The scanners, which automatically grab images of any plates they identify, are an often-overlooked layer of the surveillance that blankets Americans, along with social media, online searches, mobile-phone apps and credit-card purchases. Private companies feed snapshots of plate numbers—including time, date and location—then share that information with police departments, which are largely free to access it at their discretion.

The scope of these data pools is difficult to measure. But Vigilant Solutions, a division of Motorola Solutions Inc. and one of the largest private vendors of data and scanners, boasted a decade ago that it had 450 million plate scans in its commercial database, with 35 million new plates added each month…

Mistakes can be consequential. In August 2020, police in Aurora, Colo., got a match on a license plate belonging to a vehicle reported as stolen. They approached the car with weapons drawn and ordered a Black family—-with children ages six, 12, 14 and 17—to lay on the pavement. Video of the confrontation caused nationwide outrage and drew an apology from the city’s police chief, who said later the family, driving a minivan registered in Colorado, happened to have the same plate number as a stolen motorcycle with Montana plates.