What are ALPRs?

In this article:

  1. What are ALPR?
  2. What are the types of ALPR?
  3. ALPR Databases
  4. What Kinds of Data Do ALPRs Collect?
  5. How Does Law Enforcement Use ALPRs?

What are ALPR?

Automated License Plate Readers (ALPRs) are high-speed, computer-controlled camera systems typically mounted on street poles, streetlights, highway overpasses, mobile trailers, or attached to police patrol cars.

ALPRs capture all license plate numbers that come into view, along with the location, date and time.  The data, which includes photographs of the vehicle and sometimes its driver and passengers, is then uploaded to a central server.

Vendors say that the information collated, can be used by police to find out where a plate has been in the past, to determine whether a vehicle was at the scene of a crime, to identify travel patterns and even to discover vehicles that may be associated with each other.  Law enforcement agencies can choose to share their information with thousands of other agencies.

ALPR technology can be applied to target drivers who visit sensitive places such as health centres, immigration clinics, protests, or centres of religious worship.

Drivers have no control over whether their vehicle displays a license plate because the government requires all car, lorry and motorcycle drivers to display license plates in public view.

What are the types of ALPR?

Automated license plate readers are divided into two categories:

  1. Stationary ALPR Cameras

These are installed in a fixed location, such as a traffic light, a telephone pole, the entrance of a facility, or a motorway exit. These cameras generally capture only vehicles in motion that pass within view.

If multiple stationary ALPR cameras are installed along a single thoroughfare, the data can reveal, what direction and what speed a car is travelling.

Stationary cameras can sometimes be moved. For example, surveillance vans or truck trailers can be outfitted with ALPR systems and parked at strategic locations, such as protests or political rallies.

ALPR cameras are often used in conjunction with automated red-light and speed enforcement systems and also as a means of assessing tolls on roads and bridges.

  1. Mobile ALPR cameras

These are often attached to police patrol cars, allowing law enforcement officers to capture data from license plates whilst driving around the town or cities throughout their shifts.

In most cases, these cameras are turned on at the beginning of a shift and not turned off again until the end. Also, private vendors like Vigilant Solutions capture plate data with mobile ALPRs and then sell that data to police agencies and others.

In addition to capturing images of passing vehicles, mobile ALPR cameras are effective at capturing license plates of parked cars. For example, a patrol car may drive around a public parking lot to capture hundreds of vehicles’ plates in minutes.

ALPR Databases

Most of this ALPR data is stored in databases for extended periods of time, often as much as five years.

Law enforcement agencies without their own ALPR systems can access data collected by other law enforcement agencies through sharing systems and networks operated by private companies.

Several companies operate independent, non-law enforcement ALPR databases, contracting with drivers to put cameras on private vehicles to collect the information.

This data is then sold to companies like insurers, but law enforcement can also purchase access to this commercial data on a subscription basis.


Law enforcement agencies often pre-load a list of license plates that the ALPR system is actively looking for, such as stolen vehicles and vehicles associated with outstanding warrants.

Police officers can also create their own hotlists. If the ALPR camera scans a plate on the list, the system sends an alert to the officer in the patrol car (if a mobile reader) or the agency (if a fixed reader). Some hotlists include low-level misdemeanors and traffic offenses. Some agencies use these hotlists to generate revenue by stopping citation scofflaws.

What Kinds of Data Do ALPRs Collect?

ALPRs collect license plate numbers, location data along with the date and time the license plate was encountered.

Some systems are able to capture the make and model of the vehicle. They can collect thousands of plates per minute.  When data is combined with algorithms, the systems can reveal regular travel patterns and predict where a driver may be in the future.

The data generally does not include the driver’s name.  Law enforcement officers can however, use other databases to connect individuals with their license plate numbers.

How Does Law Enforcement Use ALPRs?

ALPR data is indiscriminately gathered, collecting information on millions of ordinary people. By plotting vehicle times and locations and tracing past movements, police can use stored data to paint a specific portrait of a driver’s life, determining past patterns of behaviour and possibly even predicting future ones, even though the vast majority of people whose license plate data is collected and stored have not been accused of a crime.

Without ALPR technology, law enforcement officers must collect license plates by hand, creating practical limitations on the amount of data collected which means choices about which vehicles are to be tracked.

ALPR technology removes those limitations and allows officers to track everyone, allowing for a faster and broader collection of license plates with far reduced staffing requirements.  Law enforcement has two general purposes for using license plate readers.

Real-time investigations

By adding a license plate to a “hot list,” officers can use ALPR to  identify or track particular vehicles in real time. Licenses plates are often added to hot lists because the vehicle has been stolen or associated with an outstanding warrant.

Officers may also add a plate number to the list, if the vehicle is seen at the scene of a crime, the owner is a suspect in a crime, or the vehicle is believed to be associated with a gang. Hot lists often include low-level offenses, too.

Historical investigations

Since ALPRs typically collect information on everyone, not just hot-listed vehicles, officers can use a plate, a partial plate, or a physical address to search and analyise historical data.

For example, an officer may enter the location of a convenience store to identify vehicles seen nearby at the time of a robbery. The officer can then look up those plate numbers to find other locations that plate has been captured.

Training materials, policies and laws in some jurisdictions instruct officers that a hot-list alert on its own, may not be enough to warrant a stop. Officers are instructed to visually confirm that a plate number is a match. Failure to manually confirm, combined with machine error, has caused wrongful stops.

Law enforcement claims that ALPR data has been used to, for example, to recover stolen cars or find abducted children. However, police have also used ALPR data for mass enforcement of less serious offenses, such as searching for uninsured drivers or tracking down individuals with overdue court fees.

ALPR data retention varies from agency to agency, from as short as mere days to as long as several years, with some entitie, including private companies, retaining data indefinitely.

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