Mobile speed cameras

Official reports have estimated that RTM (regression to the mean) is the largest effect at camera sites, but no report has ever managed to remove it.

This is different:

This is the first report in the world to have completely excluded RTM.

Full report: “THE EFFECTS OF MOBILE SPEED CAMERAS ON ROAD SAFETY“.

Quick summary:

Figure 8.1 shows what happened at these sites.

There was a large reduction at the end of the period shown in green and when it occurred is crucial. It happened a year before the cameras started operating.

Speed cameras cannot go back in time and prevent collisions when they weren’t there, so that reduction cannot be due to them. This begs the question:

What caused the reduction a year before the cameras?

The answer is “RTM”.

RTM is simply a result of the site selection process. These sites were selected after considering where collisions had occurred within the 2.5 year period called the SSP (in green). We can see that there are 4 time periods:

1. Blue: before the SSP.
2. Green: the SSP.
3. Blue: after the SSP, but before the cameras.
4. Red: after the cameras started operating.

Collisions were occurring at around their normal rate before the SSP. These sites were then chosen after a higher than normal rate during the SSP. The collision rate then dropped back to normal again after the SSP.

After the cameras started operating, collisions increased above their normal rate.

The report also compensates for trends in road safety and finds:

• a 19% increase in fatal and serious collisions after mobile speed cameras started operating.

Full report: “THE EFFECTS OF MOBILE SPEED CAMERAS ON ROAD SAFETY“.