The Role of Police Car License Plate Scanners in Modern Law Enforcement

Automated license plate readers (ALPRs), often referred to as police car license plate scanners when mounted on vehicles, are high-speed camera systems revolutionizing law enforcement. These computer-controlled devices are not limited to police cars; they can also be found on street poles, traffic lights, highway overpasses, and mobile trailers. Their primary function is to automatically capture license plate numbers of every vehicle that comes into their field of view, along with crucial data such as location, date, and time. This information, frequently including photographs of the vehicle and sometimes even occupants, is then instantly uploaded to a central server for analysis and storage.

Vendors of these systems highlight their utility in aiding police investigations. By accessing the collected data, law enforcement can trace a vehicle’s past locations, determine its presence at crime scenes, analyze travel patterns, and identify vehicles that might be connected. The ability for law enforcement agencies to share this vast network of information across thousands of other agencies significantly amplifies its potential impact.

However, the extensive data collection by police car license plate scanners raises significant privacy concerns. When aggregated, ALPR data can create a detailed and intimate picture of a driver’s life, potentially chilling activities protected by the First Amendment. The technology could inadvertently target individuals visiting sensitive locations like healthcare facilities, immigration support centers, gun stores, union offices, protest sites, or places of worship.

Despite the privacy implications, drivers have no choice but to display license plates, a government mandate for all vehicles operating in public. This makes the widespread use of automatic license plate readers to track the movements of millions of ordinary citizens, the vast majority of whom are not suspected of any wrongdoing, a particularly concerning issue in the modern age of surveillance.

Understanding How Police Car License Plate Scanner Systems Operate

Automated license plate reader systems, including police car license plate scanners, can be categorized into three main types, each serving distinct operational purposes.

Stationary or Fixed ALPR Cameras

Alt text: Stationary police license plate scanner cameras mounted on traffic light pole, monitoring vehicle traffic.

Stationary ALPR cameras are permanently installed in specific locations. These fixed points can include traffic signals, utility poles, facility entrances, or highway exit ramps. These cameras are designed to capture license plates of vehicles in motion as they pass within their field of view.

Deploying multiple stationary ALPR cameras along a roadway enables the tracking of vehicle direction and speed. With a network of these cameras, law enforcement can achieve real-time vehicle tracking. Furthermore, storing the collected data over time allows authorities to review a vehicle’s historical presence at specific locations, potentially inferring residential or workplace proximity. Smaller municipalities sometimes utilize ALPR cameras at town entrances and exits, effectively creating a digital perimeter where every vehicle entering or leaving is recorded. In some instances, police may camouflage ALPRs as ordinary objects like traffic cones or even cacti to maintain discreet surveillance.

These stationary ALPR systems are frequently integrated with automated red-light and speed enforcement systems and are also employed for electronic toll collection on roads and bridges.

Mobile ALPR Cameras: Police Car License Plate Scanners

Alt text: Mobile police car license plate scanner mounted on top of patrol vehicle, capturing license plates while on patrol.

Mobile ALPRs, the quintessential police car license plate scanners, are typically mounted on police patrol vehicles. These mobile units allow law enforcement officers to collect license plate data as they patrol city streets throughout their shifts. In most cases, these systems are activated at the beginning of a shift and remain operational until the shift concludes, continuously scanning and recording.

Beyond capturing images of moving vehicles, mobile ALPR cameras are particularly effective at recording license plates of parked cars. For instance, a patrol car equipped with a police car license plate scanner can drive through a public parking lot and capture hundreds of license plates in just minutes, creating a comprehensive record of vehicles present.

Law enforcement also employs police car license plate scanners for a practice known as “gridding.” This involves systematically driving an ALPR-equipped vehicle along every block of a neighborhood to gather intelligence on residents and vehicle presence.

Private companies like Vigilant Solutions also utilize mobile ALPRs to collect license plate data, which they then sell to law enforcement agencies and other interested parties, creating a commercial market for vehicle location data.

ALPR Trailers

Alt text: Police ALPR trailer unit deployed for temporary license plate scanning and data collection in specific areas.

ALPR technology is also available in trailer form, providing a mobile and temporary surveillance solution. Police can tow these ALPR trailers to specific locations and deploy them for extended periods. These trailer-based systems operate similarly to fixed ALPRs, collecting data without requiring permanent camera installations. The Drug Enforcement Administration (DEA) has previously used these systems, disguised as speed enforcement trailers, to monitor vehicles in areas along the U.S.-Mexico border. Furthermore, trailers and vehicles equipped with ALPRs have been strategically parked by police near gun shows and political rallies to monitor attendees and collect vehicle data.

ALPR Databases: The Repository of Scanned Data

A crucial component of ALPR systems is the database where collected data is stored. Most ALPR data is retained in these databases for extended durations, often up to five years. While some databases are maintained by police departments themselves, many are managed by private companies like Vigilant Solutions or Flock Safety. Law enforcement agencies that do not operate their own ALPR systems can access data collected by other agencies through regional sharing networks and platforms managed by these private companies. Notably, several companies operate independent, non-law enforcement ALPR databases, contracting with individuals to install cameras on private vehicles for data collection. This commercially gathered data is then sold to various entities, including insurers, and is also available for subscription by law enforcement agencies.

