Exclusive The Department for Transport (DfT) has "unwittingly" misled the public over the benefits of speed cameras for the last four years.

That was the shock admission yesterday by a DfT spokeswoman, when finally cornered by the Department’s own research. She also told us that they have finally agreed to put matters right by adding an explanation to future public statements.

The misinformation began with a report produced by the DfT (pdf) itself in 2005. On the basis of this report, it came up with the now infamous claim that speed cameras are directly responsible for reducing the level of killed and seriously injured (KSI) at camera sites by 42 per cent. Yet its own evidence barely supported half that figure.

The claim has been repeated frequently by official spokespersons and road safety campaigners, and on the DfT’s own Think! road safety website. It has also been regularly questioned by speed camera opponents, who point out that other effects should be taken into account. These include overall "trend" improvements in the KSI rate, other road safety measures put in place at accident black spots, and a statistical quirk known as "regression to the mean".