"Robin" wrote in message
...
For calibration check, they used to use a process of checking time with
a stopwatch, over a fixed distance at a fixed speed. The tyre wear
makes surprisingly little difference to the calibration. They used to
do that daily.

I've always thought that timing over a fixed distance is subject to lots of
random errors such as the reaction time of the observer to press the button,
and his judgment as to the exact instant when he has passed the fixed
features such as the white squares painted on the road.

I expect now it's done automatically electronically many times a day.
There is ample, published material from the police and the NPCC that
shows Harry is right. That is, the police use routinely - and courts
convict on the basis of evidence from - speedometers.

Yes, but are the speedometers measuring wheel rotations as for a
conventional speedo (but more accurate), or do they use GPS, or do they use
doppler measured off the road surface (the way that a mouse detects movement
over a mouse mat or desk)?

I suppose a speedo, re-calibrated fairly frequently to adjust for tyre wear,
will be the simplest and most accurate - as long as the spring tension in
the analogue meter is constant throughout the range: I'd have thought that
was the thing that is least easy to control in the manufacture of any
analogue gauge. If the display is digital then as long as the timing crystal
doesn't drift over time, you've eliminated that variable.