Is this for a real application or a school project? I know its done commonly here in the USA, but i am not sure of the exact method. Talking to a traffic engineer would likely be a place to start.
Your controller knows the state of the lights, so you could simply count cars approching the red, or change the timing of the light when traffic at the red backs up to a set distance from the light.
Around here sensors are buried under the road and dectect a large metal mass passing over them.