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Automatic number plate recognition

2007 Schools Wikipedia Selection. Related subjects: Road transport


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   The system must be able to deal with different styles of licence plates
   Enlarge
   The system must be able to deal with different styles of licence plates

   Automatic number plate recognition (ANPR; see also other names below)
   is a mass surveillance method that uses optical character recognition
   on images to read the licence plates on vehicles. As of 2006, systems
   can scan number plates at around one per second on cars travelling up
   to 100 mph (160 km/h). They can use existing closed-circuit television
   or road-rule enforcement cameras, or ones specifically designed for the
   task. They are used by various police forces and as a method of
   electronic toll collection on pay-per-use roads, and monitoring traffic
   activity such as red light adherence in an intersection.

   ANPR can be used to store the images captured by the cameras as well as
   the text from the licence plate, with some configurable to store a
   photograph of the driver. Systems commonly use infrared lighting to
   allow the camera to take the picture at any time of day. A powerful
   flash is included in at least one version of the
   intersection-monitoring cameras, serving to both illuminate the picture
   and make the offender aware of his or her mistake. ANPR technology
   tends to be region specific, owing to plate variation from place to
   place.

   The software aspect of the system runs on standard PC hardware and can
   be linked to other applications or databases. It first uses a series of
   image manipulation techniques to detect, normalise and enhance the
   image of the number plate, and finally optical character recognition
   (OCR) to extract the alphanumerics of the licence plate. ANPR/ALPR
   systems are generally deployed in one of two basic approaches; one
   allows for the entire process to be performed at the lane location in
   real-time, the other transmits all the images from many lanes to a
   remote computer location and performs the OCR process there at some
   later point in time. When done at the lane site, the information
   captured of the plate alphanumeric, date-time, lane identification, and
   any other information that is required is completed in somewhere around
   250 milliseconds. This information, now small data packets, can easily
   be transmitted to some remote computer for further processing if
   necessary, or stored at the lane for later retrieval. In the other
   arrangement there are typically large numbers of PCs used in a server
   farm to handle high workloads, such as those found in the London
   congestion charge project. Often in such systems there is a requirement
   to forward images to the remote server and this can require larger
   bandwidth transmission mediums.

   Concerns about these systems have centered on privacy fears of
   government tracking citizens' movements and media reports of
   misidentification and high error rates. However, as they have
   developed, the systems have become much more accurate and reliable.

Other names

   ANPR is sometimes known by various other terms:
     * Automatic licence plate recognition (ALPR)
     * Automatic vehicle identification (AVI)
     * Car plate recognition (CPR)
     * Licence plate recognition (LPR)

Technology

   The font on Dutch plates was changed to improve plate recognition
   The font on Dutch plates was changed to improve plate recognition

   ANPR uses optical character recognition (OCR) on images taken by
   cameras. When Dutch vehicle registration plates switched to a different
   style in 2002 one of the changes made was to the font, introducing
   small gaps in some letters (such as P and R) to make them more distinct
   and therefore more legible to such systems. Some licence plate
   arrangements use variations in font sizes and positioning – ANPR
   systems must be able to cope with such differences in order to be truly
   effective. More complicated systems can cope with international
   variants, though many programs are individually tailored to each
   country.

   The cameras used can include existing road-rule enforcement or
   closed-circuit television cameras as well as mobile units which are
   usually attached to vehicles. Some systems use infrared cameras to take
   a clearer image of the plates.

Algorithms

   Steps 2, 3 and 4: The licence plate is normalised for brightness and
   contrast and then the characters are segmented ready for OCR
   Enlarge
   Steps 2, 3 and 4: The licence plate is normalised for brightness and
   contrast and then the characters are segmented ready for OCR

   There are six primary algorithms that the software requires for
   identifying a licence plate:
    1. Plate localisation – responsible for finding and isolating the
       plate on the picture
    2. Plate orientation and sizing – compensates for the skew of the
       plate and adjusts the dimensions to the required size
    3. Normalisation – adjusts the brightness and contrast of the image
    4. Character segmentation – finds the individual characters on the
       plates
    5. Optical character recognition
    6. Syntactical/Geometrical analysis – check characters and positions
       against country specific rules

   The complexity of each of these subsections of the program determines
   the accuracy of the system. During the third phase (normalisation) some
   systems use edge detection techniques to increase the picture
   difference between the letters and the plate backing. A median filter
   may also be used to reduce the visual "noise" on the image.

