They have been installing a few per year here over the last three years, whenever they re-pave an intersection with a heavily-used bike lane. They lay something that looks like metal sensors down into cut-groves into the pavement. I'm not really sure how they work though. I do know that some of them are sensitive enough to sense full carbon bikes, which makes me wonder if some of them are actually a pole/line mounted motion-detector, and the "x" is actually just the spot that it is aimed at.