We have one and I drive it occasionally, of course I'm in the auto theft unit, so that's what it's for. It has other capabilities, but the tin foil factor is high here. For starters, it takes a picture of the front of the car, and the driver is not visible; the top of the picture is below the windshield, so it snaps a picture of the car, NOT THE DRIVER. We have a PIPS/Federal Signal system, and it's comparable to others out there.
All it does is look for a license plate shaped object in it's field of vision and converts the picture into an alpha-numeric character set, then compares it to a database that we load into the computer. Because it's an official piece of equipment and takes pictures, we cannot legally delete the pictures because it's now a public record, and subject to public records laws. I think we have to keep the pictures for 5 years.
Our purpose for having this car is to find stolen cars, and it's worked well for that purpose. Because criminals also drive, we can search for known vehicles involved in felonies, amber alerts, etc.... There are static cameras at some major shipping ports to scan vehicles going in/out, scanning for stolens, cars on terrorist watch lists, etc... It can also be used 'after the fact' to check alibis and to search a specified area during a specified time frame. There are a lot of possibilities, but basically this system to remember where it saw a CAR, not a PERSON.