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Automatic License Plate Recognition

By Dennis J. Lau

What is Automatic License Plate Recognition?

In an era of increasing threats to public safety and shrinking city budgets, judicious application of modern technologies are a force multiplier for police agencies who need to do more with less. Automatic License Plate Recognition (ALPR) is a case in point. ALPR uses cameras to capture digital images of license plates, then a computer to convert plate image into alphanumeric characters. That information, also referred to as “plate code”, can then be stored in a database and compared to other databases.

Automatic License Plate Recognition got its start in the 1990s when the British responded to a series of bomb attacks by members of the Irish Republican Army by developing London’s “Ring of Steel.” Using fixed mounted cameras in strategic locations, authorities capture the plate code of every vehicle entering London. (Coaffee, pp. 204-205) ALPR is used in parking revenue systems by recording cars as they enter and exit the lot, in Intelligent Transportation Systems to conduct Origin-Destination and Travel Time Studies by calculating the time it takes for a vehicle to pass between two points, in Cordon Studies to determine traffic patterns in and out of an area, and automatic vehicle tolling systems. (Rossetti, 2001)

State and local law enforcement has begun deploying vehicle based systems. With current computer processing speeds, police cars can be equipped with up to four cameras running simultaneously, checking the results from each plate it “sees” against a criminal data base, and alerting the officer with a preliminary match or “hit” within a couple of seconds. These systems can accurately read plates at speeds up to 160mph in closing and greater than 75mph in passing. (Senn, 2008)

How does it work?

Automatic License Plate Recognition can trace its beginnings to character reading machines used in mail sorting over 50 years ago. An ALPR system has to locate the license plate in an image of the vehicle and then perform optical character recognition (OCR) on that plate. It is comprised of a camera or cameras, an illumination source - often infrared (IR), image processing software, a host computer, a power supply, and network connectivity should the system need to communicate with external databases for its operation.

The challenge from converting mail reading technology, to grabbing characters off of fast moving vehicles, at random and variable angles and light conditions, is achieved by clever mathematics and modern computer processing power. Fuzzy logic is often used to look for where the license plate should be based on assumptions of where a plate should be located, and edges, colors and textures that would correspond to images of license plates. Algorithms, neural networks and fuzzy logic, segment out the characters in the image and covert them to alphanumeric plate code. (Jia, He, & Piccardi, 2004)
In a law enforcement application, plate code is commonly compared to a database of wanted persons and stolen vehicles. If there is a positive match, the system alerts the officer for further action.

How is it used in law enforcement?

Without a vehicle based ALPR system, a police officer check plates by calling them out over the police radio for a dispatcher to check and report back to the officer. Officers who have Mobil Data Computers (MDCs) installed in their vehicles, that are wirelessly networked to their department’s Computer Aided Dispatch (CAD) system, can play license plate bingo directly on their MDC without dispatcher assistance. A technically competent officer can “run” about 150 plates in a normal shift. With an ALPR system, the same officer can expect to have checked 3,000 -5,000 plates in a single shift. (Plourd, 2006)

The technology allows the same work to be completed with fewer officers. Retired New York Police Lieutenant, Pat Fitzgerald had this to say: “It's very efficient… for example, the city of White Plains hired one officer for 8 hours on over time to run plates (without a reader) in a particular parking lot. I did this same lot in 20 minutes.” (Senn, 2008) Other agencies have had great success assigning them to Auto Theft Task Forces. The Pennsylvania Auto Theft Prevention Authority has seen a dramatic drop in auto theft. When it began in 1996, 53,000 auto thefts were reported that year. When the authority began using mobile ALPR “in earnest” in 2007, the annual auto theft rate dropped to 28,000. As the word gets out, they’re hoping for continued declines. (McKay, 2008)

While the initial driving force behind law enforcement ALPR was to locate stolen cars, the enormous data collection activities of these systems offer public safety benefits far beyond the efficient location of wanted persons and stolen vehicles. When fixed and mobile ALPR systems are connected to robust “back office” hardware and software, ALPR data, which includes the plate code, date, time, location and color picture of every single car it saw, can be stored in a single searchable database. An industry consortium of ALPR system vendors are adopting data format standards so ALPR data from the different vendor’s systems can be shared and searched. (Reuters, 2008)

Regional ALPR data can be “mined” to solve crimes by placing the same car at multiple crime scenes. Driving an ALPR car around the area of a crime can also aid in placing a suspect at the scene and locating witnesses. LASD recently solved a serial rapist and a murder case by using ALPR data to locate the suspects. (PRNewswire, 2008) Fixed ALPR can be used as one layer in a multi-layered security approach, to protect critical infrastructure. The same plate seen at multiple sensitive key infrastructure locations could generate leads for investigation by anti-terrorism task forces, which causes some heartburn for the ACLU.

