sorry if its already been mentioned in this thread, i cbf reading through..
but with the underground speed cameras, i think it has to do with magnetic strips under the road that read a vehicles speed when it passes over it, but there still has to be a camera present on the ground as its impossible to get the cars rego plates without one..
the current speed cameras are all like this, if u notice on the road there is fresh concrete added, this is where the magnetic strips have been placed.. so once u pass this strip, the camera cant record ur speed..