WILTSHIRE CLAIMS ARE AN ABUSE OF STATISTICS
13 July 2005 - Safe Speed

The Swindon and Wiltshire camera partnership today are claiming that they have reduced crashes on the M4 motorway, but their claims are statistically invalid.

There are 4 main errors in the claims:

1) Cherry picking. They have chosen a ''comparison group'' intended to support their claims. This exaggerates the benefits.

2) Bias by selection. (Also known as regression to the mean). They started with their camera vans on the M4 because the figures were at an unusually high level. With that as a starting point, it''s perfectly natural for the figures to tend to reduce. This is a huge effect and endemic in camera partnership claims.

3) Other effects. Although the partnership claim that ''nothing else has changed'' on the M4, that''s simply not true. There are extensive roadworks at J18, and those roadworks may well be having an effect on traffic.

4) Too soon to tell. Accidents are essentially random in nature and this means that long periods of observation are required to make valid comparisons. The usual recommendation of professional traffic engineers is five years before and five years after.

Safe Speed is quite certain that the numbers published do not justify the claims made.

Paul Smith, founder of the Safe Speed road safety campaign said: ''I''ve completely lost count of the times that wild and unjustified claims have been made by camera partnerships. It''s become par for the course. It makes me incredibly angry because we depend on accurate road safety information in order to deploy resources to save lives. Claims like these cause death because they cause resources to be misallocated.''

Paul continues: ''Camera partnerships are riding a runaway gravy train that must be stopped. Speed cameras don''t save lives - they just distract everyone from real road safety. We won''t get back on track until we scrap the infernal devices.''

www.safespeed.org.uk

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