Incorrect speed indication on GO720 screen...

Only on this one particular stretch of the M3 I always see my car's speed freeze on the GO720 screen, with a horizontal bar showing above the speed value - see the attached screen-grab below. Whereas I'm actually travelling at the 50mph speed limit together with many other vehicles.
The satnav operates and displays normally in every other respect.
I've never seen the same effect anywhere else.
Anybody else seen this anomaly?
What's the explanation?
Could it be caused by interference (intentional or otherwise) in the extensive roadworks operating on that part ot the M3?
Or by an inadequate number of satellites being received?
[img]C:\Users\rhammond\Documents\TomTom\+TomTom%20temp%20(for%20speedcams%20etc)\dump218073783.bmp[/img]

A horizontal bar showing above the speed value is use to to indicate an average speed eg. Road works with SPECS Cameras.

Thanks - that explains the phenomena precisely. That section of the M3 does have such cameras and speed limit. And so that 'overlined' speed shows what my average speed is at the time as I travel through the roadworks.

I've never found where that explanation is published, either on-line or in any user manual.