Cameras before speed cameras on M25

[HammerCJ]

Does anyone know why there are always a set of cameras/detectors/something on a small white pole maybe 100yds or so before the HADECS speed cameras on the M25? What are they detecting?

[Strudul]

Do those ones not record the speed limit on the gantries?

[AyBee]

Strudul said:

Do those ones not record the speed limit on the gantries?

This. They record what the speed limit on the gantry says at the time you get caught.

[HammerCJ]

Thanks guys