Here is a Loop Detector that we install at complexes /businesses to open their gates when a vehicle wants to exit.
http://www.et.co.za/loop_detector.htmlYou have a few loops of stranded wire buried under the tar, and connected to this detector.
Power up the detector, and it detects which is the best frequency based on the loop.
Then when a vehicle (or spade, or piece of scrapmetal) goes over the loop, the inductance changes, shifting the resonant frequency, and the gate is opened.
You can also put an external trigger in to the controller, so that you have to push a button, which then activates the detector, so that you can basically Authorise the gate to open when the car reaches loop). This will prevent a shopper from just driving your car out. They first have to 'ask' you to activate the loop detector, so that they can take your car. Of course if this happens, they will either take you with them, or use your remote, or use your car as a heavy duty gate opener.
You can see I am South African.....