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Professor Benjamin Heydecker does it AGAIN!
Concerning the DfT document: "The national safety camera programme: Three-year
evaluation report"

 
Introduction

In the recently published DfT report: "The national safety camera programme: Three-year evaluation report" (click here), claims are made for the effectiveness of speed cameras. But like last year's report, major error sources remain unaccounted for.

We decided to write an open letter to Professor Ben Heydecker, apparently the lead author of the report.

Our open letter
 
 
21st June 2004
Professor Ben Heydecker
Centre for Transport Studies
University College London
Gower Street
London WC1E 6BT
 

Dear Professor Heydecker, 

Regression to the mean benefit illusion

I refer to the recently published document “The national safety camera programme: Three-year evaluation report”, of which you are apparently lead author.

You have done it again. You have allowed a large and unspecified “regression to the mean” benefit illusion to remain incorporated in the report's headline conclusions.

You know very well that the magnitude of the benefit illusion could quite possibly be larger than the entire benefit claimed for speed cameras. But that has not stopped you from making unsupportable and unscientific claims.

The report contains the words: “Because of this requirement to identify the sites as suitable for this particular safety measure, and in particular that the collision record was not the sole criterion for selection, the established statistical phenomenon of regression to the mean (also known as bias by selection) will not apply in full measure.”

But the rules for speed camera placement contain the words: “There has been a site survey by a road safety engineer and there are no other obvious, practical measures to improve road safety along this stretch of road”. I expect you appreciate that this rule strongly encourages the selection of random sites, as opposed to genuine black spot sites, and may considerably exacerbate the already large regression to the mean benefit illusion.

I remind you of your words on BBC Radio Four in the programme “More or Less” broadcast on 29th January 2004, discussing last year's report, when the following exchange occurred:

Interviewer: “So it sounds as if you were clearly aware of this problem [regression to the mean benefit illusion], but in the numbers you produced there's no adjustment for it is there? So the 36% reduction in accidents – isn't it rather likely that some of that would have occurred simply as a result of random behaviour of accidents?”

Heydecker: “Well, that is certainly possible, but we did not attempt to make any correction for that at the sites – there is a published correction that is available, but in fact it is known to reduce the accuracy of the estimate because the correction is less accurate than the original figure.”

You know very well that there are a number of ways that you could have estimated the magnitude of the regression to the mean error. For example, you could have used sites that did not receive cameras as a control group.

There are other peculiarities in the report, but the regression to the mean benefit illusion is likely to be so large that other errors are barely worthy of consideration.

You are responsible for misinforming the public and the government about a matter of public safety, and it is likely that this misinformation will result directly in loss of life by misdirecting road safety efforts. You are also bringing University College of London into disrepute. You must resign and you must issue a public statement apologising for the error and withdrawing the report.

Yours sincerely
 
 
 
 

Paul Smith

CC:  Tim Yeo MP, John Thurso MP, Alistair Darling MP, Lord Young of Graffham (UCL), Professor Malcolm Grant (UCL), Professor Mervyn Stone (UCL), Professor John Adams (UCL), Jon Moynihan (PA Consulting), Bruce Tindale (PA Consulting), John Brignall (Numberwatch), Press and
http://www.safespeed.org.uk/heydecker2.html [this page]

We also sent this new letter to the press. See (PR126)

Background

It is most important to understand the "regression to the mean" error. See this (page).

The new report is similar to a 2003 report. The same errors are incorporated. 

We know full well that Professor Heydecker is aware of the regression to the mean error because he spoke about it, (and admitted the lack of compensation or correction) on Radio 4 on 29th January 2004. (click here)

Indeed we wrote to him on the 10th September 2003 last year and asked the big question: "Do you actually deny that an uncompensated site level regression-to-the-mean benefit illusion exists within the “35% reduction at speed camera sites” headline claim of your report?" We never had any reply. See (here)

And now He's done it again!

Support for our analysis

Professor John Brignall's Numberwatch site (click here)

More about the new report

To follow...

Comments

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Copyright © SafeSpeed 2004
Created 21/06/2004. Last update 23/06/2004
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