White squares over traffic lights?

Norman_Castle

Saw these at two sets of lights today. Two flat white squares about 6" across on short poles above traffic lights. What are they?

vikingaero

Radio antennae - similar to the satellite squarials.

chris-j

In some places used to track buses and display arrival times at stops further down route.

flang

Sounds to me like they are wireless vehicle detectors (magnetometers) like these.
These are basically installed to save on cost of digging up the road/verge to lay cables for vehicle detection loops.