HI,
I developed many applications on car traffic evaluation/control. Normally we used:
- magnetic field under road
- infrared switch
- camera
- doppler
We used this in order to avoid that the sensor does not the target because the car pass far from the sensor itself.
Clearly any sensor presents advantages and disvantages (precision, require to work on the road, cost, whether problem...)
In any case no problem to design it for you.