How do intersections/traffic lights sense the presence of cars, bikes, and people?

Question

I often cycle around the city, and sometimes when I need to turn left and I am the only thing in the left-turn lane, the intersection infrastructure senses that I am there and triggers the left turn arrow.

At some busy intersections that can have a long left-turn lane, there is even a sensor to sense how many people are trying to turn left and will adjust the length of the light based on this. Where are these sensors and how do they work?

In: Engineering

Answer 1

On the road, have you see the rectangular cutouts? Those are weight sensors.

Edit: I assumed they were weight based, ignore me and carry on!