Vehicle detection loops, called inductive-loop traffic detectors, can detect vehicles passing or arriving at a certain point, for instance approaching a traffic light or in motorway traffic. An insulated, electrically conducting loop is installed in the pavement. The electronics unit applies alternating current electrical energy onto the wire loops at frequencies between 10 kHz to 200 kHz, depending on the model. The inductive-loop system behaves as a tuned electrical circuit in which the loop wire and lead-in cable are the inductive elements. When a vehicle passes over the loop or is stopped within the loop, some of the vehicle’s ferrous body material increases the loop’s inductance, in the same principle as including a metal core within a solenoid coil. However, the peripheral metal of the vehicle has an opposite effect on the inductance due to eddy currents that are produced. The decrease in inductance from the eddy currents more than offsets the increase from the ferrous mass of the engine, and the net effect is an overall reduction in the inductance of the wire loop. The decrease in inductance tends to decrease the electrical impedance of the wire to alternating current. The decrease in impedance actuates the electronics unit output relay or solid-state optically isolated output, which sends a pulse to the traffic signal controller signifying the passage or presence of a vehicle. Parking structures for automobiles may use inductive loops to track traffic (occupancy) in and out or may be used by access gates or ticketing systems to detect vehicles while others use parking guidance and information systems. Railways may use an induction loop to detect the passage of trains past a given point, as an electronic treadle.
From the Wikipedia page: https://en.wikipedia.org/wiki/Induction_loop
The relatively crude nature of the loop’s structure means that only metal masses above a certain size are capable of triggering the relay. This is good in that the loop does not thus produce very many “false positive” triggers (say, for example, by a pedestrian crossing the loop with a pocket full of loose metal change) but it sometimes also means that bicycles, scooters, and motorcycles stopped at such intersections may never be detected by them (and therefore risk being ignored by the switch/signal). Most loops can be adjusted manually to consistently detect the presence of scooters and motorcycles at the least.