The DfT has made available the full text of a letter written by Tim Stamp, its head of statistics, about regression to mean at camera sites.
On 29 November an article appeared in Local Transport Today (LTT) about a new method to assess the effectiveness of speed cameras. The “four time period” method was devised by engineer Dave Finney to account for the effects of regression to mean (RTM) at camera sites.
Extracts from a letter written by Tim Stamp to Idris Francis were quoted in the LTT article which was subsequently covered on this newsfeed.
Anti-camera campaigners, including Mr Finney and Mr Francis, have long argued that much of the casualty reduction benefit attributed to cameras is, in fact, largely due to RTM.
Click here to download the full draft of Tim Stamp’s letter.