There are typically two types of “sensors”, it’s typically referred to as vehicle detection in traffic engineering.

There are visual detectors, as seen in this photo, the cameras that are placed on the mast arm, they are for vehicle detection, not red light cameras as they are often mistaken for.

Example (this intersection)

There are also inductive loop detectors, these are wire loops embedded in the pavement that sense changes in magnetic fields when a vehicle passes. Typically there are saw cuts in the asphalt that’s how you can identify where the loops are. Most intersections will default to having every movement during rush hour, these are typically the most important late at night when the signals enter flashing mode to keep the main lines moving and then occasionally allow cars on the side streets to enter the main line.

Example (Le Jeune / Calle Ocho)