Transport Scotland is to change the way it reports casualty data at camera sites after an independent review highlighted “major errors” with the current approach (Local Transport Today).
Jennifer McCahill, assistant statistician to the Scottish Safety Camera Programme, said: “The forthcoming bulletin will incorporate a section where a sample of sites will be selected and the effects of trend and regression to the mean will be allowed for when trying to estimate the effect that safety cameras have had on road traffic accidents.”
Professor Maher’’s report says that between 2001 and 2011 the number of fatal and serious collisions across Scotland fell by an average of 4% a year and the number of all injury collisions by 3%. He adds that “there seems no reason to doubt that the same trend would have occurred at camera sites…therefore any estimate of the effect of cameras should take account of trend”.
On regression to the mean, his report said: “When the decision to install a camera is based on high numbers of collisions in the preceding period (typically three years) and this same period is used to provide the ‘before’ data in a before-after comparison, there is the danger of regression to the mean.”
Maher recommends that Transport Scotland uses a statistical method known as Empirical Bayes to identify the regression to the mean effect. This method was used by researchers in the four-year evaluation report of safety cameras for the DfT in 2005.
Citing the 2005 report, Professor Maher says the researchers suggested that for fatal and serious collisions, the 54% overall reduction observed at camera sites could be attributed to 10% trend, 34% regression to the mean, and 10% to the cameras.
Idris Francis, an independent campaigner who runs a website called ‘fight back with facts’, said: "Transport Scotland’s figures completely ignored trend, which over 10 years alone accounted for a 42% fall in Scotland KSI. The standard method is to measure KSI over three years, then install a camera and measure KSI again over the next three years – and adjust for trend, which is easy, but also for regression to the mean, which is difficult. They ignored both.
"They also ignored, as many previous analyses have done, that virtually all of the observed falls happened in the gap year between site selection and camera installation, which is absolutely critical."
I analysed the fatality stats from 1950 to 2009. This showed that due to the dependence on speed cameras, in preference to other more effective road safety measures, there were many extra deaths directly related to the number of speeding penalties, with a 98% correlation, in comparison to pre-speed-camera trends.
Dr G Luxford
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Eric, you hit the nail on the head with your law enforcement comment. Speed cameras only ever came about because those involved in law enforcement saw nearly everyone ignoring limits most of the time and they wanted to catch those people.
Nick Elmslie, New Milton
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Hugh and others – please read the annual DfT “Reported Road Accidents GB” which confirm that all fatal accidents are made known to the police but only about 28% of SI. No figures for Slight but unlikely to be higher.
How can cameras be “an efficient law enforcement tool” when they cover only 1% of rural roads, 3% of urban and perhaps 1.5% overall – at a cost of £30k – £50k pa each?
Idris Francis Fight Back With Facts Petersfield
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Hugh says:
“The cameras remain an efficient law enforcement tool”
Firstly, perhaps he would explain by what criteria he comes to that conclusion as, at best, cameras encourage lawful drivers to drive at, or below, the prevailing speed limit (plus 10% or so). They are likely to be ignored by unlawful drivers (drink/drug, stolen, cloned, etc). Secondly, this is a Road Safety website, not a law enforcement website. It is inconceivable that a camera on a pole can prevent a collision or casualty, while it is known that the same camera has contributed to deaths. Enforcing the law while damaging road safety is a flawed argument.
Eric Bridgstock, Independent Road Safety Research, St Albans
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This another good news story. When the first “Key Scottish safety camera programme statistics” were published, I emailed and spoke to both the author and the Office of the Chief Statistician pointing out that their report was seriously misleading. I requested that a simple statement precede the report to explain that the report contained no evidence of what effect their speed cameras may have had and that no work had been done to determine the effects of their speed cameras. Both bodies refused my request.
A couple of years on it seems that a group of people were far more determined than I was and have finally started the process I failed to achieve. The first step in solving a problem is recognising the problem exists!
Eventually, speed cameras will have to be run in “Randomised Controlled Trials”. This is the only way.
Dave Finney, Slough
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As it would appear that reported accidents only account for about 10% of actual accidents at a location, is there any point still trying to assess the efficacy of site-specific measures anymore? Do before and after comparisons carry any weight, if either or both figures are wildly out?
This does not only apply to speed cameras, but any intervention and would include VASs, signing and marking schemes, and even the more substantial engineering works designed to improve a road/junction with a poor accident record.
As Eric says, we have to use our common sense, zoom out from site-specific analysis based on possibly unreliable stats and consider the wider picture. The cameras remain an efficient law enforcement tool, the wider benefits of which can’t really be measured. They happen to be fixed, so the tendency is to focus too much on their effect – if any – in their immediate vicinity.
Hugh Jones, Cheshire
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Nick
My assertion is based not just on this story alone, but on nearly six years of assessment of claims for cameras, challenges to authorities and vested interests, and the application of safety engineering and common sense.
You will be covering more stories like this in the coming months.
Eric Bridgstock, Independent Road Safety Research, St Albans
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Eric
You are racing ahead here! In time, of course, you may be proved correct but your assertion that the ‘net effect of speed cameras on casualties is detrimental’ is a big leap forward from the facts reported in this story.
Nick Rawlings, editor, Road Safety GB newsfeed
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Well done to the determined campaigners who have brought this to light. As this story unfolds it will eventually be clear that there is no reason to call them “safety cameras” as they can provide no safety benefit. In fact, it will be evident that the net effect of speed cameras on casualties is detrimental.
Eric Bridgstock, Independent Road Safety Research, St Albans
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