Safe Speed Forums

Here!

http://www.dft.gov.uk/stellent/groups/d ... 10815.hcsp

The conclusions are interesting in that they rely on the headline reductions much as seen before (40% reduction in KSI) yet it mentions RTM but not the level estimated effect. P60 is the conclusion of the



it goes on about how the sample isn't representative, but it if were.. blah..



Hmmm.... we know that many partnerships use the £1.4m fatality figure to justify "cost savings" in their areas.

Apprendix H (P141) deals with the detail of the RTM calculations - this conclusion is significant, yet mentioned as "some" in the conclusions (!)


THREE FIFTHS! 60%!! of the reduction!

Gareth

_________________
Shooting is good for you and too good for some people.

Let me check my understanding here

RTTM has accounted for 3/5ths of the reduction

'Trends' have accounted for 1/5th

so the best reduction that the cameras could have had is 1/5th of the total?

I would like to see similar figure for devices such as speed actuated signs.

The conclusion that cameras have reduced KSI's is correct based on the figures but the reduction is at best 1/5th of the total.

I'd like to see the RTTM calcs of course it will all depend on how long before the installation of cameras the 'mean' is calculated from. I would argue that it has to be since the road has been in its present condition i.e trunk road or estate road etc etc this could mean 50 years or more.

In Yorkshire there was an apparant 100% reduction in people in excess of 15mph over the speed limit. This sounds highly unlikely having watched the criminals being chased on the television on Traffic Cops!

Did they just stop counting anydody 15mph+ over the limit?

The RTTM stuff is buried in gobbledygook, I suspect deliberately

They do say (in Appendix H)...

(my bold)

But there is no mention of this in the headline figures or the main body of the report.

edited

The public and meadia has largely been defrauded over RTTM. Accordingly Safe Speed issued the following PR at 7:56 this morning:

PR268: Camera report defrauds media and public

news: for immediate release

The Department for Transport (DfT) "4th year" report issued yesterday is
worded in such a way to have widely misled media and public alike about the
true effects of speed cameras.

The important headline figure of '42% reduction in killed and seriously
injured' contain a gross statistical bias - yet has already been very widely
quoted as if it were the true benefit of speed camera operation.

The gross statistical bias is called 'regression to the mean' (RTTM or RTM).
It is a large effect. Although the report attempts to suggest that it cannot
be accurately estimated on available data, two self similar estimates are
included. (Table 4.9 and appendix H)

According to table 4.9:

0.36 RTTM + 0.11 benefit = 0.47 total. Applying this to the 42% headline
conclusion suggests:

42% reduction in KSI at speed camera sites is comprised of:
32% RTTM benefit illusion and
10% camera benefit

According to appendix H

* three fifths RTTM
* one fifth trend
* one fifth benefit

Since the trend has already been calculated in the 42% headline figure, this
becomes:

* three quarters RTTM
* one quarter benefit

This would suggest:

42% reduction in KSI at speed camera sites is comprised of:
31.5% RTTM benefit illusion and
10.5% camera benefit

Paul Smith, founder of the Safe Speed road safety campaign
(www.safespeed.org.uk) said: "Regression to the mean effect is acknowledged as
providing a gross distortion of the benefits claimed. It is no surprise to me
that the report's authors make many excuses for failing to provide a
substantial and accurate estimate of the effect because clearly it will reduce
the benefit claimed by about a factor of four. The benefit of cameras
expressed in KSI isn't 42% at all - it's about 10%."

"This effect means that if we had installed garden gnomes at the roadside
instead of speed cameras they would have been about three quarters as
effective. We could have had headlines stating that 'Gnomes cut fatal and
serious crashes by 32%'"

"I would be proud to be associated with a road safety initiative that cut
fatal and serious crashes by even 10%. However speed cameras and supporting
policy have many negative side effects on wider road safety which
comprehensively swamp the benefits. The net effect of speed cameras has been
to make our roads much more dangerous."

"I am disgusted that the executive summary of the report fails to make this
adequately clear. Newspapers and broadcasters have presented the 42% figure as
if it were truly the benefit of speed cameras. Clearly it is not and the
public is being misled."

RTTM explained:

Regression to the mean arises because we like to apply a safety treatment - in
this case a speed camera - to the worst places. This means we tend to place
cameras in places where crashes are peaking. After the camera has been
installed the 'peak' conditions pass and the crash rate at the site tends to
return to its long term average.

