Lots of traffic lights in my area already have sensors (induction loops to detect waiting cars or approaching bicycles, IR motion detectors to detect pedestrians). In some cases this is really useful (eg. a traffic light for a bike path does not need to switch very often in winter, so it only switches when a bike approaches)

I think that the problem is that often there is too much traffic from all directions and even with all the sensors in the world the traffic light can‘t change that.