I need to be able to detect a parked car in the street, so there's two critical things to think about, one is the position of the sensor and the second is the sensor itself.
The first thing i thought is to put an ultrasonic sensor in the pavement, but the problem with this is that this sensor has to be exposed to rain and an ultrasonic sensor is not water resistant...is it?
So the question is: what other kind of sensor might work to detect a car?, maybe a metal detector?
My first question would be why do you want to detect it?
also is the car always parked in the same spot?
How will you get cabling to the parking spot or sensor location?
Ground loops are probably the most common method of vehicle detection, most traffic lights use them to determine traffic flow, carparks use them for everything from signaling car coming warnings to holding barrier arms up until the vehicle has passed.
I have also seen systems that detect if a door is opened even if the vehicle does not move (used in truckstops to detect if a vehicle gets broken into)