NY laid this down on his screen :
Do traffic cars still use those in preference to GPS?

I'm not familiar with current practice, but I doubt they use still use
magnet/spring speedos, or GPS.

I presume nowadays if
they do measure wheel rotations, everything is electronic, with no moving
parts other than the object that rotates at (a proportion of) wheel speed,
but now sensed by a Hall effect sensor and then rate of pulses (speed)
determined electronically - so there's no spring in the gauge to need to be
recalibrated as it loses its springiness. I wonder if they are recalibrated
periodically as a car's tyres wear down.

I expect the standard traffic car is equipped with just the same system
has our cars have - pulse taken from the ABS of one wheel, then
electronics feeding a stepper motor, except their systems will be set
to display the true speed to +/- 1 mph.

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 used to have a mate who worked on traffic, he then progressed to the
helicopter. So I able to keep in touch with their methods and etc..