How does a traffic light change?

I would suspect a laser is detecting a shorting of it's length by your car blocking it. Or it could be Elfin Magic.

I would say "mass" in either case as evidenced by my traffic light troubles during my motorcycle riding days (i.e. the light wouldn't turn for me).

I agree with kevbo, as I have heard other motorcycle riders complain about this. More and more the motion detectors are being used instead. They are cameras mounted on the traffic light post.

From what I have heard the boxes on the ground are the "trip wires" for the light. When your car's tires on on the strips the light senses a change in resistance and it trips the light. When I was riding motorcycles I was sure to stop on one of those lines and I never had a problem with a traffic light not noticing me!

The light sensors are magnetic and are triggered by changes in the magnetic field, just like the anti-shoplifting sensors at stores. Sometimes motorcycles and kit cars do not have enough metal to disrupt the magnetic field and trigger the light change.

I used to think that there were people far away watching it all and changing the lights. then I fould out that it was a senor

I can second glial and kevbo on the motorcycle troubles bit, but from what I’ve read the answer given by hossman is most correct. The Seattle Times has an article that indirectly answers the question. From the article, “As part of the sensor system, a wire is cut into the pavement in the shape of a circle or a rectangle near the stop line, Cunningham said. It senses the metal of vehicles and signals a control box on the side of the road to change the light.”