The system can enforce a number of speed limits. That is to say, it has a fixed
number of speed limits rather than being constantly variable. The speed limits are
0 (a.k.a. red X or "lane closed"), 20, 30, 40, 50, 60 and NS (the national speed
limit, currently 70mph). Each speed limit also has a user-settable threshold, so
that the actual speed at which an offence occurs is the sum of the speed limit and
this amount (eg. a 40mph limit with a 10mph threshold means it will enforce at
50+mph.). Generally the threshold is no less than 10%+2 of the speed limit, but
this is merely a guideline and it can be anything from 0-99mph, and is entirely at
the discretion of the operator.
The speed limit set is read directly from the same electrical input that sets the
speed limit displayed to the motorist on the matrix board. There is also a fibre-
optic sensor on each bulb in the matrix board that is turned into an electrical
signal so that the system can verify that the matrix board is actually displaying
what it has been told to display.
Extract taken from UK driving secrets (was writtin by an ex-cop). Hope that helps