Are the safety benefits of fixed speed cameras real, or imaginary?

A?new study says there’s no evidence that fixed speed cameras in the Thames Valley have reduced collisions. But the former operations manager for the cameras says the method is badly flawed.

Do fixed speed cameras cut accidents, or are the recorded reductions nothing to do with the cameras at all? Organisations responsible for camera enforcement have long cited impressive statistics for casualty reductions at camera sites. But rarely are the figures adjusted for the statistical phenomenon of regression to the mean (RTM). 

Authorities often selected camera sites following a higher than normal collision rate during the site selection period. Because of RTM the rate would then be...

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