What are the civil rights issues?

You can bet that the ACLU has not been silent on this issue. Different ACLU offices have freely offered their wisdom. "The NYPD has proposed the blanket, indiscriminate videotaping of millions of people," said New York Civil Liberties Union Associate Legal Director Christopher Dunn. "The NYPD should not be spending $100 million of public money to track law-abiding New Yorkers." (Gendar, 2008)

While not public safety experts, they freely comment on perceived governmental intrusions. Jeff Gamso, legal director of the American Civil Liberties Union of Ohio said, "The scanner’s gaze is too wide and it’s an infringement against the innocent drivers whose plates get captured… Using (ALPR) to scan all license plates is a civil rights violation and could lead to government abuse of the information… I think they should just knock it off." (Associated Press , 2007)

The Washington State Patrol started checking all the plates of vehicles boarding ferries. They check them for AMBER alerts , reported stolen vehicles, felony wanted persons, and suspected terrorists. ACLU Seattle chapter spokesman Doug Honig said, “The (ACLU) has no problem with the camera system being used for flagging stolen vehicles, potential child abductors and wanted felons” but not terrorists. (Port Orchard Independent - Opinion, 2008)

The running thread is concern is how long the information will be stored, how it will be used, and who will have access. Ultimately the courts may have to weigh in on these plain view observations and the need for public safety vs. a reasonable expectation of privacy.

How does the equipment from different vendors compare?

Law enforcement agencies with limited technical expertise are often challenged to select technology based equipment, systems and services. Trade shows and vendor claims can be problematic to sort through. Recognizing this dilemma the U.S. Department of Homeland Security established the System Assessment and Validation for Emergency Responders (SAVER) Program to provide local agencies, objective tests of comparable equipment. SAVER focuses on two questions: “What equipment is available?” and “How does it perform?”

In 2008 SAVER issued a series of reports on mobile license plate recognition systems. Its focus group recommendations set evaluation criteria, comparison methodology, and selected four vendors for evaluation. Systems were evaluated on a 5 point scale in the areas of capability, usability, deployability and maintainability. The PAGIS system by PIPS Technology scored the highest with an overall score of 4.4, followed by MPH-900 by ELSAG North America with an overall score of 4.0, and PlateScan by Civica Software with an overall score of 3.5. CarDetector by Vigilant Video received the overall lowest score of 3.1. (Engstrom, 2008)

If a local agency is deploying new ALPR technology, the consideration for regional compatibility with ALPR systems used by other local agencies should be considered. A good first step would be to talk to their other local agencies that are already using ALPR systems and learn from their experience. ELSAG is well established in the east, while PIPS has a strong following out west. I believe that ALPR is already an effective tool for law enforcement, and like all technology, it continues to improve rapidly with time.

Dennis J. Lau is a law enforcement official in Southern California. He completed this paper in conjunction with his course work while pursuing his BA in Criminal Justice Management.

Associated Press . (2007, July 30). ACLU objects to plate scanners in police cruisers. Retrieved March 22, 2009, from The Chronicle-Telegram:

Coaffee, J. (2004). Rings of Steel, Rings of Concrete and Rings of Confidence: Designing out Terrorism in Central London pre and post September 11th. International Journal of Urban and Regional Research , 28 (1), 201-211.

Engstrom, C. (2008). Mobile License Plate Recognition Systems Assessment Report. Washington D.C.: U.S. Department of Homeland Security.

Gendar, A. (2008, August 12). ACLU sez city plan to scan license plates bum steer. Retrieved March 22, 2009, from NY Daily News:

Jia, W., He, X., & Piccardi, M. (2004). Automatic License Plate Recognition: A review. Proceedings of the International Conference on Imaging Science, Systems and Technology (pp. 43-48). Las Vegas: CSREA Press.

McKay, J. (2008, April 9). License Plate Recognition Systems Extend the Reach of Patrol Officers. Retrieved March 22, 2009, from Government Technologies:

Plourd, K. (2006, April 18). Camera system aids Long Beach police. Retrieved March 21, 2009, from CSULB Online 49er:

Port Orchard Independent - Opinion. (2008, July 30). WSP plan irks terrorists and their enablers. Retrieved March 22, 2009, from Port Orchard Independent:

PRNewswire. (2008, November 10). Los Angeles County Advances Public Safety with Federal Signal Technology. Retrieved March 22, 2009, from$44070

Reuters. (2008, January 15). PIPS Technology Forms License Plate Recognition Industry Consortium. Retrieved March 22, 2009, from Reuters:

Rossetti, M. D. (2001). Applications And Evaluation Of Automated License Plate Reading Systems. Proceedings of the Intelligent Transportation Society of America 11th Annual Meeting and Exposition. Miami.

Senn, P. M. (2008, July 8). Picking Out Plates. Retrieved March 21, 2009, from