Automatic number-plate recognition

The system must be able to deal with different styles of license 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 license plates on vehicles. 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 license 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 the day. A powerful flash is included in at least one version of the intersection-monitoring cameras, serving both to illuminate the picture and to make the offender aware of his or her mistake. ANPR technology tends to be region-specific, owing to plate variation from place to place.

Concerns about these systems have centered on privacy fears of government tracking citizens' movements and media reports of misidentification and high error rates.

Other names

ANPR is sometimes known by various other terms:

• Automatic license plate recognition (ALPR)
• Automatic vehicle identification (AVI)
• Car plate recognition (CPR)
• License plate recognition (LPR)
• Lecture Automatique de Plaques d'Immatriculation (LAPI)

Development history

The ANPR was invented in 1976 at the Police Scientific Development Branch in the UK. Prototype systems were working by 1979, and contracts were let to produce industrial systems, first at EMI Electronics, and then at Computer Recognition Systems (CRS) in Wokingham, UK. Early trial systems were deployed on the A1 road and at the Dartford Tunnel. The first arrest due to a detected stolen car was made in 1981.

Components

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, normalize and enhance the image of the number plate, and then optical character recognition (OCR) to extract the alphanumerics of the license plate. ANPR 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, and 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 media.

Technology

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 license 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.

ANPR in mobile systems

A City of Alexandria police car equipped with mobile ANPR.

Recent advances in technology have taken automatic number plate recognition (ANPR) systems from fixed applications to mobile ones. Scaled-down components at more cost-effective price points have led to a record number of deployments by law enforcement agencies around the world. Smaller cameras with the ability to read license plates at high speeds, along with smaller, more durable processors that fit in the trunks of police vehicles, allow law enforcement officers to patrol daily with the benefit of license plate reading in real time, when they can interdict immediately.

Despite their effectiveness, there are noteworthy challenges related with mobile ANPRs. One of the biggest is that the processor and the cameras must work fast enough to accommodate relative speeds of more than 100 mph (160 km/h), a likely scenario in the case of oncoming traffic. This equipment must also be very efficient since the power source is the vehicle battery, and equipment must be small to minimize the space it requires.

Relative speed is only one issue that affects the camera's ability to actually read a license plate. Algorithms must be able to compensate for all the variables that can affect the ANPR's ability to produce an accurate read, such as time of day, weather and angles between the cameras and the license plates. A system's illumination wavelengths can also have a direct impact on the resolution and accuracy of a read in these conditions.

Installing ANPR cameras on law enforcement vehicles requires careful consideration of the juxtaposition of the cameras to the license plates they are to read. Using the right number of cameras and positioning them accurately for optimal results can prove challenging, given the various missions and environments at hand. Highway patrol requires forward-looking cameras that span multiple lanes and are able to read license plates at very high speeds. City patrol needs shorter range, lower focal length cameras for capturing plates on parked cars. Parking lots with perpendicularly parked cars often require a specialized camera with a very short focal length. Most technically advanced systems are flexible and can be configured with a number of cameras ranging from one to four which can easily be repositioned as needed. States with rear-only license plates have an additional challenge since a forward-looking camera is ineffective with incoming traffic. In this case one camera may be turned backwards.

Algorithms

Steps 2, 3 and 4: The license plate is normalized for brightness and contrast, and then the characters are segmented to be ready for OCR.

There are six primary algorithms that the software requires for identifying a license plate:

1. Plate localization – 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. Normalization – 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 (normalization), 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

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

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.
• Lack of coordination between countries or states. Two cars from different countries or states can have the same number but different design of the plate.

Must be able to recognize
foreign license plates as such.

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.

On some cars, tow bars may obscure one or two characters of the license plate. Bikes on bike racks can also obscure the number plate, though in some countries and jurisdictions, such as Victoria, Australia, "bike plates" are supposed to be fitted. Some small-scale systems allow for some errors in the license plate. When used for giving specific vehicles access to a barricaded area, the decision may be made to have an acceptable error rate of one character. This is because the likelihood of an unauthorized car having such a similar license plate is seen as quite small. However, this level of inaccuracy would not be acceptable in most applications of an ANPR system.

Imaging Hardware

At the front end of any ANPR system is the imaging hardware which captures the image of the license plates. The initial image capture forms a critically important part of the ANPR system which, in accordance to the Garbage In, Garbage Out principle of computing, will often determine the overall performance.

License plate capture is typically performed by specialized cameras designed specifically for the task. Factors which pose difficulty for license plate imaging cameras include speed of the vehicles being recorded, varying ambient lighting conditions, headlight glare and harsh environmental conditions. Most dedicated license plate capture cameras will incorporate infrared illumination in order to solve the problems of lighting and plate reflectivity.

