You want to cense cars and not animals.
1.)Place a rubber hose across the road. Plug the far end. Close end has a pressure censor. The hose should not be airtight, it needs a small hole [leak]. When a car drives over the hose, the air pressure changes.
2.) I have a commercial device. There is a “coil” in a PVC tube buried under the road. The coil oscillates. The frequency is not stable, it slowly drifts over time. When a car passes over the coil the frequency changes fast. It is built on a ‘metal detector’ idea.
3.) I now am playing with flux-gates for another projects. It appears I can detect large metal objects 20 feet away. Too complex but fun.

I can supply details if you need it.