The Parking Pass Garage Attendant started out for me as a project a buddy Chris of mine did for his garage. I made it first for a roommate who couldn't seem to keep it within the walls in a single garage, and it worked out great. I noticed that the sound bounced off the wind shield sometimes, and realized this is probably why Chris went with a 2 part system, however still worked well good enough to stop my roommate from hitting the walls.
The Parking Pass is designed around the Ping))) sensor, which uses sound chirp to detect objects, but listening to the sound reflecting off objects. The chirp is a specific frequency, so it's easier to pick up that just a background sound. There is a chirp created, and then listens for that chirps reflection and then determines where something is, just like echo location.
There is an interesting thing about working with sound waves, they can be deflected by smooth objects; easily concluded is that a windshield isn't the best part of a vehicle to use to bounce a sound off of. The front of anything is pretty great to use, the bumper is solid as well if you want to use a 2-part setup or use another indication method. Through testing, the unit worked well, but I had only programmed one setting, I could use another method to create a way to ID each car or use a sweep to detect better. I like that last option, attach a servo and sweep for cars. Keeping it small and sleek is the key and still easily to install and set.