sensor for illegal parking detection
Don't get discouraged, there are many ways to do this, but I would rate a PIR as being one of the worst options because even a bird or a squirrel could set it off. Plus, once the car has stopped you can't detect it's presence anymore with a PIR sensor, it has to be moving. I would recommend either using something like a webcam and OpenCV for this or if you're set on using an Arduino and physical sensors, multiple laser or multiple ultrasonic sensors placed 5-10 feet apart so that they both have to trip at once for positive detection of a vehicle instead of something else. You're going to have to spend some time experimenting and tuning. If you want something very reliable and you own the place, you could always do what the public street works does and cut a loop into the pavement and install a large steel cable and use inductance, but that may be overkill. ;)

Some other ideas:


Paint a pattern on the ground using high contrast paint and detect
when the pattern is completely covered.  Maybe there is something you
can do with IR reflective paint and use a camera with no IR filter,
this way you could make a pattern invisible to the eyes.
A carbon monoxide detector - Detects dinosaur powered vehicles, but
is nice to the environmentally conscious.


About a decade ago now I created my own parking detection using a Raspberry Pi and a webcam and some Perl code I wrote (non-opencv). I basically just detected the shadow contrast under the car and it worked very well. Originally I tried detecting red colors in the tail lights, but this was more troublesome due to other lighting in the environment. Especially at night. In my case, I didn't have direct access to set up physical sensors and lasers would have been too intrusive, so visual analysis made sense.