alt Hacker NewsIf your police force installed officers in parked cruisers at every intersection to record license plate numbers and then digitized that information so you can build up a dossier of times and locations for a given individual without probable cause that would be both illegal and a huge waste of resources. ALPR just do it at scale so it's cheaper; still illegal without cause.
The legality stationing officers to record plates and building a DB is not settled law, and my interpretation is that it's more likely legal than not. Observing things in public is normally legal, and the extent to which scale changes this is very much to be seen.