An automated license plate reader — ALPR for short — is a camera plus software. The camera photographs vehicles as they pass. The software reads the plate characters, stamps the image with a time and a location, and writes the whole thing to a database.
That is the entire trick. There is no magic. What makes ALPRs interesting is not any single photo but the pile of photos: a record of which cars were where, and when, accumulated automatically, at every intersection with a pole.
Vendors and police departments generally describe these systems as investigative tools — a way to find a stolen car, or a vehicle connected to a specific crime, without staffing every corner. Civil liberties groups generally describe the same systems as bulk location tracking of people who are not suspected of anything, retained by default.
Both of those descriptions can be accurate at the same time, which is exactly why the argument does not resolve itself. The details that actually decide the question are boring and local: how long is data retained, who can search it, is a case number required, who else gets a copy, and who audits the searches.
That's why this site is mostly going to be about paperwork. The paperwork is where the policy actually lives.