Best way for identifying cars in parking bays

I want to be able to identify a car in a roofed parking bay similar to the image below

I have thought of two ways but I'm not sure which would be the best.

The first option is using passive UHF RFID. I feel like this would be a very robust system, and require very little maintenance. The biggest problem with this is UHF RFID readers are very expensive and the project would require 20+ of these.

The second option is using QR codes read by a camera mounted on the roof. This option is less expensive as the Arduino cameras are quite cheap, but I feel like it wont be as robust and require a lot more maintenance.

What would you suggest?

Do you mean "identify", or simply "detect"?

Identify. I want to know which car is within this parking bay.

I think any radio -based solution will have to first solve out the interference issues from cars near the spot.

Another point is data transmission, if needed.

A very uncommon solution would be to use a color detector + a distance detector, assuming that rarely the color of the car and length of the car will coincide for two distinct cars. So, set these two values for each place, assign it to a certain car and set an alarm to sound if another car park there.

Well, in this option, a grey car cannot be differentiated from a big grey elephant, if it happens to be of the same length. :-).

I think beacons is a reliable yet cheap solution.

It's possible to use a camera with computer vision to read the license plate of cars. That's nice because it doesn't require anything special on the cars. You would need to work out camera angles that have a good line of sight on the license plates.

Another thing to consider is the consequences of misidentification.

Yep.... this might require camera and machine learning to truly recognise cars of various kinds. Preferrably advanced learning where the machine can recognise a car in the way that a human does.

As for qr codes and/or rfid.... the issue might be not every car will have it.... unless all are required to have it in that car park.

Local council here installed some detector/sensors a while back as a trial.
Idea was to map time parked and identify vehicle.
As far as I could make out it was a device about the size of a small downlight and they anchored it into the bitumen surface.
No idea how it works but bound to be some info out in Google somewhere.
Maybe takes a snapshot of the number plate.