Safe Speed Forums

The following letter from the local camera partnership manager appeared in the North Wales Daily Post on Thursday 10th February 2005:



I sent the following letter for publication:

Lies, damn lies and camera partnership statistics

Is it possible that Alan Hughes believes what he writes? (letters, Thursday)
Or does he wilfully misrepresent the facts to preserve his own employment?

To any statistician the naivety required to believe that "accident reductions
at speed camera sites" represents a road safety improvement is breathtaking.

Accident reductions should be expected at speed camera sites, but far from
representing the beneficial effect of the camera, these accident reductions
are mainly an expected side effect of the site selection method.

Suppose, somewhere, a bend in the road has one crash per year on average. This
crash rate is not sufficient to justify a camera. Eventually, through random
chance, that same bend has three crashes in a year. The criteria are met and a
camera is installed. Now the crash rate at the bend continues at the long term
average of 1 per year, but the camera records say the camera was installed
when the bend had three crashes. Doubtless Alan would make the claim that the
camera had reduced the crash rate by 66% from 3 to 1, but clearly this is an
abuse of the facts - the bend's crash rate hasn't changed - instead a bad year
triggered the installation of the camera.

This effect is extremely well known to road safety professionals and is called
"regression to the mean". If Mr Hughes doesn't fully understand the effect he
is incompetent. If he does fully understand, he is attempting to delude the
public and is therefore dishonest. Either way he should resign.

_________________
Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety