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News extra: Issue 590 17 Feb 2012

New report asks: are speed camera effects on collisions real or illusory?

Speed cameras in the Thames Valley have had no impact on cutting injury collisions, according to a report that uses a new method to isolate the impact of cameras from other effects. But the area’s former road safety manager says the research is flawed.

Andrew Forster

Over the last 20 years speed cameras have become a familiar part of the roadside furniture in the UK. But what effect have they had on injury collisions? Assessments have been complicated by the need to isolate the effects of the cameras from wider trends in collisions and the statistical phenomenon of ‘regression to the mean’ (RTM).    

RTM relates to the fact that sites with high numbers of collisions in one time period would be expected to have fewer...

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Your Comments:

18 Feb 2012

Having studied road casualty data, trends and claims for thousands of hours over some 12 years I find Mr. Finney's analysis entirely credible. I am currently uodating my own analysis of 4.7m injury accidents from 1991 to 2007 to 6m from 1985 to 2007 since the earlier data became available and comparing the 3 years after installation noit only with the 3 years before but also with the 3 years before that, to highlight the extent to which selection bias skews the results and gives rise to the largely bogus claims of effectiveness we have seen for many years.

It is in any case clear that these claims have to be bogus because it is beyond rational dispute that speee cameras cannot lead to falls in accident numbers far greater than ever involve speeds above limits in the first place - 5% for slight, 6% for all, 9% for SI, 105 for KSI and 14% for K - approx. Even these figures are inherently overstated however because they include accidents where speeding "might have been" involved as well as those where it probably was. Further and in any case, in many of those accidents, speeding might only have been a minor or indeed irrelevant factor compared to the many other triggering factors. For all these reasons the potential benefits of cameras are relatively trivial at camera sites, (2% of our roads) and utterly trivial in terms of national figures.

And as Sherlock Holmes pointed out, once you have eliminated the impossible what it left must be the explanation. Including selection bias, drivers diverting to avoid cameras etc.

The whole scheme has been based from the beginning on wishful thinking, seriously flawed analysis, snake-oil salesmen promising cash flow, empire building and in far too many cases active dislike of motor vehicles.

Is Mr. Owen, in flatly denying selection bias prior to the DfT criteria (which he presumably accepts do introduce such bias) claiming that those earlier sites were chosen using a blindfold and a pin? Presumably (and we must hope) not - presumably they were selected on some basis and the only logical one would have been recent crash history - inevitably introducing the selection bias he seeks to deny, even if not exactly the same as if the DfT criteria had been used.

Only two months after Thames Valley cameras were switched off in 2010, Mr. Owen, then head of the then Partnership, issued reports and Press Release claiming a 200% or 300% increase in speeding at those sites. However closer examination of the figures in the context of likely traffic flow soon showed that this corresponded to compliance falling from 99.8% to 99.4% or so! I made a formal complain of Misconduct in Public Officet to Thames Valley Police at the time about this seriously misleading report, but they were not interested - of course.

As an enginer for 50 years, when my business and survival, let alone success, depended not only on being right most of the time but also - crucially - on admnitting that I was wrong when I was - I have been uttterly shocked over these past twelve years by the grotesque errors of fact and analysis have found in road safety analysis, particularly with respect to speed camera claims. To give only two examples - I proved that DfT claims that cameras were (marginally) more cost effective than vehicle activated signs were wrong by a factor of 50, and that ther Commons reply that daytime running lights would add 5% to fuel bills was a typing error - for 0.5%. All this and more is fully detailed in my web site

. albeit as yet far from complete.

21 Feb 2012

I must say that Andrew Forster has written a very good article.

He has taken the trouble to read my report, research other reports and speak to the authors. Speed cameras, and road safety generally, are difficult to evaluate due to the lack of scientific tests, poor standards of test design and difficulty in obtaining data but Mr Forster has made a very good job of explaining the evidence available to date.

Your efforts are appreciated, thank you.

ps, the report itself is a bit "heavy" so I wrote a brief opening page: