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PUBLISHED: 11:40 27 March 2008 | UPDATED: 13:08 06 May 2010

SIR, — Your correspondent Steve Bridgstock (Herts Advertiser, March 20), rightly challenges the statistical basis of the claims that speed cameras save lives . The truth is even more slanted than the case he makes. He correctly states that an accident bl

SIR, - Your correspondent Steve Bridgstock (Herts Advertiser, March 20), rightly challenges the statistical basis of the claims that speed cameras "save lives". The truth is even more slanted than the case he makes.

He correctly states that an accident black-spot, the basis for installing a camera-site, is defined by "at least four serious accidents in a three-year period".

This implies four separate incidents, which would seem fair enough, but the truth is rather different. The "black-spot" criteria is in fact satisfied by single incident provided four fatalities or seriously injured were involved. In other words, a single accident can give rise to the installation of a speed-camera. Even if there has not been an accident in the previous 20 years or even longer, it makes no difference. Small wonder then that safety seemingly then improves. What in fact happens is that the statistics have simply "regressed to norm", hardly surprising if the "black-spot" assessment was founded on a single random event.

D.H. LANDAU,

Melbourne Close, St Albans.

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