Hotlists: Targeted License Plate Surveillance

Law enforcement agencies frequently pre-load “hotlists” into ALPR systems. These lists contain license plate numbers of vehicles of interest, such as stolen vehicles or vehicles associated with outstanding warrants. Police officers can also create their own hotlists for specific investigations. When a police car license plate scanner or any ALPR camera scans a plate that matches an entry on the hotlist, the system immediately sends an alert to the officer in the patrol car (for mobile readers) or to the relevant agency (for fixed readers). Some hotlists include plates associated with low-level misdemeanors and even traffic violations. In some cases, agencies utilize these hotlists to generate revenue by targeting and stopping individuals with outstanding citations.

Types of Data Collected by Police Car License Plate Scanners

Police car license plate scanners and other ALPR systems collect a range of data beyond just license plate numbers. Along with the plate number, the systems record location data and the precise date and time of each plate encounter. Advanced systems can also capture the make and model of the vehicle. These systems are incredibly efficient, capable of scanning thousands of plates per minute. One vendor boasts a dataset exceeding 6.5 billion scans, growing by an astonishing 120 million data points each month, highlighting the sheer volume of data being collected.

By combining data points, ALPR systems can determine the direction and speed of travel through triangulation. Over time, the accumulated data paints a comprehensive picture of a vehicle’s travel history. Algorithms applied to this data can reveal regular travel patterns and even predict future locations of a driver. The data also exposes all vehicles that have visited a particular location, offering detailed insights into movement patterns.

While the data typically does not include the driver’s name directly, law enforcement officers can utilize other databases to link license plate numbers to individual identities.

In addition to license plate data, the photographs captured by ALPR systems can reveal images of the vehicle itself, its drivers and passengers, and the surrounding environment. Some systems even create “vehicle fingerprints” that include details such as vehicle color, make, model, physical damage, and bumper stickers, further enriching the data collected.

Law Enforcement Applications of ALPR Technology

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A time-lapse visualization of the data collected by Oakland Police Department vehicles mounted with license plate readers.

ALPR data collection, including that from police car license plate scanners, is indiscriminate, gathering information on millions of ordinary individuals. By mapping vehicle times and locations and tracking past movements, police can use stored data to construct detailed profiles of drivers’ lives. This allows them to identify past behavior patterns and potentially even predict future actions, even though the vast majority of individuals whose data is collected are not suspected of any crime. Before ALPR technology, officers had to manually record license plates, limiting the volume of data collected and requiring them to make choices about which vehicles to track. ALPR technology removes these limitations, enabling the tracking of everyone and facilitating faster, broader data collection with reduced staffing needs.

A 2021 report by the EFF analyzing data from 63 California law enforcement agencies found that a mere 0.05% of the data collected by ALPRs was relevant to a public safety interest at the time of capture, underscoring the vast scale of indiscriminate data collection.

Law enforcement primarily uses license plate readers, including police car license plate scanners, for two main purposes:

Real-Time Investigations

By adding a license plate to a hotlist, officers can utilize ALPR systems for real-time identification and tracking of specific vehicles. License plates are commonly added to hotlists for reasons such as vehicle theft or association with outstanding warrants. Officers may also add plates if a vehicle has been observed at a crime scene, if the owner is a suspect, or if the vehicle is linked to gang activity. However, it is important to note that hotlists often include vehicles associated with low-level offenses as well.

Historical Investigations

Since ALPRs, including police car license plate scanners, collect data on virtually all vehicles, not just those on hotlists, officers can search and analyze historical data using a plate number, partial plate, or physical address. For example, in the event of a convenience store robbery, an officer could input the store’s location into the system to identify vehicles present nearby at the time of the incident. They could then further investigate those plate numbers to uncover other locations where the same plates have been recorded, potentially establishing patterns or connections.

It is important to note that training materials, policies, and laws in some jurisdictions advise officers that a hotlist alert alone may not justify a traffic stop. Officers are typically instructed to visually confirm the plate number match. Failures in manual confirmation, combined with machine errors, have led to wrongful stops and detentions.

While law enforcement emphasizes the use of ALPR data in recovering stolen vehicles or locating abducted children, it has also been used for mass enforcement of less serious offenses, such as identifying uninsured vehicles or tracking individuals with overdue court fees, highlighting the scope of its application beyond serious crime.

The data retention period for ALPR data varies significantly across agencies, ranging from just a few days to several years. Furthermore, private companies may retain the data indefinitely, raising long-term privacy concerns.

Vendors of ALPR Technology

Vigilant Solutions (a subsidiary of Motorola Solutions) and Flock Safety are leading vendors of ALPR technology in the United States, providing systems used in police car license plate scanners and other ALPR deployments. Other notable vendors in the market include Rekor, Elsag, Axon, Perceptics, and Jenoptik.

