License Plate Privacy Act?

There is a bill before the Massachusetts state legislature to prevent the state government from using license plate scanning technology to collect and share license plate information: which cars were where when. The Boston Globe has a perfectly reasonable op-ed arguing in favor of the bill.

The NSA scandals show what happens when we give vast powers to government agencies to spy in the dark: mission creep and the potential for serious abuses of power. We hope that police, who themselves object to being tracked as they go about their workdays, will join with the ACLU to call for sensible legislation to regulate the use of license plate readers and the collection, retention, and data-mining of our sensitive location information.

But there is an important difference from the NSA data. The micro data is inarguably public. Lots of people see lots of cars on the street every day and could notice the license plate numbers if there was any reason to do so. I wonder, therefore, about citizens' ability to collect this information for themselves. People can buy license plate reading cameras and set them up on the side of their houses or on their front lawns.  They can organize themselves to crowdsource to create a national database. Maybe the Boy Scouts can give a merit badge to scouts who tie a hundred plate numbers of cars parked in neighborhood driveways to their owners' names. Can this be outlawed too? If not, do we really want to keep out of police hands a database that the public could assemble for itself? If this kind of data aggregation can be outlawed, how would it be done? Would there be some quantitative threshold, so private citizens could retain small amounts of data but would need a license to collect a lot? What if the database is distributed so nobody has possession of the whole database, but it is possible to query it? In other words, are there certain kinds of information that will in the future have to be treated the way we now treat nitrogen fertilizer – OK to buy, sell, and possess in small quantities, but subject to heavy regulation when handled in bulk?