Difficulties

   There are a number of possible difficulties that the software must be
   able to cope with. These include:
     * Poor image resolution, usually because the plate is too far away
       but sometimes resulting from the use of a low-quality camera.
     * Blurry images, particularly motion blur
     * Poor lighting and low contrast due to overexposure, reflection or
       shadows
     * An object obscuring (part of) the plate, quite often a tow bar, or
       dirt on the plate
     * A different font, popular for vanity plates (some countries do not
       allow such plates, eliminating the problem)
     * Circumvention techniques

   Early ANPR systems were unable to read white or silver lettering on
   black background, as permitted on UK vehicles built prior to 1973.
   Enlarge
   Early ANPR systems were unable to read white or silver lettering on
   black background, as permitted on UK vehicles built prior to 1973.

   While some of these problems can be corrected within the software it is
   primarily left to the hardware side of the system to work out solutions
   to these difficulties. Increasing the height of the camera may avoid
   problems with objects (such as other vehicles) obscuring the plate, but
   introduces and increases other problems such as the adjusting for the
   increased skew of the plate.

   Many countries now use licence plates that are retroreflective . This
   returns the light back to the source and thus improves the contrast of
   the image. In some countries, the characters on the plate are not
   reflective, giving a high level of contrast with the reflective
   background in any lighting conditions. A camera that makes use of
   infrared imaging (with a normal colour filter over the lens and an
   infrared light-source next to it) benefits greatly from this as the
   infrared waves are reflected back from the plate. This is only possible
   on dedicated ANPR cameras, however, and so cameras used for other
   purposes must rely more heavily on the software capabilities. Further,
   when a full-colour image is required as well as use of the
   ANPR-retrieved details it is necessary to have one infrared-enabled
   camera and one normal (colour) camera working together.
   Blurry images make OCR difficult – ANPR systems should have fast
   shutter speeds to avoid motion blur
   Enlarge
   Blurry images make OCR difficult – ANPR systems should have fast
   shutter speeds to avoid motion blur

   To avoid blurring it is ideal to have the shutter speed of a dedicated
   camera set to 1/1000th of a second. Because the car is moving, slower
   speeds could result in an image which is too blurred to read using the
   OCR software, especially if the camera is much higher up than the
   vehicle. In slow-moving traffic, or when the camera is at a lower level
   and the vehicle is at an angle approaching the camera, the shutter
   speed does not need to be so fast. Shutter speeds of 1/500 can cope
   with traffic moving up to 40 mph (64 km/h) and 1/250 up to 5 mph (8
   km/h).

   On some cars, towbars may obscure one or two characters of the licence
   plate. Bikes on bike racks can also obscure the number plate, though in
   some countries and jurisdictions, such as New South Wales, "bike
   plates" are supposed to be fitted.

   Some small-scale systems allow for some errors in the licence plate.
   When used for giving specific vehicles access to a barriered area the
   decision may be made to have an acceptable error rate of one character.
   This is because the likelihood of an unauthorised car having such a
   similar licence plate is seen as quite small. However, this level of
   inaccuracy would not be acceptable in most applications of an ANPR
   system.

Circumvention techniques

   Vehicle owners have used a variety of techniques in an attempt to evade
   ANPR systems and road-rule enforcement cameras in general. One method
   increases the reflective properties of the lettering and makes it more
   likely that the system will be unable to locate the plate or produce a
   high enough level of contrast to be able to read it. This is typically
   done by using a plate cover or a spray, though claims regarding the
   effectiveness of the latter are disputed. In most jurisdictions, the
   covers are illegal and covered under existing laws, while in most
   countries there is no law to disallow the use of the sprays.

   For the 407 toll route in Ontario, Canada, police have caught several
   advanced techniques that some motorists have attempted. One driver had
   a setup that allowed him to lift a wire from the driver's seat that
   would show a different plate as he was cruising through the camera
   zones. Other users have attempted to smear their licence plate with
   dirt or utilise covers to mask the plate.

