What are the black tar circles in the road at traffic lights? Are they sensors of some kind?

Question

What are the black tar circles in the road at traffic lights? Are they sensors of some kind?

Answer 1

Just writing something so I can come back later as I have always been curious of this too.

Answer 2

Yes, there is a sensor imbedded into the asphalt that detects when a car approaches.  They are called "detector loops" or "induction loops."  They tell the traffic signal when a car is waiting at the red light.

These sensors are inserted into the road by making a cut into the asphalt in the shape of a circle or square, putting the wire down, then covering it with a sealer (which is a slightly different color than the asphalt).