http://www.tnn.co.uk/UKNews/plonearticl ... 0113475052
( It's our PR268 word for word (as far as I can tell):
http://groups.yahoo.com/group/SafeSpeedPR/message/117 )
Camera Report Defrauds Media and Public
According to Safe Speed, The Department for Transport (DfT) '4th year' report issued yesterday is worded in such a way to have widely misled media and public alike about the true effects of speed cameras.
The important headline figure of '42% reduction in killed and seriously injured' contain a gross statistical bias - yet has already been very widely quoted as if it were the true benefit of speed camera operation.
The gross statistical bias is called 'regression to the mean' (RTTM or RTM). It is a large effect. Although the report attempts to suggest that it cannot be accurately estimated on available data, two self-similar estimates are included. (Table 4.9 and appendix H)
According to table 4.9, 0.36 RTTM + 0.11 benefit = 0.47 total. Applying this to the 42% headline conclusion suggests:
42% reduction in KSI at speed camera sites is comprised of 32% RTTM benefit illusion and 10% camera benefit
According to appendix H:
Three fifths RTTM.
One fifth trend.
One fifth benefit.
Since the trend has already been calculated in the 42% headline figure, this becomes:
Three quarters RTTM.
One quarter benefit
This would suggest:
42% reduction in KSI at speed camera sites is comprised of 31.5% RTTM benefit illusion and 10.5% camera benefit.
Paul Smith, founder of the Safe Speed road safety campaign (
www.safespeed.org.uk) said: "Regression to the mean effect is acknowledged as providing a gross distortion of the benefits claimed.
!It is no surprise to me that the report's authors make many excuses for failing to provide a substantial and accurate estimate of the effect because clearly it will reduce the benefit claimed by about a factor of four.
"The benefit of cameras expressed in KSI isn't 42% at all - it's about 10%.
"This effect means that if we had installed garden gnomes at the roadside instead of speed cameras they would have been about three quarters as effective.
"We could have had headlines stating that 'Gnomes cut fatal and serious crashes by 32%
"I would be proud to be associated with a road safety initiative that cut fatal and serious crashes by even 10%. However speed cameras and supporting policy have many negative side effects on wider road safety that comprehensively swamp the benefits.
"The net effect of speed cameras has been to make our roads much more dangerous.
"I am disgusted that the executive summary of the report fails to make this adequately clear.
"Newspapers and broadcasters have presented the 42% figure as if it were truly the benefit of speed cameras. Clearly it is not and the public is being misled."
RTTM explained:
Regression to the mean arises because we like to apply a safety treatment - in this case a speed camera - to the worst places. This means we tend to place cameras in places where crashes are peaking. After the camera has been installed the 'peak' conditions pass and the crash rate at the site tends to return to its long term average.
It should be obvious that many of these peaks arise through random chance and random peaks always pass.
You can read the TNN article about the DfT's publication here:
http://www.tnn.co.uk/UKNews/plonearticl ... 1058894905
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