   Novelty frames around Texas licence plates were made illegal on 1
   September 2003 by Senate Bill 439 because they caused problems with
   ANPR devices. That law made it a Class C misdemeanour (punishable by a
   fine of up to US$200), or Class B (punishable by a fine of up to
   US$2,000 and 180 days in jail) if it can be proven that the owner did
   it to deliberately obscure their plates.

   There are some custom car rear panels with an inset for the licence
   plate at an angle, which changes the alignment of characters relative
   to the reading grid. Since most U.S. states no longer require new
   plates each year, perhaps the easiest way to disable recognition is
   simply to allow the reflective paint on the plates to become degraded
   by age and therefore unreadable.

   If an ANPR system cannot read the plate it can flag the image for
   attention, with the human operators looking to see if they are able to
   identify the alphanumerics. It is then possible to do lookups on a
   database using wildcard characters for any part of the plate obscured,
   and use car details (make and model, for example) to refine the search.

   In order to avoid surveillance or penalty charges, there has been an
   upsurge in car cloning, particularly in London. This is usually
   achieved by copying registration plates from another car of a similar
   model and age. This can be difficult to detect, especially as cloners
   may change the registration plates and travel behaviour to hinder
   investigations.

Police enforcement

   Closed-circuit television cameras such as these can be used to take the
   images scanned by automatic number plate recognition systems
   Enlarge
   Closed-circuit television cameras such as these can be used to take the
   images scanned by automatic number plate recognition systems

   After the licence plate has been identified it can then be
   cross-referenced against a police database. The primary objectives of
   this are to identify vehicles that have been stolen, used in a crime or
   are in violation of some other law. Some systems are also linked to
   insurance databases to monitor if the vehicle is currently insured.

   On 18 November 2005 British police constable Sharon Beshenivsky was
   shot and killed during a robbery in Bradford. The CCTV network was
   linked in to an ANPR system and was able to identify the getaway car
   and track its movements, leading to the arrest of six suspects. At its
   launch in May, Ch Supt Geoff Dodd of West Yorkshire Police, called the
   ANPR system a "revolutionary tool in detecting crime".

Glutton System in Northern Ireland

   In 1997 a system of one hundred ANPR cameras, codenamed GLUTTON, was
   installed to feed into the automated British Military Intelligence
   Systems in Northern Ireland. Further cameras were also installed on the
   British mainland, including unspecified ports on the east and west
   coasts.

Project Laser in the United Kingdom

   In March 2005, plans were announced to set up a nationwide system of
   over 2,000 automatic number plate recognition cameras in the United
   Kingdom. Starting in 2006 Britain will become the first country in
   which every journey of every vehicle is monitored and recorded.

   This followed the successful rollout of Project Spectrum in which all
   43 Police Forces in England and Wales were supplied by the Home Office
   with an ANPR capable mobile unit, and a 'Back Office'. A subsequent
   series of trials were then commenced in 2002 when the Vehicle and
   Operator Services Agency (VOSA) was given funding by the Home Office to
   work with the Police Standards Unit and develop "Project Laser" using
   the equipment supplied under Project Spectrum. With the aim of running
   the ANPR system nationwide, it was initially trialled by nine police
   forces and ran between 30 September 2002 and March 2003. Those police
   forces were:
     * Greater Manchester
     * North Wales
     * Avon and Somerset
     * Northamptonshire
     * The Metropolitan Police Service
     * Kent
     * West Yorkshire
     * Staffordshire
     * West Midlands.

   The second phase of the project ran between 1 June 2003 and 21 June
   2004 and involved 23 police forces in total. The DVLA is also involved
   with Project Laser, using the system to gather details on unregistered
   and unlicensed vehicles and those without a valid MOT certificate or
   insurance cover.

     "Eventually the database will link to most CCTV systems in town
     centres, meaning that all vehicles filmed on one of the many cameras
     protecting Bedford High Street, for instance, can be checked against
     the database and the movements of wanted cars traced to help with
     serious crime investigations."
     — Bedfordshire Police

   The project was seen as a success despite a Home Office report showing
   that the Driver and Vehicle Licensing Agency (DVLA) trial had an error
   rate of up to 40%, with claims that the system was contributing

     "…in excess of 100 arrests per officer per year – ten times the
     national average…"
     — Police Standards Unit.

   Further findings went on to show that the error rate dropped to 5% when
   infrared systems and updated software were used.

