Modelling of Accident Data from Fixed Safety Camera Sites in London Mike Maher University College London

Background to the talk Around 500 fixed safety cameras installed since 1990

Analysis of London data Data available for camera sites operated by TfL annual data on FSCs and PICs at cameras 1990 – 2011 .. plus totals for all non-camera sites TfL press release, Sept 2014: “Cameras have proved successful in reducing road casualties in recent years. 58% reduction in KSIs at camera sites”

Analysis of London data Data available for camera sites operated by TfL annual data on FSCs and PICs at cameras 1990 – 2011 .. plus totals for all non-camera sites TfL press release, Sept 2014: “Cameras have proved successful in reducing road casualties in recent years. 58% reduction in KSIs at camera sites” But – similar reduction at non-camera sites too camera sites non-camera sites

Method of analysis Clear that trend must be allowed for comparison with annual totals at non-camera sites to gauge what would have happened But regression to the mean (RTM) also sites selected on basis of previous accident record typically number of accidents in three year period the site selection period (SSP) generally selected sites show a reduction in after period, even if treatment ineffective DfT national safety camera study (2005)

4-year evaluation report: FSCs before 1.05 after 0.48 FSCs/site/year: (-54%) Overall reduction = 0.57 = 0.10 + 0.36 + 0.11 trend RTM camera 54% = 10% + 34% + 10% 50% allowing for trend 19% allowing for trend + RTM relative to what would have been:

Four time periods method Report by Allsop (2013) for RAC Foundation For each camera, years divided into periods before site selection period (SSP): typically 3 years transition (year camera installed): specified post-installation (camera period) So - if camera installed in mid-2000 SSP assumed to be 97-99 before period is then 90-96 camera period is 01-10

Accs/yr (adjusted for trend) SSP RTM ASBiC Camera effect Pre-SSP Post-installation Installation of camera time

Timing of the SSP? No record in data of actual SSP for any site Not safe to assume the three preceding full years Different SSP for each site? if SSP is earlier, some RTM in the before period hence inflates camera benefit important to get timing of assumed SSP right Or play safe and find a range to cover all SSPs Model to estimate factors Fd : d = t – s(i)  

Way to identify range of SSP d = t – installation year d = 0 is installation year d > 0 is camera period d < d* is before period d = d*, … -1 is range of SSP Clear signs of inflated level for more than 3 years Best choice of d* = -6. SSP*: d = (-6, -5 … -1)

Form of data used For each camera (eg KSIs at camera 240: installed in 2000) Before SSP* After No. years 4 6 11 Site total 5 8 Regional total 27934 36197 37784 4/37784 Estimate of camera effect = = 0.591 5/27934

Form of model Binomial model to split accidents between periods Probability factored in camera period by  Otherwise given by regional totals (trend) Then maximise log likelihood wrt     

Results Max Likelihood Estimate β = 1.014 (se = 0.030) 95% CI: (0.958, 1.074) So, no significant camera effect.

Conclusions and further work? No apparent camera effect modelling confirms initial graphical evidence important to allow for trend and RTM and to avoid making spurious claims not just TfL, but Scotland, Wales … But other modelling possibilities to investigate are the actual SSPs known for each site? 6-year SSP* used to encompass possible SSP so some data being lost was there a change in selection procedure at some point? monthly data available?

Thank you! Any questions?