In Dayton, Ohio, black trash bags now flap over license plate readers. Police there aren’t sure the cameras have stopped working. City officials don’t know if they can simply take them down under their contract. So they improvised. Low-tech barriers against a high-tech system that has quietly spread across thousands of communities. This scene repeats elsewhere. Evanston, Illinois, did the same last year. Cities cancel contracts with Flock Safety yet find themselves tangled in agreements that make swift removal difficult. The bags signal deeper frustration. They mark growing alarm over a private firm that collects vehicle data on a massive scale, stores it in its own cloud, and makes that information searchable by law enforcement nationwide. Flock Safety operates thousands of automated license plate readers. The company pulls readings into centralized servers. Even small-town departments gain access to a vast database of movements.…