Vigilant Solutions, through its sister company Digital Recognition Network, gains access to privately collected data through partnerships with repossession companies. These companies passively collect ALPR data using their own vehicles, which is then made available through Vigilant Solutions. Similarly, Flock Safety has established partnerships with numerous homeowners associations, obtaining data from neighborhood ALPR systems that is then provided to law enforcement. Both Vigilant Solutions and Flock Safety offer data-sharing capabilities, enabling law enforcement agencies to share data across the country, creating a vast interconnected surveillance network.

Privacy and Civil Liberty Threats Posed by ALPRs

ALPR technology, including police car license plate scanners, is a powerful surveillance tool with the potential to significantly infringe upon individual privacy and violate the rights of entire communities.

Instances of abuse of this technology by law enforcement agencies have been documented. For example, in New York, police officers systematically recorded license plate numbers of vehicles parked near a mosque. In Birmingham, UK, police disproportionately targeted a Muslim community with CCTV surveillance while misleading the public about the project’s scope and intent. Data obtained from the Oakland Police Department by the EFF revealed that ALPR-equipped vehicles were disproportionately deployed in low-income communities and communities of color, raising concerns about biased surveillance practices.

Furthermore, individual officers have misused law enforcement databases, including license plate information and motor vehicle department records, for personal reasons. In one case, a Washington, D.C. police officer was convicted of extortion after using license plate data to blackmail vehicle owners parked near a gay bar. More recently, a police officer in Kechi, Kansas, was arrested for allegedly accessing a Flock Safety ALPR database to stalk his estranged wife, demonstrating the potential for misuse even with systems designed for law enforcement purposes.

Beyond deliberate misuse, ALPR systems are not infallible and can misread license plates, leading to serious and unjust consequences. In a notable case in San Francisco, a woman was wrongly pulled over at gunpoint due to a license plate reader error. In another incident in Aurora, Colorado, a group of Black youths were traumatized after police incorrectly identified their vehicle as stolen based on ALPR data. These incidents highlight the risks of relying solely on ALPR technology without human verification and the potential for technology-driven errors to disproportionately impact marginalized communities.

The long-term storage of aggregated ALPR data exacerbates privacy risks, making it more invasive and vulnerable to misuse and data breaches. Even Customs & Border Protection, a well-resourced federal agency, experienced a data breach at its ALPR vendor, Perceptics, resulting in the online publication of sensitive data. Implementing sensible data retention limits, clear policies on data access within agencies, and robust audit and control processes are crucial steps to mitigate these issues. Ideally, a strong privacy protection measure would be for police to retain no data at all when a scanned vehicle does not match a hotlist, minimizing the collection of data on law-abiding citizens.

Moreover, ALPR technology, including police car license plate scanners, can be used to target vulnerable populations such as immigrant communities and individuals seeking or providing reproductive health services, raising concerns about the weaponization of surveillance for political or discriminatory purposes.

EFF’s Advocacy and Legal Work on ALPRs

The Electronic Frontier Foundation (EFF) has been actively engaged in investigating and addressing the privacy threats posed by ALPR technology through public records requests, litigation, and legislative advocacy since 2012.

ALPR Litigation Efforts

EFF, in partnership with the ACLU of Southern California, initiated legal action against the Los Angeles County Sheriff’s Department and the Los Angeles Police Department after these agencies refused to release ALPR data. The agencies argued that the data was exempt from the California Public Records Act as investigative records. This argument was challenged, with a lawyer for the LAPD and a California Supreme Court Justice acknowledging the “Orwellian” implications of considering all residents under investigation. In a significant victory in 2017, the California Supreme Court ruled in favor of EFF and ACLU, directing the case back to the Superior Court and establishing a precedent for public access to ALPR data.

In 2021, EFF and the ACLU further challenged the Marin County Sheriff’s Office in Lagleva v. Marin County Sheriff. This lawsuit addressed the sheriff’s practice of sharing ALPR data with out-of-state agencies, including Immigration & Customs Enforcement (ICE), violating state laws governing ALPR use and prohibiting the sharing of criminal justice data for immigration enforcement. A settlement was reached where the sheriff agreed to cease data sharing outside of California, limiting the scope of data dissemination.

Beyond California, EFF has filed amicus briefs in lawsuits concerning excessive ALPR data storage in Virginia, Massachusetts, and Nevada, advocating for stronger privacy protections against unwarranted surveillance.

Promoting ALPR Accountability and Transparency

In 2015, the California legislature enacted S.B. 34, a bill aimed at enhancing ALPR data protection. This law mandates ALPR users to implement data protection measures, maintain access logs, hold public meetings before initiating ALPR programs, establish usage and privacy policies, and maintain access logs. It also prohibits public agencies from selling, sharing, or transferring ALPR data to non-public entities.

EFF has coordinated volunteer efforts to collect ALPR policies across California and to expose agencies failing to comply with S.B. 34. Through projects like Data Driven and Data Driven 2, EFF has independently filed public records requests with numerous agencies to shed light on their ALPR data usage practices, promoting transparency and accountability.

In 2019, EFF successfully advocated for a California State Legislature audit of law enforcement agencies’ compliance with S.B. 34. The subsequent California State Auditor report substantiated many of EFF’s concerns, revealing agency failures to adhere to the law, implement adequate policies, and instances of excessively broad data sharing, highlighting the ongoing need for oversight and reform in ALPR usage.