Hm. Maybe we're talking about different things. The detectors I mean are fairly simple: inductive loops in rectangles cut into the road, that detect the metal body of a car (or metal frame of a bike). They don't seem technologically advanced or expensive (but I don't know the cost). This article claims they're common: https://auto.howstuffworks.com/car-driving-safety/safety-reg...
Sorry, now I see what you mean!! Yes underground inductor loops are already pretty common. They make it possible to do things like preventing a change of lights if nobody is waiting on the red. The limitation is it can only detect the car that is physically on top of the inductor loop. They can also be less than ideal in some situations e.g. someone on a side road turning right who triggers a change of lights, needlessly stopping the main road. It’s typically a great improvement over nothing at all, but there is additional (massive) improvement.
I thought you were initially referring to Adaptive Traffic Signal Controllers [1]. They detect vehicle congestion in real time and adapt routing strategies to maximize throughflow. They also coordinate directly with other traffic lights instead of relying on manual timings. Which are often determined through lengthy and data intensive “signal studies”. These have shown to provide, for example, substantial time savings, fuel and emission savings, and there have been measurable positive effects on safety.
