1. The Effectiveness of Average Speed Cameras a report commissioned by the RAC Foundation
Richard Owen

2. Background Road Safety Analysis Richard Owen Co-authors
Not-for-profit company limited by guarantee registered in England
Independent specialists in collision and casualty analysis, evaluation, online analysis systems, intervention design, training and more
Richard Owen
Former manager at Thames Valley Safer Roads Partnership
Specialist in spatial analysis, GIS, and project management
Co-authors
Professor Richard Allsop - Emeritus Professor of Transport Studies at UCL
Dr George Ursachi – RSA Analyst

3. History of Speed Cameras in GB
2000 – 2007 Focus on casualty reduction
Government sets installation criteria
4 Collisions (KSI) per km in 3 years
8 Collisions (PIC) per km in 3 years
Speed as a ‘causation factor’
85th Percentile speeds > 10% + 2mph e.g. 35mph in 30mph limit
20% of drivers exceeding the speed limit
The aim of this report is not to review this in any great detail but it important to understand that the programme was very much data-led with a focus on collision cluster sites

4. Evidence for Casualty Reduction
42%
KSI
50%
Fixed
35%
Mobile
22%
PIC
4100 Sites Analysed, results very impressive, although they didn’t analyse the handful of ASC separately

5. Evidence for Casualty Reduction
Regression to Mean
36% at Fixed Sites
43% at Mobile Sites
Regression to mean describes the reduction to ‘normal’ levels of collisions at sites that have been selected due to high collision frequencies
The Four year evaluation report estimated the impact of RTM on collision reductions for fixed and mobile sites
Time

6. RAC Foundation Objectives
To create a national database/inventory of ASC sites of various kinds in Great Britain
To establish a suitably large and appropriate control group of sites to enable an understanding of the difference in collision reduction between potential ASC sites with and without such enforcement
To establish levels of occurrence of collisions before and after ASC installation (with consideration given to site-selection period, pre-installation and post-installation periods)

7. How we collected the data
Support from manufacturers
Support from authorities (Police, local authorities, camera partnerships)
Installation dates
Site selection periods
Prior enforcement
Other information
Collision data independently sourced

8. Analytical problems
We need to know if some sites are not suitable for analysis
Input from authorities was crucial here
It is possible that other changes could have occurred but weren’t recorded

9. Site Selection Periods
In order to allow for RTM we need to know about the site selection periods so their collisions can be excluded from the analysis
We used 25 years collision data for each site with a figure provided for each individual month which was then allocated to a specific period
The month the camera was installed was excluded

10. Map sample
Excluded

11. 29% PIC Comparison sites GB Collisions 2005 - 2015
When reviewing the effectiveness of an intervention over the long term you need to consider what other trends on vehicle and user safety could have had on collision rates.
In Great Britain between 2005 and 2015 there was a 29% reduction in all injury collisions for example so this needs to be adjusted for at our camera sites.
We used information about similar roads (those with the same classification) in the same area and compared changes on collision rates there against our sites.

12. Control sites Cameras considered but never installed
9 sections, 25km of roads
As we were designing the analysis methodology after the installation had taken place it was not possible to introduce randomised control trials. The use of this methodology on roads is a difficult exercise anyway as there are so many variables that can’t be controlled for.
We found 9 stretches of road that were considered for treatment with ASC but ultimately not progressed. In our report we measured changes after the initial scoping phase was undertaken to see if anything happened.

13. Installation History
Very few permanent systems were installed in the years of DfT control. There was some expansion in 2009 and 2010 but then no large increase again until According to Jenoptik there has been another big year in It will be possible to keep the database up-to-date in the future with support.

14. Standard “3 Before vs 3 Recent” Analysis
50%
FSC
Approach adopted by most authorities
Doesn’t take into account trend
Doesn’t allow for Regression to Mean
25%
PIC
The traditional analysis approach ignores RTM and SSP and just focussed on the three years before installation and the three years after
If we apply this methodology to the data these are the results we get

15. Generalised Linear Model
Monthly data for each site in each period
Takes into account collisions on other similar roads
Estimates the effect of the SSP
Estimates the effect of installation
The GLM was designed by Professor Richard Allsop and uses the tens of thousands of data points collected in the analysis.
It isn’t important to understand the model (although you can read the report) but it is important to understand that it produces two sets of results; one for the SSP and one for the installation effect

16. 16% PIC 36.4% FSC Results Site-selection period effect
24.9%
FSC
36.4%
FSC
16%
PIC
16.7%
PIC
The site selection criteria appears to have resulted in site selection bias i.e. Collisions were unusually higher at camera sites during the site selection period
Overall installation effect
Site-selection period effect

17. Results
No difference in collision reduction rates at sites installed pre-April 2007 versus after
No significant difference in effectiveness on low speed (20 – 40 mph) and high speed (50 – 70 mph) sites
Candidate Sites – No significant change in collisions post-consideration
Groupings of sites were considered in the report:
Those installed during DfT management vs those after – no significant difference
Those installed at sites with different speed limits - no significant difference
The candidate sites did not show a reduction in collisions post-consideration (only 35) and this could well have arisen by chance

18. What this means
The presence of Average Speed Cameras reduces the frequency of injury collisions, even when other mitigating factors are taken into account
When analysing the long-term impact of road safety interventions, consider the influence of general trend
If you select sites for treatment based on high collision rates, not all of the subsequent reductions can be attributed to the intervention