TPA response to RoadPeace

July 12, 2010 1:04 PM

Last week the TPA launched its third report in partnership with the Drivers’ Alliance about speed cameras. The report has received some criticism from RoadPeace.  They have written a response with a number of criticisms, none of which stand up to scrutiny:
"As TPA only consider national casualty figures, the impact of safety cameras is inevitably lost in the noise from other factors."
We have used national figures to avoid the problems with local studies that look at accident rates before and after cameras are installed. Cameras are often installed after an exceptional spate of accidents.  After such a spate of accidents the casualty rate will normally fall back to the long term average without any intervention; research that views such a fall as the result of installing a camera will overestimate their effectiveness.  Simple regression to the mean is mistakenly ascribed to the effect of cameras.

Evidence at the local level does not contradict the TPA's findings, though.  Swindon cut back its use of cameras drastically and saw no increase in casualties.
"[The TPA's] analysis (conveniently) omits the data from 2008 and 2009."

When the research was produced, we used the latest available data from the Department for Transport. Road transport passenger kilometres for 2008 or 2009 are not yet available. It is crucial to adjust raw casualty figures for the number of passenger kilometres.  Otherwise it is impossible to fairly compare safety on the roads over the years as use increases and decreases.

"Using the TPA methodology, the overall casualty rate in 2008 fell by 9% (and the corresponding death rate by 18%)."
Even if we accept this claim, and the data is not provided in their response, it coincides with a decline in the amount raised from fines shown in our report, as safety camera partnerships were no longer allowed to recoup the costs of processing fines.  It does not undermine our finding that speed cameras and the wider focus on enforcing speed limits is not an effective safety policy.
"It is also biased in the division of the period of study. Of the 5,562 camera sites available in March 2007, only 13% had been established by 1996 and 35% by 2000."
The early nineties saw a shift in road safety policy towards focussing overwhelmingly on enforcing speed limits.  The first speed cameras were introduced in 1992.  And in 1991 the government launched its first £1 million television advertising ‘Kill you speed not a child’. By 1997 the advertising budget centred around the slogan ‘Speed Kills’ had risen to £3.5 million. The critical question is whether the choice to focus on enforcing speed limits was effective as a road safety policy, our statistical study suggests it was not.

To narrow the data to 2001 onwards would unfairly truncate the sample to ignore almost a full decade in which speed cameras rose to prominence and moved to the centre of road safety policy. It is inconsistent for RoadPeace to criticise us for not including data for 2008 and 2009 – two years for which data does not exist – but then argue for the deliberate omission of crucial data which is in existence from 1992 to 2000 – a period of nine years, for which the information is available.

The early nineties saw a shift in road safety policy towards focussing overwhelmingly on enforcing speed limits.  The first speed cameras were introduced in 1992.  And in 1991 the government launched its first £1 million television advertising ‘Kill you speed not a child’. By 1997 the advertising budget centred around the slogan ‘Speed Kills’ had risen to £3.5 million. The critical question is whether the choice to focus on enforcing speed limits was effective as a road safety policy, our statistical study suggests it was not.

The alternative approach suggested by RoadPeace, of looking at the rate of improvement in road safety since 2001, has two key problems:

 1.     It does not give a series that is sufficient for a reliable statistical test of speed camera effectiveness.

2.     Starting from the completion of the camera network ignores the more important timing of the shift in policy.


It is good to see that our report has fed into the continuing debate over the effectiveness of cameras.  But the attacks from RoadPeace are unfair and our results remain robust.

Last week the TPA launched its third report in partnership with the Drivers’ Alliance about speed cameras. The report has received some criticism from RoadPeace.  They have written a response with a number of criticisms, none of which stand up to scrutiny:
"As TPA only consider national casualty figures, the impact of safety cameras is inevitably lost in the noise from other factors."
We have used national figures to avoid the problems with local studies that look at accident rates before and after cameras are installed. Cameras are often installed after an exceptional spate of accidents.  After such a spate of accidents the casualty rate will normally fall back to the long term average without any intervention; research that views such a fall as the result of installing a camera will overestimate their effectiveness.  Simple regression to the mean is mistakenly ascribed to the effect of cameras.

Evidence at the local level does not contradict the TPA's findings, though.  Swindon cut back its use of cameras drastically and saw no increase in casualties.
"[The TPA's] analysis (conveniently) omits the data from 2008 and 2009."

When the research was produced, we used the latest available data from the Department for Transport. Road transport passenger kilometres for 2008 or 2009 are not yet available. It is crucial to adjust raw casualty figures for the number of passenger kilometres.  Otherwise it is impossible to fairly compare safety on the roads over the years as use increases and decreases.

"Using the TPA methodology, the overall casualty rate in 2008 fell by 9% (and the corresponding death rate by 18%)."
Even if we accept this claim, and the data is not provided in their response, it coincides with a decline in the amount raised from fines shown in our report, as safety camera partnerships were no longer allowed to recoup the costs of processing fines.  It does not undermine our finding that speed cameras and the wider focus on enforcing speed limits is not an effective safety policy.
"It is also biased in the division of the period of study. Of the 5,562 camera sites available in March 2007, only 13% had been established by 1996 and 35% by 2000."
The early nineties saw a shift in road safety policy towards focussing overwhelmingly on enforcing speed limits.  The first speed cameras were introduced in 1992.  And in 1991 the government launched its first £1 million television advertising ‘Kill you speed not a child’. By 1997 the advertising budget centred around the slogan ‘Speed Kills’ had risen to £3.5 million. The critical question is whether the choice to focus on enforcing speed limits was effective as a road safety policy, our statistical study suggests it was not.

To narrow the data to 2001 onwards would unfairly truncate the sample to ignore almost a full decade in which speed cameras rose to prominence and moved to the centre of road safety policy. It is inconsistent for RoadPeace to criticise us for not including data for 2008 and 2009 – two years for which data does not exist – but then argue for the deliberate omission of crucial data which is in existence from 1992 to 2000 – a period of nine years, for which the information is available.

The early nineties saw a shift in road safety policy towards focussing overwhelmingly on enforcing speed limits.  The first speed cameras were introduced in 1992.  And in 1991 the government launched its first £1 million television advertising ‘Kill you speed not a child’. By 1997 the advertising budget centred around the slogan ‘Speed Kills’ had risen to £3.5 million. The critical question is whether the choice to focus on enforcing speed limits was effective as a road safety policy, our statistical study suggests it was not.

The alternative approach suggested by RoadPeace, of looking at the rate of improvement in road safety since 2001, has two key problems:

 1.     It does not give a series that is sufficient for a reliable statistical test of speed camera effectiveness.

2.     Starting from the completion of the camera network ignores the more important timing of the shift in policy.


It is good to see that our report has fed into the continuing debate over the effectiveness of cameras.  But the attacks from RoadPeace are unfair and our results remain robust.

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