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Speed Camera Claim Shame 2008-09-10

Posted by clype in Gizmo, Health, Statistics.
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Government claims of the value of speed cameras in preventing death and injury have been greatly exaggerated, claim scientists.

A statistical quirk meant that estimates of about 100 lives saved/year should be cut by half, according to researchers.

The assessment methods used to evaluate speed camera success failed to take account of random ‘bad luck’ at notorious accident spots, it was claimed.

Dr Linda Mountain, from ‘The University of Liverpool’s Department of Engineering’, said:

‘Although some parts of the road network are undoubtedly more dangerous than others, there is also a degree of randomness in where accidents occur – driver error, bad luck etc – which means that an accident can happen anywhere.’

A study forming the basis of the Government claims looked at trends at all 4 100 camera sites in the UK.

It found that over a period of four years speed cameras were responsible for a 22 per cent drop in the number of all accidents. The number of people killed or seriously injured was said to have fallen by 42 per cent, amounting to around 100 lives saved/year.

Dr.Mountain re-examined the evidence, conducting a new study looking at road accident numbers before and after cameras were installed at 215 sites.

This time an alternative analysis method was used called Empirical Bayes which took into account the element of uncertainty.

Allowing for the randomness element — known as regression-to-the-mean— showed a fall in accidents of 19 per cent. Not allowing for this element would have indicated a reduction of 50 per cent.

Speaking at ‘The British Association Festival of Science‘ at ‘The University of Liverpool‘, Dr.Mountain said:

‘I think it’s not reasonable to say that 100 lives/year are being saved. I would have thought a smaller figure would be the correct one. The number has been overestimated by 50 per cent on the basis of our data.’

MORE INFORMATION ON THIS:

‘Regression to the mean’ explanation in terms of speed cameras, Safespeed.org

Wikipedia, “Regression Fallacy”

Examples given: ‘The frequency of accidents on a road fell after a speed camera was installed. Therefore, the speed camera has improved road safety.

Speed cameras are often installed after a road incurs an exceptionally high number of accidents, and this value usually falls (regression to mean) immediately afterwards. Many speed camera proponents attribute this fall in accidents to the speed camera, without observing the overall trend.

Some authors have claimed the alleged “Sports Illustrated Cover Jinx” is a good example of a regression effect: extremely good performances are likely to be following by relatively less extreme ones, and athletes are chosen to appear on the cover of Sports Illustrated only after extreme performances. Assuming athletic careers are partly based on random factors, attributing this to a “jinx” rather than regression as some athletes reportedly believed would be an example of committing the regression fallacy’

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