I don't have a problem with agencies gathering data by camera and LP scanners. The question is how they use it and how long they keep the data. If it's stored in raw form, to be searched later as part of an investigation, then keeping it for a week or month seems reasonable, but not any longer. OTOH, if they mine and/or collate the data by license plate or whatever, then I'd insist on a legitimate government purpose, or that all such files are destroyed within 24hours, except if there's a live criminal investigation.

The problem isn't as much that the police will misuse the date, they have enough to do. But the existence of this data means it's open to mining by third parties, or could be obtained by subpoena for use in things like divorce.

In any case, with greater ability to gather and save all manner of data, we've reached (or passed) the point where we need a national dialog to establish policies protecting privacy at all levels.