Some traffic lights are controlled by wire loops under the road that act like a giant metal detector. If there's no vehicle detected waiting on the side street the light may stay red for a long time. They don't always detect bicycles.
This is an older system. Newer installations almost universally use cameras instead.
It's easy to see if that's the case. I can see where they are.
There's this intersection where I live where the options are left, straight, 45 degree right turn, and 90 degree right turn. Those making a right usually go to the right half of the lane and there are sensors under there in addition to the ones on the left. It also has a bike symbol there as a sign that it's a designated bicycle route, but motor vehicles can be in the lane when it's about 100 feet from the intersection. And almost all intersections with lights around here have these. I haven't seen one that doesn't have one. I have seen some bike routes where there was even a button for bicyclists to press.
The method of exposing the bicycle was to lean it diagonally to expose more metal close to the sensor. Not sure what would happen with a carbon fiber bike though, although the frame is really only a small amount of metal even though it has the largest area. Not sure if the other metal parts on a carbon fiber bike would be enough. There are other ways too. I've heard of round loops that sense a bicycle vertically, although that's something for anyone to avoid hitting. There are also some bicycle/motorcycle dedicated loop sensors that are far more sensitive than those used for cars. I've even heard of some devices that are designed to trigger the sensors.
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