Full schematics and firmware for microcontroller can be found on project page here:
http://www.elektronika.ba/843/inductive-loop-vehicle-detector-v2-1/
PCBs: http://www.elecrow.com/inductive-loop-vehicle-detector-v21-p-1458.html
Inductive Loop Detector works by detecting an inductance change in wire loop (coil) that is buried in pavement.