So much will depend on the flexibilty that you have in fixing the sensors and the budget allocation and the importance of the job.

The choice of sensors is very wide - IR beams, visible light beams, Inductive sensors ( ruled out by you), In motion weighing systems, Acoustic dopplers and Image recognition systems with CCD cameras. And being a car, thermal imaging is also a possibility.

This is confusing : "temporarily powering it up, look for a car, and then power it down again"

If so what is the trigger for the "look for a car " event ? At this rate you may miss lots of cars!?

Appears interesting.

Raghu