According to a recent Governing article, six states (Arkansas, Maine, Maryland, New Hampshire, Utah, and Vermont) have enacted laws that restrict or prohibit police from using license plate reader technology.
These automated license plate readers are mounted either on a police car or on a fixed position (e.g., bridge). The technology allows police to check tens of thousands of plates in an hour, which a 2010 study by the Center for Evidence-Based Crime Policy showed increased arrests for car theft and deterred people from stealing vehicles.
Under good conditions, the readers are accurate over 90% of the time, but under less than ideal conditions (e.g., bad weather) the percentage drops to below 80%. This can result in false matches.
But critics of the readers, including the American Civil Liberties Union, have raised privacy concerns, since the technology allows police to track any car. According to the Governing article, the ACLU is concerned with the lack of rules about reader use, as well as data retention periods, with police departments keeping data anywhere from 48 hours to five years.