Transport for London claims are FRADULENT says Safe Speed
Transport for London (TfL) today are claiming that a 50% drop in 'Killed or seriously injured' (KSI) crashes at speed camera sites means that speed cameras have saved 1,500 from death or serious injury in the last five years.
Safe Speed points out that this claim does not account for a major statistical bias known as 'regression to the mean' and is a cynical attempt to DEFRAUD THE PUBLIC.
The Department for Transport road safety good practice guide defines 'regression to the mean' as follows:
"5.119 This effect, sometimes called bias by selection, complicates evaluations at sites with high accident numbers (blackspot sites) in that these sites have often been chosen following a year with particularly high numbers occurring. In practice their accidents will tend to reduce in the next year even if no treatment is applied. Even if three-year accident totals are considered at the worst accident sites in an area, it is likely that the accident frequencies were at the high end of the naturally occurring random fluctuations, and in subsequent years these sites will experience lower numbers. This is known as regression-to-the-mean."
The Department for Transport 4th year report into the speed camera program estimates regression to mean effect across a small subset of sites in appendix H. It found that 75% of the apparent benefit was nothing more than a statistical bias. [3]
Paul Smith, founder of www.safespeed.org.uk said: "Of the 1,500 reduction in KSI claimed, Transport for London know very well that around 1,125 are due to nothing more than a statistical bias. To claim the full 1,500 is a cynical attempt to defraud the public. They are clearly more interested in saving face than saving lives."
"Of course the remaining 375 or so would be a saving well worth having - if it was actually delivered. But of course it isn't. They have not accounted for reductions in traffic volumes at speed camera sites, nor the benefit of other engineering works, nor any of the 40 negative side effects [4] arising from speed cameras."
"There is no doubt in my mind that speed cameras don't save lives - they cost lives. When I see organisations like TFL trotting out fraudulent statistics to justify their failed policies it makes my blood boil."
"As it happens our best estimate for regression to the mean benefit illusion at an average speed camera site is 50% - exactly the same figure that TfL are claiming."
