When it comes to large-scale, untargeted, collection and retention by traffic cameras, etc. then it may come into the realm of what is "reasonable."
The data something like this collects is so vast that it actually encroaches into the parts of your life you would reasonably expect to be private. For instance, you could determine that I'm seeing a psychiatrist, that I'm cheating on my wife or that I have cancer. That's private information you don't expect to be exposed by simply driving around.
To push the limit of the broad interpretation of your public data, consider the following scenario. Imagine in the not-to-distant future that a technology exists that allows people to scan, from a distance, the electromagnetic field produced by your brain and use that data to map your thoughts and memories. Would me simply stepping into public be a tacit agreement between me and the government to forfeit that privacy?
I think people definitely do have a reasonable expectation that they aren't being followed everywhere they travel, and if you framed it in that way many would see it as a violation of privacy.
It may be true that, in the US, one cannot reasonably expect privacy in the various instants of their public existence, but when you aggregate those instants into a timeline then a reasonable person very much does have an expectation of privacy (otherwise stalking would be perfectly acceptable in society).
Outside the US, I would not be surprised if other cultures have developed a much higher expectation of privacy (or at least discretion and confidentiality) than the US.
Taking the point a bit farther - we have no expectation of privacy for the exterior of our homes. Does this, then, make it OK for the government to use audio recording devices that can pick up sound waves based on the vibrations of the home's panes of glass (because this can actually be done in some cases)? Similarly, the exterior IR signature of our home is theoretically in public view, but should we let the government monitor which rooms are occupied based on changes in the heat signature of the walls?
The collection of GPS records from your phone is still quite legal and this would be the license plate scanning equivalent.
Even in the absence of a trespass, however, Supreme Court precedence recognizes a search when the government violates a subjective expectation of privacy that society recognizes as reasonable.
So, if the Government monitors your location by enlisting factory or owner-installed vehicle tracking devices or GPS-enabled smartphones, that could well be a search, but it's still a fuzzy and undecided area of the law.
Sotomayor's concurring opinion in U.S. v. Jones is well worth a read if you are interested in this sort of thing.
"[Location data] generates a precise, comprehensive record of a person’s public movements that reflects a wealth of detail about her familial, political, professional, religious, and sexual associations ... trips to the psychiatrist, the plastic surgeon, the abortion clinic, the AIDS treatment center, the strip club, the criminal defense attorney, the by-the-hour motel, the union meeting, the mosque, synagogue or church, the gay bar and on and on. The Government can store such recordsand efficiently mine them for information years into the future. And because GPS monitoring is cheapin comparison to conventional surveillance techniques and,by design, proceeds surreptitiously, it evades the ordinary checks that constrain abusive law enforcement practices: limited police resources and community hostility.
"Awareness that the Government may be watching chills associational and expressive freedoms. And the Government’s unrestrained power to assemble data that reveal private aspects of identity is susceptible to abuse. The net result is that [location tracking] making available at a relatively low cost such a substantial quantum of intimate information about any person whom the Government, in its unfettered discretion, chooses to track may alter the relationship between citizen and government in a way that is inimical to democratic society.
The real reason this is probably allowable is that you have no expectation of privacy while driving around. People see you. Cops can tail you everywhere you go.
What we really need is some sort of anti dragnet amendment.
I would feel better about this if the license plate readers simply pinged law enforcement on a real-time match. The storage and potential data mining aspects are where it crosses the line. Stasi stuff there.
The system should be unable (by design) to store information on plates without warrant.. as you mention, in terms of a ping... No historical logging of data outside that scope.
Cops are able to start following someone... they aren't able to go back in time and start following someone three weeks ago.
If I go shoot up a theatre, the police would allowed to put an active trace on me and query their database.
They wouldn't be allowed to just search for drug users by investigating who goes near drug corners.
I find it interesting that the supreme court recently ruled that GPS trackers are a form of search. I suspect that in a few years this will go before them, but given the differences (namely that nothing is attached to your vehicle) I'm a lot less confident that they'll rule against it. I REALLY hope they will, but I'm a bit doubtful.