Inductive Loop Detector works by detecting an inductance change in wire loop (coil) that is buried in road. It does that by measuring the frequency of the internal oscillator which powers the mentioned coil. When a metallic object moves over the coil it changes its inductance and that in turn changes the oscillator frequency which is measured by the microcontroller. It is used for vehicle access control at door and barrier controls, for monitoring the occupancy and for vehicle counting in parking garages, for traffic light installations and traffic controllers, for direction and speed detection of vehicle traffic (if dual channel version is used), in car-wash plants, etc.
Because there is no complete solution to this problem with schematics and firmware available for download anywhere on the Internet. At least I couldn't find one. Please prove me wrong :-)