assessing what is an
appropriate local speed limit, as they are felt to better reflect what
the majority of drivers perceive as an appropriate speed for the road.
The Department shares this view and therefore recommends that mean
speeds be used in future assessments of appropriate speed limits."

Understanding speed limits and the 85th percentile rule:
http://www.safespeed.org.uk/speedlimits.html

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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

Yes, sorry, possibly I did jump to conclusions there, and you're right the median speed is that exceeded by 50% of drivers. However, I can't read the document without feeling that this new principle is INTENDED to encourage lower speed limits than heretofore - the passage I quoted in bold suggests the buzzing of a very large bee in someone's bonnet.

Did they let Transport 2000 write this for them?

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Keep right on to the end of the road ...

If the population is normally distributed then the mean and the median have the same value. Speed distibution isn't usually far from normal.

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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

True, and in a population of thousands (rather than tens), one or two lunatics won't skew the result very much.

Then again, perhaps we should ALL drive like lunatics to ensure the mean speed IS higher than before when the powers that be come to selecting their new limit

Good PR Paul, I hope the press pick it up and give it the exposure it deserves.

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Keep right on to the end of the road ...

A good thread and a good press release Paul.

This is my take on it:

Imagine a scenario: A section of road has for a long time been policed by a speed camera, lets use a SPECS camera for this exercise. For the whole of that time people have got used to it's presence and prosecutions have fallen to a negligible level. The accident rate however has not fallen so a review of the speed limit is carried out. Even using the 85 percentile rule the speed limit will be lowered. Using the latest suggested method it will go even lower. So as the use of speed cameras is proliferated it is inevitable that the limits will be ratcheted down, or am I missing something.

Max

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Tailgaters - Please Pass
You have an Accident to go to!

I would not be surprised if the 85th percentile speed under (say) a 40mph specs installation was 40 or 41mph. I'd expect to be somewhere around the 85th percentile speed myself and I tend to regulate my speed as shown on the speedo to a couple of miles per hour over the posted limit.

With mean speed, something entirely different would be found.

Mean speed is totally unsuitable for speed limit setting.

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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

See comments from "Owen" (who I believe is or has been a professional highway engineer) in this thread.

The implication is that, in most cases, setting a speed limit to be above the mean speed will produce the same result as looking at the 85th percentile speed and then rounding down if up to 7 mph above a 10 mph increment.

I must say I'm not at all convinced by this, and feel the government would not have made the change unless they believed it would lead to lower speed limits being set.

Also the comment in the report that "Practitioners' thinking has evolved since then and many have expressed concern that 85th percentile speed can be heavily influenced by excessive speeds travelled by a minority of drivers" is highly questionable. If we were talking about a 95th percentile speed this might be valid, but to suggest that a full 30% (taking those on either side of the 85th percentile) are travelling at inappropriately high speeds is ludicrous.

_________________
"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

I would agree - 'Practitioners' obviously believe the figures are scewed leading to an 'undesirably high' limit. This new measure is aimed at allowing them to set the limit much lower.

In all honesty, I don't think that 'practitioners' have been using the 85th percentile for a while now.

My council even include 'social' reasons in setting limits. How does that help to set the maximum speed that is safe for the road, which is what the speed limit is meant to reflect.

There are some A roads in Surrey that have had their limits changed 3-times in the last 3 years. NSL - 50 in plances - 50 everywhere - 40 in places - 40s are then extended and new 30s appear.

That means a driver could now face a Ban for travelling at a speed which was legal 3 years ago. How can a speed that was safe then not be safe now - traffic can only have grown 3%. They wont have sufficient accident data to back the reduction in such a small amount of time.

And while we're at it, it's about time the consultation process was changed. To errect an A5 notice in 10pt on a lamp post after the speed limit sign has been errected is hardly consultation.

In addition with Darling's new zero tollerance approach - should we not be seeing the limit rounded up such that the majority of people are legal not illegal.

If you replaced every rural 30, 40 and 50 with an NSL tommorow I really doubt speeds would increase.

I agree - the previous guidelines have been completely ignored by councils for some years, so what the government are doing is in effect legitimising what has gone on.

And while some warnings remain about the risk that setting inappropriately low limits will not of itself do much to reduce speeds, and may erode respect for limits in general, they are much watered down.

A couple of years ago I wrote a letter of complaint to a local council who had decided to reduce a one-mile stretch of rural NSL dual carriageway (with one or two sharpish bends) all the way to 40. They cheerfully admitted that they were free to drive a coach and horses through the official guidelines, and that this reduction was part of a "holistic" approach to reducing casualties. (Yes, I thought, you're a bunch of a**eholes )

At the same time as cutting the limit, they installed street lighting all along this stretch. No doubt accidents will have fallen, the real reason being the street lighting, but it will be claimed as a triumph for speed limit slashing

_________________
"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

Perhaps are focus should be getting councils to rewrite their speed management policy.

Almost all see speed as something that should be reduced and quote the flawed 'the faster we go the more accidents we haveand 'the slower we go the less likely we are to kill' B*ocks

I also like the way that many of the local transport plans that councils have contain targets for reducing speed limits. They then cheerfully say "in the last year we achieved 100% of our target of reducing 15 speed limits by 10 mph". How on earth does that contribute to road safety?

If anyone would like to give their feedback directly to the DfT. You can do so by emailing:

caroline.britt@dft.gsi.gov.uk

There is an on-line form, but her details are not too hard to extract from the source

Has anyone been able to get a copy of the TRL spreadsheet for speed limit calculation?

A picture of it is included in the briefing sheet.

http://www.dft.gov.uk/stellent/groups/d ... 032871.doc

It seems to take all sorts of social, environmental and noise factors into account.

It also seems to suggest that there is a road safety improvement through introducing lower limits.

Surely the role of the speed limit (as enforced by speed camera partnerships) is to identify the max. safe speed for the road. If we take all other factors in to account - surely it's no longer appropriate to enforce those that exceed the limit as criminals.

Found it

http://www.trl.co.uk/1024/mainpage.asp?page=57

I'm in the process of picking the model apart