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Privacy Rights in the Digital Age

License plate reader system

by Paul Riermaier

A technological device typically consisting of a camera that captures images of vehicle license plates, and software that converts the license plate image into machine-readable text. License plate reader (LPR) systems, or automatic license plate readers (ALPRs), are a form of technology increasingly used by law enforcement and private enterprises to detect, capture, and read vehicle license plates. In the law enforcement realm, once license plates are captured and converted into machine-readable text, they are checked against databases known as hot lists that identify vehicles of interest. For instance, a hot list may be a database of stolen vehicles or vehicles licensed to drivers with outstanding warrants for arrest. When an LPR system is mounted on a police cruiser, license plate information can be captured and checked against hot lists nearly instantly, alerting a police officer if a vehicle of interest is nearby. LPR systems have long been used, but their prevalence is increasing among law enforcement, state transportation departments, and private entities, and the information that is captured is being retained longer and used in more ways.

LPR technology was invented in the United Kingdom in 1976. The technology was widely deployed in London in 1997 through the use of fixed cameras capturing license plates of vehicles entering a security cordon that was established in response to terrorist bombings. Use of LPR systems spread in the United Kingdom, leading to the creation of a national data repository, and by 2010, the repository was receiving around 10 million license plate captures daily from 5,000 cameras.

In the United States, law enforcement agencies have readily adopted and deployed LPR systems. There is no comprehensive study showing exactly how many law enforcement agencies use LPR systems, but a number of studies provide a snapshot of the prevalence of their use. For instance, a 2012 Police Executive Research Forum survey of seventy law enforcement agencies, with an average size of 949 sworn officers, showed that 71 percent of agencies used LPR systems. In response to a series of public records requests made in 2012, the American Civil Liberties Union (ACLU) received 26,000 pages of documents from 293 local police departments and state agencies detailing use of LPR systems.

When deployed, LPR systems can be either fixed or mobile. Fixed LPR systems are permanently installed and continuously monitor the same area. Fixed LPR systems are often attached to roadway infrastructure, such as signs or bridges. Mobile LPR systems are mounted to vehicles, typically law- enforcement vehicles, and capture license plate data as the vehicle drives around.

However they are deployed, all LPR systems generally capture an image of the license plate, a record of the machine-readable license plate number, the time the image was captured, and the location of the vehicle when the image was captured, in the form of global positioning system (GPS) coordinates. An LPR system's effectiveness is based on its ability to capture license plate images, to read those images accurately, and to match license plates against the database records in hot lists. A study of LPR systems in the United Kingdom found that fixed LPR systems correctly read captured license plates 95 percent of the time, while mobile LPR systems read plates correctly 85 percent of the time.

Historically, LPR systems were used in conjunction with hot lists to find stolen vehicles, vehicles registered to individuals with outstanding warrants, vehicles associated with drug purchases, or vehicles associated with Amber Alerts. But as LPR systems proliferate and the cost of data storage decreases, information collected from LPR systems is increasingly being retained for lengthy periods of time and, in some cases, aggregated among regional or national partnerships. Privacy advocates are concerned that as LPR systems move beyond their traditional purpose of looking for vehicles included on hot lists and start collecting, storing, and aggregating data on a vehicle as it travels throughout a network of LPR systems, it will be possible to track an individual's movements.

Private entities are also aggregating information collected from LPR systems. Using this information, companies are compiling regional or national databases of information that can be used to track the entirety of a vehicle's movement as captured by LPR systems. Often these databases are made available to companies that repossess vehicles when owners fail to make payments on car loans; the databases are typically also made available to law enforcement agencies.

The increased prevalence of LPR systems and the growing ease of permanently or semipermanently saving and aggregating the information gathered has caused concern among privacy advocates. Privacy advocates' concerns derive in part from the limited regulation of the use of LPR systems. A small number of states have passed laws or enacted rules governing the use of LPR systems, and their treatment varies. For example, in New Hampshire, the law generally prohibits the use of LPR systems but provides a few acceptable uses, including investigation of specific crimes. In California, the California Highway Patrol may collect and store LPR information for sixty days, and it may not sell or otherwise make the information available to private entities. As LPR systems proliferate, it is likely that more state legislation addressing the acceptable use of LPR systems will be enacted and that the courts will have more occasion to review the use of LPR systems.

Further Reading

1 

Crump, Catherine, et al. “You Are Being Tracked: How License Plate Readers Are Being Used to Record Americans' Movements.” American Civil Liberties Union (July 2013).

2 

Dryer, Randy L., and S. Shane Stroud. “Automatic License Plate Readers: An Effective Law Enforcement Tool or Big Brother's Latest Instrument of Mass Surveillance? Some Suggestions for Legislative Action.” Jurimetrics Journal 55, no. 2 (Winter 2015): 225–274.

3 

Newell, Bryce Clayton. “Local Law Enforcement Jumps on the Big Data Bandwagon: Automated License Plate Recognition Systems, Information Privacy, and Access to Government Information.” Maine Law Review 66, (2014): 397–435.

4 

Police Executive Research Forum Staff. “How Are Innovations in Technology Transforming Policing?” Washington, DC: Police Executive Research Forum, January 2012.

5 

Roberts, David J., and Meghann Casanova. “Automated License Plate Recognition Systems: Policy and Operation Guidance for Law Enforcement.” Washington, DC: U.S.

6 

Rushin, Steven. “The Judicial Response to Mass Police Surveillance.” University of Illinois Journal of Law, Technology and Policy 2011, no. 2 (2011): 281–328.

See also: Electronic surveillance; Fourth Amendment to the U.S. Constitution; Law enforcement

Citation Types

MLA 9th
Riermaier, Paul. "License Plate Reader System." Privacy Rights in the Digital Age, edited by Christopher T. Anglim & JD, Salem Press, 2016. Salem Online, online.salempress.com/articleDetails.do?articleName=PRDA_0132.
APA 7th
Riermaier, P. (2016). License plate reader system. In C. Anglim & JD (Ed.), Privacy Rights in the Digital Age. Salem Press. online.salempress.com.
CMOS 17th
Riermaier, Paul. "License Plate Reader System." Edited by Christopher T. Anglim & JD. Privacy Rights in the Digital Age. Hackensack: Salem Press, 2016. Accessed May 30, 2026. online.salempress.com.