But it’s not just police who use automatic license-plate reading technology. Cameras like these, which usually cost between 20 and 30 thousand dollars, are used to process fees on toll roads across the country, keep track of customers in parking lots and garages, and trawl city streets for cars whose owners are behind on payments and flag the vehicles for repossession.

The systems have proved useful to their public and private buyers, and the technology is proliferating. Recent statistics about their use are hard to come by, but a survey of police agencies conducted in 2011 showed that 71 percent of departments used license-plate readers, and that 85 percent of departments planned to increase their use over the next five years.

But police don’t have to do all the work themselves. A company called Vigilant Solutions claimed in a 2015 press release to maintain “the largest commercially gathered LPR dataset available to U.S. law enforcement.” That dataset allows police to access private license-plate scans, but does not allow law enforcement to share their data with companies.

In 2015, Vigilant said its dataset contained more than 3 billion scans, and was growing at a rate of more than 100 million scans a month. Since then, the dataset has grown dramatically: A Vigilant spokesperson said Friday that the dataset now includes 4.2 billion sightings, and is growing at a rate of 120 million data points a month.

As more vehicles and utility poles are outfitted with plate readers, and databases like Vigilant’s swell at increasing rates, some groups might feel the effects of the increased scrutiny more than others.

Data about the actual use of of license-plate readers is thin, but two examples stand out. In a 2014 investigation into automatic plate readers for The Boston Globe, Shawn Musgrave found at least ten repossession companies in Massachusetts that used license-plate readers to do their job. (Todd Hodnett, the director of government affairs for Vigilant Solutions, says he estimates about one in four repo companies nationally operate license-plate readers.)

And with 200 to 400-dollar bounties for locating cars that were stolen or are in default, some of those companies focused their search on the most lucrative neighborhoods. Two Massachusetts companies told Musgrave that they expressly targeted low-income housing developments, since it’s likely that a disproportionate number of residents in those areas are behind on auto payments, their cars ripe for repossession.

Police, too, have used license-plate readers heavily in low-income areas. The Electronic Frontier Foundation submitted a request in 2014 for information about the Oakland Police Department’s use of license-plate readers. When the advocacy organization analyzed the data it got back, it found that the readers were deployed disproportionately often in low-income areas and in neighborhoods with high concentrations of African-American and Latino residents.