Many countries now use license 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 active infrared imaging (with a normal colour filter over the lens and an infrared illuminator 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 or impossible. 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 shutter 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 1/500th of a second can cope with traffic moving up to 40 mph (64 km/h) and 1/250th of a second up to 5 mph (8 km/h). License plate capture cameras can now produce usable images from vehicles traveling at 120 mph (190 km/h).

To maximize the chances of effective license plate capture, installers should carefully consider the positioning of the camera relative to the target capture area. Exceeding threshold angles of incidence between camera lens and license plate will greatly reduce the probability of obtaining usable images due to distortion. Manufacturers have developed tools to help eliminate errors from the physical installation of license plate capture cameras

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. Other users have attempted to smear their license plate with dirt or utilize covers to mask the plate.

Novelty frames around Texas license plates were made illegal in Texas on 1 September 2003 by Texas Senate Bill 439 because they caused problems with ANPR devices. That law made it a Class C misdemeanor (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.

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.

In order to avoid surveillance or penalty charges, there has been an upsurge in car cloning. 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 behavior to hinder investigations.

In principle, it may be possible to foil infrared detection simply by heating the license plate to a sufficient temperature so that the infrared brightness of the license plate exceeds that of the reflected signal that would otherwise be detected in the camera, but it would need to almost visibly glow red to work. Other possible options include IR emitting LED's around the license plate which would serve to "blind" cameras.

Police enforcement

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

Germany

On 11 March 2008, the Federal Constitutional Court of Germany ruled that the laws permitting the use of automated number plate recognition systems in Germany violated the right to privacy.

Hungary

Several Hungarian Auxiliary Police units use a system called Matrix Police in cooperation with the police. It consists of a portable computer equipped with a webcam that scans the stolen car database using automatic number plate recognition. The system is installed on the dashboard of selected patrol vehicles (PDA based handheld versions exist as well) and is mainly used to control the license plate of parking cars, as the Auxiliary Police doesn't have the authority to order moving vehicles to stop. If a stolen car is found, the formal police is informed.

United Kingdom

Main article: Police-enforced ANPR in the UK

The UK has an extensive (ANPR) automatic number plate recognition CCTV network. Effectively, the police and Security services track all car movements around the country and are able to track any car in close to real time. Vehicle movements are stored for 5 years in the National ANPR Data Center to be analyzed for intelligence and to be used as evidence.

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.

USA

In the USA, ANPR systems are more commonly referred to as LPR (License Plate Reader or License Plate Recognition) technology or ALPR (Automatic License Plate Reader/Recognition) technology.

One of the biggest challenges with ALPR technology in the US is the accuracy of the Optical Character Recognition (OCR)—the actual identification of the characters on the license plate. Many variables affect OCR accuracy, starting with the fact that each state has multiple license plate designs that must be recognized by the ALPR system. Also, the shape of the characters, color of the plates and whether the characters are raised or flat can affect accuracy. Many times the letter D is mistaken for a Q or an O and some colors, especially reddish tones, are hard to read and as a result many system vendors will quote accuracy rates that are misleading. So potential buyers must be wary of any quoted % rates and ask what they mean as you will find that 90% really means 90% (N-1 or N-2). This little detail in the brackets (which will not be either obvious or explained up front) allows the vendor to get all characters in a plate correct except one (N-1) or two (N-2).

From time to time, states will make significant changes in their license plate protocol that will affect OCR accuracy. They may add a character or add a new license plate design. ALPR systems must adapt to these changes quickly in order to be effective.

Another challenge with ALPR systems is that some states have the same license plate protocol. For example more than one state may use three letters followed by four numbers. So each time the ALPR systems alarms, it is the user’s responsibility to make sure that the plate which caused the alarm matches the state associated with the license plate listed on the in-car computer.

Many states in America are deploying ALPR systems to aid a diverse range of missions. One of the most common is that of removing drivers with suspended or revoked licenses from the streets, because these drivers are more likely to cause accidents.

Another effective application of ALPR is the recovery of stolen vehicles. ALPR technology increases the odds of recovery because these systems can cross check license plate numbers against lists of stolen plates and vehicles many times faster than if done manually. This capability is very beneficial to the auto insurance industry and in Arizona, insurance companies are helping to fund the purchase of ALPR systems for their local law enforcement agencies.

Other ALPR missions include parking enforcement and identifying individuals who are delinquent on city or state taxes. By cracking down on these offenses, many cities and states are significantly increasing their revenue.

A recent initiative by New York State deployed LPR systems toward the mission of catching organized car thieves by tracing suspect plates back to forged documents.

