This morning I noticed that 2 overhead gantries with older style speed camera/detectors have been upgraded between the M3 and M4.
The new boxes are about the size of a roadside Gatso and mounted in the gantries. I have no reason not to suspect that these are now live speed cameras and maybe digital. How accurate that are going to be set for giving out tickets I don't want to find out.

The gantries I have noticed so far are:-
From M3 northbound to M4, gantry 1 just after you enter the M25 and 9 just before the Terminal 4/5 exit.
Also heading south on the M25 the last gantry before you exit for the M3.

Beware!

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RTFM!!
Normal people believe that if it ain't broke, don't fix it. Engineers believe that if it ain't broke, it doesn't have enough features yet.