As others are indicating, this is tech exists, and in some places it's widely deployed, where in some others it's nowhere to be found. I've seen them in several US cities and suburbs, in many states, usually along major arterials and covering the busiest intersections.

Missouri Department of Transportation's St Louis District has one of the best FAQs [1] on this tech that shows how these cameras work: exactly as you'd suspect from some OpenCV proof-of-concept.

The cameras are a more robust replacement of the in-pavement induction loops, which are much more widely deployed, but give you a subset of the information at a higher cost.

[1] http://www.modot.org/stlouis/links/signalcameras.htm