Regression-to-the-mean

Some critics of speed cameras have specifically argued that crash reductions found at speed camera sites are an overestimation, since they include regression-to-the-mean effects. The regression-to-the-mean effect is the statistical phenomenon that roads with a high number of crashes in a particular period are likely to have fewer during the following period, even if no measures are taken; this is just because of random fluctuations in crash numbers. Since speed enforcement often takes place at roads with large numbers of crashes, the effects of the intervention may be overestimated if there is no appropriate statistical control.

However, research by Gorell and Sexton [30] showed that also after controlling for regression-to-the-mean, the estimated reduction in all injury collisions due to 77 speed cameras in London was 12%, and the reduction in fatal and serious injury crashes 21%. Similarly, after correction for regression-to-the-mean, Mountain et al.[46] found that, within a distance of 250 metres of the speed cameras, the overall effect of a sample of 62 speed cameras was a 25% reduction in all injury crashes.