On Wed, 26 Jan 2005 09:43:01 -0000, Alan B. Pearce wrote:

> >detector + camera taking two picures a certain time apart and
> >lines on the road for distance measurement. (This is in the UK anyway)
> 
> The UK is odd ball in this - in quite extensive travelling around the world
> the UK is the only place I have come across this. Everywhere else one
> picture on a calibrated camera is enough.

Corroboration of evidence is a basic tenet of English law, and a policeman and his radar/lidar gun counts as 
such, as does an unattended radar camera and the two photos of the vehicle crossing a measured set of lines on 
the road.  It is different in Scotland, by the way, where a policeman on his own can estimate the speed and it 
can be accepted without corroboration.

The thing is that calibrating the camera doesn't account for such things as a large flat-backed lorry, where 
the signal can reflect from it onto traffic in the other direction, and then back again to the camera, giving 
an apparent speed which is the sum of that of the lorry (in the photo) and the other traffic (not visible).  
There was a case here recently where the driver challenged the accusation, and the pair of photos showed he 
was doing 18mph, but the camera's radar said something like 55mph.  He was acquitted, of course.  The camera 
wasn't out of calibration, but the reflected signal it detected wasn't "correct".

Cheers,


Howard Winter
St.Albans, England


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