Traffic signal cameras for traffic light control are common. They used to be dumb monochrome analog cameras connected to simple "non-pavement object detected in box" processing units. There's been considerable mission creep since. Here's the promo video from Econolite's current product.[1] HTDV, WiFi, car, truck, bus, bicycle and pedestrian detection, connects to control center if the bandwidth is available.

CALTRANS has most of their highway cameras available from their web site.[2][3] CALTRANS has been at this since the 1980s, and the cameras are mostly somewhat old and low-rez.

[1] https://www.econolite.com/products/detection/autoscope-visio...

[2] http://cwwp2.dot.ca.gov/vm/streamlist.htm

[3] http://cwwp2.dot.ca.gov/vm/iframemap.htm

Yes, optical detectors are often used for traffic control.

Traffic cameras which stream visible-light video are also very common. https://www.weatherbug.com/traffic-cam/

I'm not sure why you've been downvoted. Traffic-aware traffic lights are generally implemented using induction loops built into the road surface: https://en.wikipedia.org/wiki/Induction_loop

Today, many have cameras for other reasons. But this is not ubiquitous and the prevalence of these cameras varies from location to location. Many traffic lights don't have cameras (many don't even have induction loops, and work off timers instead.)

In areas where the ground is covered in a foot of snow and ice all year those aren't particularly effective and they do use devices that look kinda like cameras and everyone assumes are cameras.

They're actually just small radar devices. It's a much simpler problem to solve across a wide variety of conditions to simply point radar at the ground and when the distance suddenly drops 4-8' assume a vehicle is there.

These devices don't have any sort of lens assembly which should be your first clue, but they also are pretty easily differentiated if you own a radar detector as well...

How often is this done? Do they also detect if a bike is at the intersection? Will they eventually cycle the light even if nothing is detected (just in case the sensor is not working)?

They are at almost every intersection in my state. I’ve heard people in bikes complaining about having hard time getting detected sometimes. I suppose it depends on how the light is programmed if it has a max time before it cycles, but I’ve had to go through intersections on a red light because I waited minutes and it didn’t turn green.