In addition to the real-time processing of license plate numbers, some ALPR systems in the US collect data at the time of each license plate capture. Data such as date and time stamps and GPS coordinates can be reviewed in relation to investigations and can help lead to critical breaks such as placing a suspect at a scene, witness identification, pattern recognition or the tracking of suspect individuals.

Average Speed cameras

Main article: Speed limit enforcement

ANPR is used for speed limit enforcement in the UK, Italy, The Netherlands and Dubai (UAE). This works by tracking vehicles' travel time between two fixed points, and calculating the average speed. 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.

UK

The longest stretch of average speed cameras in the UK is found on the A77 road in Scotland, with 30 miles (48 km) being monitored between Glasgow and Ayr. In 2006 it was confirmed that speeding tickets could potentially be avoided from the 'SPECS' cameras by changing lanes and the RAC Foundation feared that people may play "Russian Roulette" changing from one lane to another to lessen their odds of being caught. however in 2007 the system was upgraded for multi-lane use and in 2008 the manufacturer described the "myth" as “categorically untrue”. There is no evidence that average speed cameras actually reduce accident rates long term, with many motorists arguing that average speed check systems encourage bunching.

Italy

In Italian Highways has developed a monitoring system named Tutor covering more than 1244 km (2007). Further extensions will add 900 km before the end of 2008. The Tutor system is also able to intercept cars while changing lanes.

China

In some regions of China on hilly roads near rivers control system is made without any cameras other automated systems. Policeman gives printout with smallest time to arrive in next checkpoint, where another policeman checks if average speed is not exceeded. Charge is for each minute arriven too early. Some dining and amusement services offers to spend some time in the middle of the road. 只有少部分地方使用该方法，例如青藏公路上有多个这样的点。在南方很少听说有这样的方法来限速。高速公路上一般有流动的测速点和固定的测速点可以自动拍照。

Traffic control

Video tolling at Schönberg, Austria

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

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$2.55 per month are not charged the "Video Toll Charge" of C$3.60 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:

• Bridge Pass for the Saint John Harbour Bridge in Saint John New Brunswick
• CityLink & Eastlink in Melbourne, Australia
• Gateway Motorway and Logan Motorway, Brisbane, 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), Virginia (formerly Smart Tag), and other States.
• TollTag in North Texas.
• I-pass in Illinois
• Pike Pass in Oklahoma.
• OGS (Otomatik Geçiş Sistemi) used at Bosporus Bridges and Trans European Motorway entry points in İstanbul, Turkey.
• The M50 Westlink Toll in Ireland
• Hi-pass in South Korea

Charge zones – the London congestion charge

The London congestion charge scheme uses 230 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 6:00 p.m., Monday to Friday. Fines for traveling within the zone without paying the charge are £60 per infraction if paid before the deadline, doubling to £120 per infraction thereafter.

There are currently 1,500 cameras, which use Automatic Number Plate Recognition (ANPR) technology in use. 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.

Stockholm congestion tax

In Stockholm, Sweden, ANPR is used for the congestion tax, owners of cars driving into or out of the inner city must pay a charge, depending on the time of the day.

Usage

Several companies and agencies use ANPR systems. These include Vehicle and Operator Services Agency (VOSA), Police Information Technology Organisation (PITO) and Transport for London.

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?"

Systems with a simple review step are thought by some to eliminate this argument. Then the machine reports data - date, time, speed measurement and license plate - a good system records a photo of the event - so a person presented with the data is making an accusation. You will get a copy of the data when you go to court.

Similar criticisms have been raised in other countries. Easterbrook also argues that this technology is employed to maximize revenue for the state, rather than to promote safety. The electronic surveillance system produces tickets which in the US are often in excess of $100, and are virtually impossible for a citizen to contest in court without the help of an attorney. The revenues generated by these machines are shared generously with the private corporation that builds and operates them, creating a strong incentive to tweak the system to generate as many tickets as possible.

Older systems had been notably unreliable. 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 offense. 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 motorist drives away without paying for their fuel.
• 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
• Drive Through Customer Recognition, to automatically recognize customers based on their license plate and offer them their last selection, improving service to the customer.
• To assist visitor management systems in recognizing guest vehicles.

Measuring ANPR system performance

It is not uncommon to read claims of Automatic Number Plate Recognition read rates in excess of 98%. The experience of system operators is that overall read rates for license plates are 90% to 94% in ideal conditions with excellent modern systems. In some older systems overall performance rates are rumoured to be between 60% and 80%. ANPR is a developing technology which is coming of age and functional applications are expanding on a steady basis. Although there has been a significant improvement in recent years in the performance of ANPR systems, how operators can assess and monitor ANPR/LPR systems has not advanced as much.

See also

• Facial recognition system
• Road Policing Unit
• SPECS (speed camera)
• Closed circuit television