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Automated License Plate Readers
To Better Protect Individuals’ Privacy, Law Enforcement Must Increase Its Safeguards for the Data It Collects

Report Number: 2019-118



An automated license plate reader (ALPR) is a camera that captures color images of license plates within its field of view. Fixed cameras are mounted on stationary objects, such as light poles, while mobile cameras are mounted on moving objects, such as patrol cars. Software extracts the license plate numbers from the images and stores the images, plate numbers, and dates, times, and locations of the image captures in a searchable database. An ALPR system consists of the cameras, the software that reads and converts images of license plates into data, and the searchable database that stores the data. Although the primary focus of each image is the license plate, the image may also show part of the vehicle itself, including individuals within the vehicle, depending on the camera’s position. ALPR technology has existed since the 1970s, yet widespread adoption by U.S. law enforcement agencies began only in the mid‑2000s. Law enforcement agencies generally view ALPR technology as a valuable tool in achieving their missions.

We conducted a statewide survey of 391 police and sheriff departments, and the survey confirmed that ALPR use is widespread in California: 230 police and sheriff departments currently use an ALPR system, and 36 plan to use one. Table 1 provides an overview of the ALPR systems of the four law enforcement agencies we reviewed as part of this audit.

Table 1
ALPR Systems of Four Audited Law Enforcement Agencies
Fresno 231 0 8 Vigilant Solutions, LLC 2016
Los Angeles 13,000 3 393 PIPS Technology* 2007
Marin 38 0 3 Vigilant Solutions, LLC 2010
Sacramento 539 33 27 Vigilant Solutions, LLC 2012

Source: Analysis of reports on ALPR systems as of 2019 and the agencies’ survey responses.

* Los Angeles uses PIPS Technology cameras and a user interface from Palantir Technologies, Inc.

An ALPR system is both a real‑time tool for law enforcement agencies and an archive of historical information. After the ALPR system identifies a license plate number in an image, it compares the plate number to stored lists of license plate numbers from vehicles of interest, called hot lists. Figure 1 shows how an ALPR system uses hot lists to search stored images. Local law enforcement agencies create their own hot lists and also obtain hot lists from state and federal agencies. For example, the California Department of Justice (Justice) provides hot lists to local agencies that include license plate numbers associated with missing persons, gang members, and suspected terrorists. We use the term ALPR data to describe all the information stored in an ALPR system, including license plate images and hot lists. Regardless of whether a license plate number matches a plate on a hot list (a hit), an ALPR system stores the plate image in a database, creating a searchable archive. Officers may search the database in various ways. For example, they may search for a full license plate number to locate a specific vehicle, search for a partial license plate number to locate a group of vehicles, or search for all vehicles recorded at a particular location at specific times.

Figure 1
How ALPR Systems Work

A flow chart that shows how an ALPR system works, from the initial image capture to the alert sent to law enforcement if the image generates a hit.

Source: Analysis of David J. Roberts and Meghann Casanova, Automated License Plate Recognition Systems: Policy and Operational Guidance for Law Enforcement, International Association of Chiefs of Police, Washington, D.C., 2012.

Law enforcement agencies can share ALPR data with other public agencies. In the ALPR systems we observed, the agency could choose to share ALPR images only, to share hot lists only, or to share both. Accessing ALPR images shared from other jurisdictions enables agencies to search a broader area, such as across county and state lines. In addition, even if an agency does not operate ALPR cameras itself, it can, through sharing agreements, access ALPR images other agencies collect. Our statewide survey showed that among agencies that operate ALPR systems, roughly 84 percent share their images. Sharing hot lists also enables broader search coverage. For example, an agency could share a hot list that provides license plates linked to wanted individuals with other entities in the region. These entities would then receive hit alerts if their cameras detected those plates.

ALPR Vendors Most Commonly Used in California

Law enforcement agencies typically contract with a third‑party vendor for an ALPR system. In our statewide survey, most—70 percent—of those that have an ALPR system reported using a company called Vigilant Solutions, LLC (Vigilant). Figure A.1 in Appendix A summarizes these responses. Three of the agencies we reviewed—the Fresno Police Department (Fresno), Marin County Sheriff’s Office (Marin), and Sacramento County Sheriff’s Office (Sacramento)—contract with Vigilant. The Vigilant ALPR system provides a user interface to search license plates and the option to share ALPR images and hot lists with other agencies through the Vigilant system. Fresno, Marin, and Sacramento all store their ALPR images on Vigilant’s server, which is a cloud service, and share their images with other agencies that subscribe to Vigilant’s services. Roughly 22 percent of the survey respondents that have ALPR systems use a company called PIPS Technology. One of the agencies we audited in depth, the Los Angeles Police Department (Los Angeles), purchased its cameras from PIPS Technology, but it stores the images on its own server. Los Angeles uses a software platform called Palantir for the user interface that allows for searches of its ALPR images, and it shares its ALPR images with other agencies in the region that use the Palantir user interface.

