Discussion elsewhere of Texas reminded me of these that I saw some time ago.
Basically, it is a stoplight on "backwards" ie. facing the direction of travel for the lane it is mounted next to (and opposite direction from every other light on the mast).
They seem to be mounted on the pole, and as best I can tell they are to give advance warning of a red light to traffic coming around a corner where they become visible before the rest of the lights for that direction of travel.
Anyone seen these someplace besides Texas?