What is the principle of vehicle detector?

What I know can be divided into two types: microwave car detector and coil car detector. The principle of coil car detector is very simple: the conductor (car) cuts the magnetic induction line, which produces the change of magnetic flux, thus producing the induced current, and the car detector amplifies the induced current through the relay to produce the gravity detection car. The price is cheap, but there is a certain construction period because of the broken road and the laying of coils.

As a newcomer, I don't know much about this aspect, but I know a little about the principle of Smartsensor 125. This is a dual radar vehicle detector, which simply means projecting two detection areas on the detection road surface, which is equivalent to placing two coils on the detection road surface. Vehicles passing through the detection area will give a microwave signal to 125, and relevant data will be obtained through data processing. The price is more expensive, so it is necessary to set up columns, but pavement construction is not needed.