When I was a student on campus I had to move my car every 90 minutes to avoid being ticketed. That got old fast, so I developed a solution utilizing a Raspberry Pi and a cell phone power bank. To better understand the solution to this problem, it helps to first understand how the ticketing process works.
- Whenever parking enforcement passes by a vehicle in the free parking zone, they simply make a chalk mark on the tire to signify that a countdown has begun on the vehicle.
- The countdown is simply how much time a vehicle can remain parked in the free parking zone, e.g. 90 minutes, without being in violation.
- After the free parking period has elapsed, the parking enforcement returns to the area to look for any vehicles with chalked tires.
- If a vehicle is found with a chalked tire, then the vehicle is in violation and a ticket is issued.
- Parking enforcement is inconsistent, meaning some days you could leave your car for 4 hours in a 90 minute zone and never get a ticket. Other days you could get a ticket for parking 5 minutes over the limit.
- Key takeaway: Parking enforcement must always pass by a vehicle twice in order to issue a ticket.
Now for the secret sauce. All parking enforcement carry a ticket printing device on their belts and these devices all give off a unique and discoverable Bluetooth signature. All the devices in a particular city have the same five letter prefix. Below it is 'XXZFJ', in another city I observed it to be 'XXRAJ'.