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PostPosted: Sat Nov 30, 2013 00:12 
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DfT endorses new method to measure speed camera impacts
Allsop: revised methodology released this weekAllsop: revised methodology released this week

Photo:Andrew Forster

The DfT’s head of statistics has endorsed a new method for measuring the effect of speed cameras on road casualties, which aims to isolate the effect of the cameras from random variations.

The ‘four-time period’ method has been used by Slough-based engineer Dave Finney and Professor Richard Allsop of University College London in reports examining camera effects.
Finney explained his method in a report analysing data from mobile speed camera sites in the Thames Valley Safer Roads Partnership area (LTT 23 Nov 12). He separated the casualty data into four time periods:

• a pre-Site Selection Period when no cameras were present
• the Site Selection Period (SSP) – the period of time in which accident/casualty levels influenced the decision that a camera should be sited at the specific location
• ASBiC (After site selection but before installation of the camera)
• after camera installation

Finney ignored the SSP period data because this would contain the influence of selection bias – i.e. an abnormally high numbers of accidents (the ‘regression to the mean’ – RTM – effect).

He therefore combined casualty rates in the pre-SSP and ASBiC periods to determine the mean collision rate before the start of speed camera operations. This was then compared with the rate after camera installation.

Finney said the method proved “beyond reasonable doubt that the entire reduction in killed and serious injury casualties at mobile speed camera sites actually occurred due to ‘regression to the mean’, and not as a result of mobile camera operations”.

Allsop also used a four-time period method (though with a slightly different treatment of time periods) in a report advising citizens about how to interpret casualty data at speed camera sites, published by the RAC Foundation this summer (LTT 14 Jun). The report included analyses of data from fixed camera sites and concluded that cameras did have an observable effect on cutting personal injury collisions.

Tim Stamp, the DfT’s head of statistics profession, endorses the four-time period method in a letter to retired engineer and speed camera critic Idris Francis.
“The basic conceptual framework for taking account of regression to the mean (RTM) in safety camera analysis, as used by both Professor Allsop and Mr Finney, strikes me as straightforward and logical,” says Stamp.

Coincidentally, the RAC Foundation this week published a revised version of Allsop’s report. “The main change I have made is in the recommendation I make (to users of data who do not have knowledge of the SSPs for the cameras they are investigating) about the assumption they need to make about the SSP to apply my method,” Allsop told LTT.
“I recommended previously that they assume the SSP to be the last three calendar years before establishment of the camera. In the light of helpful comments and analysis by Professor Mike Maher [of the University of Leeds], fuller consideration of Dave Finney’s work and sets of recorded SSP for several partnership areas, I now recommend that they assume the SSP to be the first three of the last four calendar years before establishment of the camera.”

Allsop has reworked his calculations with the revised assumption. His estimate of the reduction in personal injury collisions attributable to cameras across the nine partnership areas studied (previously 15%) is now 14%, and the estimate for fatal and serious collisions (previously 27%) is now 22%.

Explaining that the changes bring his method closer to Finney’s, Allsop told LTT: “I stand by my assessment that my method largely allows for regression to the mean. I recognise that some of the reduction my method attributes to the effect of cameras may still stem from RTM, but I don’t believe that this can be more than a small proportion.”

In his letter to Idris Francis, Tim Stamp says a large-scale study of speed camera effects could be invaluable. “As site-level camera data continues to be made publicly available, it’s easy to see real value in a robust analysis that encompasses a wider range of camera sites and geographic areas, and that takes account of RTM and other relevant factors such as trend. You may well be interested in contributing to such an analysis.”

But Stamp adds: “It’s very clear that any results and conclusions will carry most weight and influence if they are presented in objective and dispassionate terms, with their methods, assumptions, calculations and underlying data all clearly described and made available for scrutiny.
“Publication following independent peer review would be by far the best way to ensure this outcome.”

Guidance on use of speed camera transparency data – updated November 2013 is available at
http://tinyurl.com/nnllbuj

Discuss speed restrictions and camera enforcement at LTT's 'Time for 20mph' Conference in February 2014
If RTTM cannot be agreed on, then the results cannot be relied on. At least Allsop recognises that there are different way to consider facts. His own report is hardly that supportive of speed cameras anyway!
Yet no one here at this point discusses the value of the economy and how that is influencing these figures and years too and this is hardly a small effect!

Edited to add link to the Prof Allsop Update of RAC Foundation Speed Camera Data Revised Nov 2013 :
http://www.racfoundation.org/assets/rac ... ov2013.pdf

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PostPosted: Sat Nov 30, 2013 10:51 
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There doesn't seem to be any correlation between change in collisions and average speed. Ergo the very method by which cameras are said to work clearly isn't the case...

Shame the report doesn't mention general reduction in injuries via safer cars. Or change in traffic levels.

What is needed is the data to be modelled against all of that as well as adding in the RTTM, SSB corrections.

One mentioned a reduction in PIC by 14% but it didn't say over how long that was. If it was more than several years then it would be unlikely to be down to anything other than change in traffic volumes or improving car safety or differing route choices.


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PostPosted: Wed Dec 04, 2013 03:23 
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The data has been thoroughly dissected by Idris, Eric and Dave Finney plus others, who have been holding, a long series of communications with prof Allsop and this revision seems to be as a direct result of that, from what I have observed. These figures are still in dispute and are still being argued profusely.
The economic factors are not accounted for whatsoever. The fact too that people are moving closer to their work is having a knock on effect too and that includes less experience and in less 'known vehicles' (although to a much less resultant effect).

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