M4 cameras: too early to call?
Road safety organisation Safe Speed has refuted yesterday's claims by the Swindon and Wiltshire camera partnership (SWCP) that the M4's speed cameras, which were switched on in April, have reduced crashes on the M4 motorway. According Safe Speed, its claims are statistically invalid.
There are four main errors in the claims, said Safe Speed:
- Cherry picking. The SWCP has chosen a 'comparison group' intended to support its claims. This exaggerates the benefits.
- Bias by selection. (also known as regression to the mean). SWCP started with its 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.
- Other effects. Although the partnership claims that 'nothing else has changed' on the M4, that's simply not true. There are extensive road works at J18, and those road works may well be having an effect on traffic.
- 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. The cameras only went live in April.
Safe Speed said it was "quite certain" that the numbers published do not justify the claims made.
Paul Smith, founder of Safe Speed's 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."
Smith said: "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."
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