It should be obvious that many of these peaks arise through random chance and
random peaks always pass.

<ends>


Notes for editors
=================

I urge you to bring this vital information to the attention of the public. I
urge you to consider how it has been be possible for this issue to have been
largely glossed over. I urge you to consider who is responsible, and what
their motivation has been.

My congratulations to the few have already identified and reported the problem.

RTTM primer:
http://www.safespeed.org.uk/rttm.html

Yesterday's DfT report:
http://www.dft.gov.uk/stellent/groups/d ... 610816.pdf

The Times recognises the problem:
http://www.timesonline.co.uk/article/0, ... 85,00.html
(We don't agree with the figures the Times used.)

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

My input. I won’t even bother with the details of RTTM

Something obviously stinks when reading the very first words in the report: PA Consulting have been charged with handling the speed camera rollout. This report on camera effectiveness is written by PC Consulting. Now tell me there won’t be an element of conflict of interest! BTW, Linda Mountain’s unedited press release clearly shows she is biased in support for speed cameras.


Roads with recent speed limit reductions are sometimes accompanied by speed cameras, so that’s not really a shock.


Only one conclusion can be drawn from that – cease mobile enforcement, fixed cameras are so much more effective!


WTF? “probably”? Don’t they know? Did they do any work determining the effects of RTTM at all? This statement alone shows what a crock of sh*te this report really is.


That’s a lot of people (43 partnerships, some with over 100 employees), all with substantial funding. Who has been commissioned to communicate the dangers of bad driving (let alone the safety of excess speed)? No-one. SafeSpeed is the closest, no, the only source we have for hearing anything different.


How loaded is that statement? Of course they should be used for casualty reduction, but we all know they’re not. I suspect a very different response would be achieved if the question was ‘are safety cameras used as a method of reducing casualties?’

Funny how the Dodgyscope isn’t given a mention.


I can only assume that ‘bias on selection’ effects haven’t even been mentioned, common tricks such as adding a pedestrian crossing/barrier or improving a junction at a camera site, but the camera gets all the glory.

So far I’ve only got to page 9 (like most people I’m sure), no need to go further. What a joke.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Another source of error which is overlooked is the normal inter-annual variation. This could account for much more than the 'remaining' 10%.

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler

DFT says :


I think that about sums it up

Who says the remaining 8%/10% is due to the Cameras.

All they have shown so far is that 3/5ths of the reduction is due to RTTM, 1/5th to long term trend and 1/5 th to other factors. Given that where there has been a high rate of accedents other measure may have been taken at the same time (eg improved lighting, improved signage, road engineering, a reduction in the speed limit, etc). To say the reduction is due to a speed camera just because it is there is highly dubious but no more than we should suspect unless there was a proper control comparison with sites with similar KSI's but no camera's.

Some of our newer visitors may appreciate the following Safe Speed page, published in 2002: http://www.safespeed.org.uk/gambling.html

When I first wrote that page, I'd figured out what they were up to, but I'd never heard of RTTM.

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

Thx for this Paul.

Would this make a good line of attack when this story is picked up by the local weekly papers, especially if a there is a local site which has both has a camera installed and other improvements were made at the same time. A local journalist could then easily create an article on the lines of 'What's caused the rediction, the Camera or the new junction' to discredit Scamera claims.

True, the rest could be entirely down to traffic diverting to another route.

In fact the report I found on the HOV lane in Leeds suggested an initial 20% and long term 10% of traffic diverted. I did not see anything in the (4 year) report to suggest they had monitored traffic levels before and after.

I don't know, all you cynics... you should listen to the people who know what they're talking about.

edited

Paul - there are a couple of broken links (to dft pages) on that page

Hmmm. I don't see the results for Staffordshire in the list in the PIC /Fatals tables. Are other pratnerships missing? The data is mentioned elsewhere in that they said they used it but it is not in these summary tables. They were a pratnership from 2000. Conspiracy or incompetance?

Staffordshire appears to be misfiled under 'South and mid Wales' in table 3.9.

It also shows an increase in KSI at camera sites. (Along with Avon and Somerset, Cheshire, Fife and Grampian.)

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

Staffordshire residents are used to being "misfiled"!

However the increase in KSIs is interesting - Stafforshire is very heavily "camera'ed" - is there any pattern as regarding the figues against density of cameras?