Where are the traffic light sensors located that can tell when to change? How does it work?

Question

Where are the traffic light sensors located that can tell when to change? How does it work?

Answer 1

It's usually an induction loop underneath the pavement.  It's a metal detector, like the ones at the airport.  

You'll also find induction loops being used on freeways.  In this case, the purpose is not to change traffic lights, but to detect the speed at which traffic is flowing.  This is how they make those traffic maps that show you where the traffic jams are.

If you ride a motorcycle, there's products you can buy that create a magnetic field to trip the sensor.  Contrary to popular belief, the problem with motorcycles isn't that they weigh too little, it's that they don't have enough metal.

Answer 2

typically the sensors are mounted on poles, and use low range radar to detect if a car is there, waiting...

Answer 3

We have pressure sensors in TN. They are embedded under the concrete or asphalt behind the big white lines. That and timers too.

Answer 4

When you roll up to a light look at the 3 steal sensor Rings on the ground. That is were the sensors electric eye is pointed.

Answer 5

There are "triggers" embedded into the asphalt which sense weight. When sufficient weight is applied, a signal is sent to create the necessary cycle, which vary at each intersection.