   During the second phase of the project around 28 million number plates
   were spotted in total, with 1.1 million (3.9%) of these matching an
   entry in one of the databases. 180,543 vehicles were stopped (101,775
   directly because of the ANPR system), leading to 13,499 arrests (7.5%
   of the total) and the issue of 50,910 fines (28.2%). 1,152 stolen
   vehicles (worth £7.5 million in total), £380,000 worth of drugs and
   £640,000 worth of stolen goods were also recovered. The primary goal of
   the second phase was, however, to see how well the costs of the ANPR
   system could be covered. The final conclusion was that less than 10% of
   the expenditure incurred was recouped, with the Home Office claiming
   that the failure of drivers to pay fines contributed to this low
   figure, and continued to recommend the system be deployed throughout
   the UK. Report (PDF)

   Funding is now in place for the construction of the National ANPR Data
   Centre capable of holding 50 million ANPR reads per day. This should be
   complete by March 2006 and will form the basis of a vehicle movement
   database. .

   There are now suggestions that the use of the network could be extended
   to catch drivers using mobile phones illegally, and those failing to
   wear seat belts .

SPECS cameras

   Another use for ANPR in the UK is for speed cameras which work by
   tracking vehicles' travel time between two fixed points, and therefore
   calculate the average speed. Currently the only such system which is
   accepted as reliable by the courts is the SPECS brand. These cameras
   are claimed to have an advantage over traditional speed cameras in
   maintaining steady legal speeds over extended distances, rather than
   encouraging heavy braking on approach to specific camera locations and
   subsequent acceleration back to illegal speeds. Accident rates have
   tended to be reduced substantially in places where SPECS cameras have
   been installed. They are, however, significantly more expensive than
   traditional cameras.

   The longest stretch of SPECS cameras in the UK is found on the A77 road
   in Scotland, with 30 miles being monitored between Glasgow and Ayr.

Traffic control

   Many cities and districts have developed traffic control systems to
   help monitor the movement and flow of vehicles around the road network.
   This had typically involved looking at historical data, estimates,
   observations and statistics such as:
     * Car park usage
     * Pedestrian crossing usage
     * Number of vehicles along a road
     * Areas of low and high congestion
     * Frequency, location and cause of road works

   CCTV cameras can be used to help traffic control centres by giving them
   live data, allowing for traffic management decisions to be made in
   real-time. By using ANPR on this footage it is possible to monitor the
   travel of individual vehicles, automatically providing information
   about the speed and flow of various routes. These details can highlight
   problem areas as and when they occur and helps the centre to make
   informed incident management decisions.

   Some counties of the United Kingdom have worked with Siemens Traffic to
   develop traffic monitoring systems for their own control centres and
   for the public. Projects such as Hampshire County Council's ROMANSE
   provide an interactive and real-time web site showing details about
   traffic in the city. The site shows information about car parks,
   ongoing road works, special events and footage taken from CCTV cameras.
   ANPR systems can be used to provide average driving times along
   particular routes, giving drivers the ability to choose which one to
   take. ROMANSE also allows travellers to see the current situation using
   a mobile device with an Internet connection (such as WAP, GPRS or 3G),
   thus allowing them to be alerted to any problems that are ahead.

   The UK company Trafficmaster has used ANPR since 1998 to estimate
   average traffic speeds on non-motorway roads without the results being
   skewed by local fluctuations caused by traffic lights and similar. The
   company now operates a network of over 4000 ANPR cameras , but claims
   that only the four most central digits are identified, and no
   numberplate data is retained .

Electronic toll collection

Toll roads

   The FasTrak system in Orange County uses ANPR and radio transponders
   Enlarge
   The FasTrak system in Orange County uses ANPR and radio transponders

   Ontario's 407 ETR highway uses a combination of ANPR and radio
   transponders to toll vehicles entering and exiting the road. Radio
   antennas are located at each junction and detect the transponders,
   logging the unique identity of each vehicle in much the same way as the
   ANPR system does. Without ANPR as a second system it would not be
   possible to monitor all the traffic. Drivers who opt to rent a
   transponder for C$2.00 per month are not charged the "Video Toll
   Charge" of C$3.45 for using the road, with heavy vehicles (those with a
   gross weight of over 5,000 kg) being required to use one. Using either
   system, users of the highway are notified of the usage charges by post.

