Do you see those weird-looking things on the cop car around the lights? Those are surveillance cameras. I noticed this car (car 1272) driving very slowly around. I stopped and asked them about the devices on the roof.The officers told me that the cameras were reading in license-plate numbers and checking to see if any of them were stolen cars, cars recently involved in robberies, etc. I asked if they were looking for some short list of cars in particular or whether it checked against some big database of such cars. The answer is: the latter.
They said there was only one car like this per police district (there are 10 districts in San Francisco). Interestingly, they didn't seem too happy about getting assigned to it that shift - it seems like it's not a very valued job.
From Wikipedia, ANPR, aka ALPR:
Automatic number plate recognition (also called automatic license plate recognition) is a mass surveillance method that uses optical character recognition on images to read the license plates on vehicles.
There you have it in the first sentence: a mass surveillance system. The car is going out not to look for anything in particular (say, a red 2003 four-door Toyota Corolla with a specific license plate), but sucks in many, many license-plate numbers and matches them against a database. That database could include what they say it includes - or, without any notice to us, it could be expanded to include, say, "cars suspected of belonging to undocumented aliens", "cars owned by Arabs", "cars owned by anti-capitalists", or even "cars that have been near two or more anti-war protests". In addition, the data about which license plates were found where may be discarded now, but of course we have no assurance about that, or what might be done with the data in the future.
It would be more efficient for the police to set up surveillance cameras at fixed locations that scan license plates and match them against their databases automatically. But this would probably be met with a certain amount of public opposition. Only by sneaking this in in a small, pilot project as part of routine police patrolling are they able to achieve this type of mass surveillance.
Of course, once they do introduce license-plate scanning at fixed locations - or add it, publicly or not, to the processing at existing fixed surveillance camera locations - they can point out, rightly, that they are simply making the existing system of license-plate scanning more efficient.
That, my friends, is the slippery slope in effect.