Key Elements Law Enforcement Agencies Must Include in Their ALPR Usage and Privacy Policy

Source: Analysis of state law.

State Laws Governing ALPR Systems and Data Sharing

With few exceptions, California law requires public agencies that operate and use ALPR systems to implement a usage and privacy policy. The Legislature passed Senate Bill 34 (Statutes of 2015, Chapter 532) (SB 34), effective January 1, 2016, to establish requirements regarding the operation and use of ALPR systems. This law generally requires public agencies, including law enforcement agencies, that operate or use an ALPR system to maintain reasonable security procedures and practices to protect ALPR data, to implement a usage and privacy policy, to make that policy available to the public, and to post that policy on its website should the agency have one, among other provisions. The text box describes required elements of an agency’s ALPR usage and privacy policy.

SB 34 does not specify retention periods for ALPR data, although another state law limits the California Highway Patrol (CHP) to retaining its ALPR images for no more than 60 days, unless those images are being used for felony investigations or as evidence. Agencies implementing ALPR programs after January 1, 2016, must also provide an opportunity for public comment before implementing the program.

In 2018 another state law took effect that limits the information law enforcement agencies can share for immigration enforcement purposes and requires Justice to issue guidance to state and local law enforcement agencies regarding these limitations as they apply to law enforcement databases. In October 2018 Justice issued this guidance, which can also serve as best practices for law enforcement agencies on how to lawfully share ALPR images. The guidance encourages law enforcement agencies that maintain databases to inquire about the purpose for which the other law enforcement agency intends to use the information contained in the database. If a law enforcement agency intends to use the information for immigration enforcement purposes, Justice states that law enforcement agencies should require, as a condition of accessing the database, an agreement that stipulates that access will be made only in cases involving individuals with criminal histories, or for information regarding the immigration or citizenship status of an individual. Beyond this guidance and the hot lists Justice provides to local law enforcement agencies, as we describe earlier, Justice plays no other role in ALPR programs.

State law requires law enforcement agencies to maintain reasonable security procedures and practices to protect ALPR data from unauthorized access, destruction, use, modification, or disclosure. These requirements mean that ALPR data are sensitive. For comparison purposes, the California Department of Technology Office of Information Security defines sensitive data for state agencies as information that requires special precautions to protect it from unauthorized use, access, disclosure, modification, loss, or deletion. In addition to ALPR images and hot lists, a law enforcement agency can enter other information into its ALPR system, such as personal information and criminal justice information. Personal information is information that identifies or describes an individual, including name or physical description. SB 34—whose purpose was, in part, to institute reasonable privacy standards for the operation of ALPR systems—requires that ALPR data be protected with reasonable operational, administrative, technical, and physical safeguards to ensure their confidentiality. Thus, personal information in an ALPR system also requires appropriate and reasonable safeguards. Criminal justice information, as defined by the Criminal Justice Information Services Division (CJIS) of the U.S. Federal Bureau of Investigation (FBI), refers to data necessary for law enforcement and civil agencies to perform their missions. This includes information about vehicles associated with crimes, when accompanied by personal information.

When CJIS provides criminal justice information to law enforcement agencies, it requires those agencies to comply with a minimum set of information technology (IT) security requirements to protect the information, and these requirements can serve as best practices for agencies to follow. Because an agency can enter personal information and criminal justice information into its ALPR system, either as part of a hot list or as a comment added as part of a license plate search, all ALPR data are sensitive and require appropriate safeguards.

Privacy Concerns Related to ALPR Systems

Although law enforcement agencies collect ALPR images in public view, and there is no reasonable expectation of privacy regarding a license plate, the use and retention of those images raises privacy concerns. The agencies we reviewed accumulate a large number of images in their ALPR systems. For example, Sacramento recorded 1.7 million images in one week, and Los Angeles currently has more than 320 million images in its ALPR database that it has accumulated over several years. The majority of these images do not generate hit alerts. For example, data from the Los Angeles system show that at the time of our review only 400,000 (0.1 percent) of the 320 million images Los Angeles has stored generated an immediate match against its hot lists for vehicles associated with car thefts, felonies, or warrants. However, the stored images provide value beyond immediate hit alerts, as law enforcement personnel can search the accumulated images to target the whereabouts of vehicles at particular times or locations. This storage, retention, and searching of the images, although valuable to law enforcement, has the potential to infringe on individuals’ privacy.

