Vehicle tracking and direction processing
If you can alter the road then a pair of inductive loops in the road surface will do.

They will each act basically like a metal detector. And most cars have a half ton of metal in the engine bay and transmission box.

If you can't do that then a set of pneumatic tubes strung along the road surface is a decent alternative. As the car driver over them the pressure will increase momentarily and you can log that. They are prone to damage and need replacing regularly but the tubes themselves are cheap enough.