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.
I like the idea, I can not say much about the design as it's almost not seen ;) Yet I can determine that You are industrial/embedded guy (experienced) , including I see a lot of commercial-wise design PCB or not just hobby one. But on topic - I like the project, therefore give it a skull, still I'm in a doubt about IF your solution can measure speeding vehicles as it's states, cause the MCU you are using is not very high-perf. , which means not very high freq output, not very high sampling (i.e not low sample time). With just a few words - It's has an idea, the design to me (from what's seen) looks very found , yet I'm not sure the implementation can do what it's meant to do. Regardless I recognize your effort and amazing effort you've put - KUDOS ! :-)
And I wish you luck going forward with your project. I'm not sure I'm going to follow to see the design files , HW, SW , etc. But it's up to you to move forward by accel. the design + making it Open ;-)
Good Luck = )
UPDATE: Well yeah I saw the explanation about the Timers and as expected your FREQ is low, sampling time is high too, but you got results , which is why I'll state again it's promising. Moreover I believe you have 2 options - go for PIC24, dsPIC33 to be in a known ARCH/field. OR switch to some Cortex-M1/M3, cause M0 with fast Operating FREQ and fast enough peripheral. Also you have the ATXMega option which is amazing MCU Architecture, sadly not very used.
Moreover I expected to see HW(IC) based oscilator and again it was there, but if you move to a diff. MCU with more capabilities You can even adapt your Oscillator FREQ and so on. Generated from the MCU is what I mean + gives you capabilities / flexibility. It's not something very hard , cause you have to feed the signal to at least 2 Op-amps OR 1 precisely selected. You can't feed an Op-amp directly from the MCU pin as the current draw will be too much and distort your signal ;) i.e freq. If you do only switching it will be OK, still that implies some more HW/IC analog parts involved.
Last, but not least I assume that's hand-mand/custom COIL . Go for Wireless charging coil ;-) Their range is 110-190kHz and I believe you can work with that frequency looking your description. Moreover you can always optimize code (the firmware) to achieve good sampling times on that freq (if there's a problem, because I don't believe there would be as it's in your device description). So wireless charing coils are good for more than one thing - for more than what they are build ;) #Hackthhem :D
Again Good Luck ! I like your efforts, I like the outcome and what I like even more that this can serve as a good foundation moving forward to a next(gen) Design :-)