Organizations such as the American Civil Liberties Union (ACLU) have criticized law enforcement agencies’ collection of ALPR images because of the risks it poses to privacy. The ACLU stated that increasing numbers of cameras, long data retention periods, and sharing of ALPR images among law enforcement agencies allow agencies to track individuals’ movements in detail, and it has voiced concerns that such constant monitoring can inhibit the exercise of free speech and association. The ACLU has also raised concerns that law enforcement officers could inappropriately monitor the movements of individuals such as ex‑spouses, neighbors, and other associates. There have been occurrences of officers misusing law enforcement databases like those that contain ALPR images. In 2016 the Associated Press conducted a review that found more than 325 instances between 2013 and 2015 in which law enforcement officers who misused databases were fired, suspended, or resigned, and more than 250 instances of reprimands or lesser discipline related to such misuse. For example, the Associated Press reported on a police sergeant in Ohio who pleaded guilty to stalking his ex‑girlfriend after he searched law enforcement databases for personal information about her and also the woman’s mother, her close male friends, and students from a course she taught.

Law enforcement has recognized the privacy concerns posed by the operation of ALPR systems, yet it has also pointed to the usefulness of the systems. For example, the Police Executive Research Forum (police research forum) and the Mesa Police Department (Mesa) in Arizona conducted a study of the effectiveness of ALPR systems for Mesa’s auto theft unit in 2011. They found that officers got nearly three times as many stolen vehicle hits and made about twice as many vehicle recoveries when using an ALPR system, compared to officers performing manual license plate checks. Law enforcement has also found ALPR systems useful for investigations. For example, the assistant chief of the Minneapolis Police Department told the police research forum in 2012 that the department located a vehicle associated with a domestic kidnapping case by searching ALPR images. With regard to the retention of ALPR images, the International Association of Chiefs of Police (chiefs’ association) acknowledged the tension between long retention periods and privacy. The chiefs’ association noted that a reluctance to destroy records may stem from investigators’ experience that seemingly irrelevant or untimely information may acquire new significance as an investigation brings further details to light. However, the chiefs’ association also recognized the privacy risks of ALPR images. In a 2009 report, it stated that mobile ALPR cameras could record license plate numbers of vehicles parked at addiction counseling meetings, doctors’ offices, and staging areas for political protests. The chiefs’ association argued that establishing policies regulating ALPR programs could mitigate privacy concerns, and it produced a report in 2012 offering guidance on developing such policies.

Federal Guidance on Privacy Protection

As far back as 1973, the federal government acknowledged that individuals’ privacy needs to be protected from arbitrary and abusive record‑keeping practices. The U.S. Department of Health, Education, and Welfare, as it was then known, identified principles for the fair collection, use, storage, and dissemination of personal information by electronic information systems. Over time the principles were adapted into information practices. According to the U.S. Government Accountability Office, a revised version of the information practices was published in 1980 by the Organization for Economic Cooperation and Development (OECD)—an international organization that works with governments, policymakers, and citizens on social, economic, and environmental challenges—and with some variation, these practices form the basis of privacy laws in the United States and around the world. The OECD updated its eight information practices in 2013, and California’s lawmakers included many of these information practices in SB 34. For example, the OECD’s information practices describe the importance of an organization specifying the purposes for which it is collecting and using data; keeping data reasonably safe from the risk of unauthorized access, destruction, use, modification, and disclosure; being open about policies involving data; and being accountable for complying with the information practices.

The U.S. Supreme Court (court) has not directly decided a case that we could find addressing ALPR images, although it has decided cases involving other electronic surveillance. Because license plates are in plain view, the collection of license plate images by law enforcement is not a per se violation of the Fourth Amendment’s prohibition against unreasonable searches and seizures. However, the court has found that certain electronic data that reveal individuals’ movements over an extended period of time, if gathered, do at some point impinge on privacy. The court has specifically addressed these issues with respect to the use of global positioning system (GPS) data and cell‑site location information, which is location information linked to cellphone use. Cell‑site location information—similar to ALPR images—provides data on an individual’s continuous movements over a potentially unlimited period of time. In a 2018 case involving cell‑site location information, the court stated that “[a] person does not surrender all [privacy] protections by venturing into the public sphere.” The court continued, “With access to [cell‑site location information], the Government can now travel back in time to retrace a person’s whereabouts,” and noted that the information was collected on everyone, not only “persons who might happen to come under investigation.” Thus, even though case law on electronic data that enable tracking of individuals’ movements over an extended period of time is still evolving, the court has recognized that privacy implications exist for such data, which can include ALPR images.

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