Count car weels without using visible light (camera/laser etc) and without interaction
Three ideas spring to mind:


Doppler radar should work well; one can pretty reliably detect the microdoppler caused by the rotation of wheels with a directional antenna (and preferably a high frequency microwave radar)
acoustics should be pretty reliable, too, given a bit of signal processing
vibrational analysis (which is essentially, acoustics) on the concrete should work just as well.


Other than that, a simple photoelectric through-beam sensor would work; with a rain / dirt shield, that should be pretty reliable too. I don't fully buy "light can't work in dirt", as that's a kind of sensing you find in industrial environments extremely often.