Sensor for tracting a car over it

What senson would be the best to have for a device in the ground somewhere to track how many cars they pass over it? Any suggestions to try?
I would preffer to be something magnetic, is it possible?

thank you!

There was a superb and extensive discussion of sensing cars entering a parking garage in the old forum. Lots of good ideas there.

link for it? i can't find anything.

The semaphores does that, a wire loop in the road is used to sense if a car is above it or not, or if a car passed it.

can you provide any information(or link) about the wire loop. How big must it be, can it be power from the arduino or i must build a different circuit.