   There are numerous other electronic toll collection networks which use
   this combination of Radio frequency identification and ANPR. These
   include:
     * CityLink in Melbourne, Australia
     * FasTrak in California, United States
     * Highway 6 in Israel
     * Tunnels in Hong Kong
     * Autopista Central in Santiago, Chile (site in Spanish)
     * E-ZPass in New York, New Jersey, Massachusetts (as Fast Lane), and
       other States.
     * Pike Pass in Oklahoma.

Charge zones – the London congestion charge

   The London congestion charge scheme uses two hundred and thirty cameras
   and ANPR to help monitor vehicles in the charging zone
   Enlarge
   The London congestion charge scheme uses two hundred and thirty cameras
   and ANPR to help monitor vehicles in the charging zone

   The London congestion charge is an example of a system that charges
   motorists entering a payment area. Transport for London (TfL) uses ANPR
   systems and charges motorists a daily fee of £8 paid before 10pm if
   they enter, leave or move around within the congestion charge zone
   between 7 a.m. and 6:30 p.m., Monday to Friday. Fines for travelling
   within the zone without paying the charge are £50 per infraction if
   paid before the deadline, doubling to £100 per infraction thereafter.

   Two hundred and thirty CCTV-style cameras, of which 180 are installed
   at the edge of the zone, are currently in use. The 50 cameras within
   the zone are intended to pick up cars that are missed on entry and/or
   exit, as well as those that are moving solely within the zone. There
   are also a number of mobile camera units which may be deployed anywhere
   in the zone.

   It is estimated that around 98% of vehicles moving within the zone are
   caught on camera. The video streams are transmitted to a data centre
   located in central London where the ANPR software deduces the
   registration plate of the vehicle. A second data centre provides a
   backup location for image data.

   Both front and back number plates are being captured, on vehicles going
   both in and out – this gives up to four chances to capture the number
   plates of a vehicle entering and exiting the zone. This list is then
   compared with a list of cars whose owners/operators have paid to enter
   the zone – those that have not paid are fined. The registered owner of
   such a vehicle is looked up in a database provided by the DVLA. A
   government investigation has found that a significant portion of the
   DVLA's database is incorrect. Furthermore, it is now the car owner's
   reponsibility to report to the DVLA if they sell their car.

Controversy

   The introduction of ANPR systems has led to fears of misidentification
   and the furthering of 1984-style surveillance . In the United States,
   some such as Gregg Easterbrook oppose what they call "machines that
   issue speeding tickets and red-light tickets" as the beginning of a
   slippery slope towards an automated justice system:

          "A machine classifies a person as an offender, and you can't
          confront your accuser because there is no accuser... can it be
          wise to establish a principle that when a machine says you did
          something illegal, you are presumed guilty?"

   Similar criticisms have been raised in other countries. Easterbrook
   also argues that this technology is employed to maximise revenue for
   the state, rather than to promote safety.

   The fallibility of older systems was alarming, with one critic of the
   London congestion charge scheme noting "Misread plate after misread
   plate appeared on the screen – of every 10 that appeared at least four
   were incorrect." This can lead to charges being made incorrectly with
   the vehicle owner having to pay £10 in order to be issued with proof
   (or not) of the offence. Improvements in technology have drastically
   decreased error rates, but false accusations are still frequent enough
   to be a problem.

   Other concerns include the storage of information that could be used to
   identify people and store details about their driving habits and daily
   life, contravening the Data Protection Act along with similar
   legislation (see personally identifiable information). The laws in the
   UK are strict for any system that uses CCTV footage and can identify
   individuals.

Other uses

   ANPR systems may also be used for/by:
     * Section control, to measure average vehicle speed over longer
       distances
     * Border crossings
     * Filling stations to log when a driver drives away without paying
     * Car parks or road entry systems to control access
     * A marketing tool to log patterns of use
     * Traffic management systems, which determine traffic flow using the
       time it takes vehicles to pass two ANPR sites
     * Traffic management systems, which determine traffic flow using the
       time it takes vehicles to pass two ANPR sites
     * - How ANPR can be used
     * - Where ANPR can be used

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   with only minor checks and changes (see www.wikipedia.org for details
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