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New Developments In ALPR

Automated License Plate Recognition (ALPR) technology has been the "oh, wow" gadget for the past few years, and with good reason. ALPR can identify, screen, and record the license plates of more vehicles than an entire squad of police officers could handle, and can do it with greater accuracy and no fatigue factor.

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PHOTOS: View our gallery, "Plate Hunters," for additional images of real-world plates.

Automated License Plate Recognition (ALPR) technology has been the "oh, wow" gadget for the past few years, and with good reason. ALPR can identify, screen, and record the license plates of more vehicles than an entire squad of police officers could handle, and can do it with greater accuracy and no fatigue factor.

Since the last time POLICE reported on ALPR, there haven't been any major changes in the technology. What has happened is more in the area of refinement and ease of use. At first examination, it's easy to dismiss changes like this as inconsequential. But I see them in the same way I saw the introduction of electrically powered windows on patrol cars. The windows opened and closed the same way, but life got a lot easier when I didn't have to reach over to the other side of the car to do it.


The current generation of ALPR handles much more information at one time than did previous versions. Early ALPR installations read and recognized license plates in much the same way a human would, although the throughput was higher. The system would "see" a plate, resolve the number through optical character recognition, then compare the result to a "hot list" of license plates loaded into memory. When that task was complete, the process would start looking for a new plate to read.

More powerful dual and quad-core microprocessors with multithreading capabilities now make it possible for the system to function as if there were several ALPR modules working alongside one another, reading multiple lanes of license plates simultaneously. This increase in speed and efficiency will only get better as new hardware comes online to handle the faster processing of data.

Affinity Plates

Processing speed is more important than ever before, because states seem to be trying to stymie ALPR systems with new license plate designs. Public demand coupled with the allure of new revenue sources has increased the number of "affinity" license plates available to vehicle owners.

Affinity plates are those issued to members of volunteer fire departments, military veterans, contributors to special conservation funds, or to show support for any number of other causes or groups. Some states have hundreds of types of affinity plates, since it takes as few as 25 subscribers to convince the motor vehicle department to issue the special series.

There is very little consistency to the way these license plates are enumerated and coded in each state's vehicle registration files. For example, say that two states decide to issue plates commemorating service in the U.S. Army. State A starts the license plate sequence with 0001 and enters the registration information in its database as registration type AV, for "Army Vet." State B also starts its affinity plate sequence with 0001, but prefixes the number with "AV," placing those two letters to the left of the numbers with the letters stacked vertically. In the state's vehicle registration database, that plate is listed as "AV0001" with the registration type as "PC" if it's on a passenger car and "PU" if it's on a pickup truck.

In both cases, the leading "AV" characters are the only clue that the license plate won't look like the standard plates issued by that state, since most affinity plates also carry special colors, lettering, or the logo or crest of the commemorative target. The ALPR recognition engine has to determine if that emblem is a readable character, or something to be ignored.

Printed Plates

Another way that states are reducing costs is by distributing affinity license plates that are printed, rather than stamped or embossed. These are much cheaper to produce, but they aren't as durable, aren't as reflective, and they're easier to counterfeit. For several years, Oregon has been distributing a Crater Lake affinity plate that is printed, rather than stamped. To say these are unpopular with law enforcement is an understatement. One Oregon trooper said, "They're killing us with those things."[PAGEBREAK]

The ALPR camera that scans one of these plates is likely to suffer a read error if the affinity plate's layout is not already coded into its recognition algorithm. Computers do very well when they are asked to process regimented, well-ordered information; when they have to perform pattern recognition, not so much.

Most people with some familiarity of American culture are going to see the outline of a soldier, or the seal of the U.S. Army, or a Stars and Stripes logo and associate it with a government or patriotic concept. The word "service," "serve," or "veteran" provides more cues for context. The computer sees the large numbers and letters in the context of the colors of the license plate and possibly the state name. Without some strict recognition rules coded into its software, the ALPR scanning software may arrive at an incorrect conclusion on what that license plate represents, if it can resolve the information at all.

ALPR vendors are constantly updating their software to accommodate the changes, but with 50 states, 10 provinces, three territories, and a mix of Indian Nations and special government agencies all issuing their own license plates, they're always going to be behind the curve.

Vertical Stacking

A related challenge for ALPR is when states arrange license plate characters vertically, rather than horizontally. Traditional license plate formats have all the characters the same size, reading left to right. Some affinity plates start or end with two or three characters stacked vertically, followed by larger characters in the standard arrangement.

The newest software versions from some ALPR vendors can read these vertically stacked characters and resolve them for comparison to their internal hot lists, but this arrangement tends to lower the recognition and accuracy rates for any manufacturer. Bob Pinzler with PlateScan says that the company's latest software reads stacked characters reliably.

Team Optimization

A common method of deploying ALPR involves fixing a mobile ALPR unit to one spot, scanning traffic traveling under an overpass or other vantage point. When the unit registers a hit of a stolen or other wanted vehicle, the ALPR operator contacts a chase car or motorcycle officer with the description and direction of travel.

ELSAG's newest software, called the Tactical Operations Center, networks the output of the ALPR unit via a Web interface. Other units running the software in a Web browser on their in-car computers see the alerts at the same time as the ALPR operator, making the radio calls unnecessary. The chase officers have the added advantage of seeing the scanned image, the ALPR unit's interpretation of the license plate (e.g. does the "hit" match the picture?), and the reason the plate is in the hot list.

ALPR units will probably rely on pre-loaded hot lists of license plates for the foreseeable future. The technology limitation here isn't with ALPR. The ALPR units could easily send queries of each recognized license plate to NCIC or another online database via radio. The bottleneck is with NCIC, or rather each state's switcher, which has to process each request and its reply.[PAGEBREAK]

A single ALPR unit might generate as many requests in a unit of time as all of that state's regular users put together, overwhelming the system. This is why any hit from an ALPR hot list should be verified as still current before taking enforcement action. A stolen car might be recovered and released, or a warrant cleared, between the time the hot list is loaded on the ALPR system and the hit is obtained in the field.


Hot lists are a great way to find suspects, even when they're not managed by a police agency. Vigilant Video uses an unusual marketing plan to leverage ALPR units sold to private industry to benefit law enforcement agencies.

The company sells its ALPR products to "repo men" who look for vehicles that are to be repossessed when the owners default on their loans. The repo men cruise streets, apartment buildings, and business parking lots, scanning the plates of parked cars and comparing the results to their own internal hot list of wanted vehicles. These are locations where vehicle operators may leave, but will probably return on a regular basis. Owners of the ALPR systems must agree to upload their scanned vehicle data to Vigilant Video's servers for use by law enforcement.

Vigilant Video calls its database the National Vehicle Location Service, and it's free to any law enforcement agency with a National Law Enforcement Telecommunications System (NLETS) Originating Agency Identifier (ORI). A paid subscription service provides added benefits, including proactive searching of records from hot lists uploaded by the subscriber. If an investigator has the license plate of a vehicle used in a crime, the Vigilant Video database may be able to tell him or her where that vehicle has been, or where it can be found. The database is ever-expanding, with 12 million new records added each month.

The information contained in the dataset is extremely valuable. In December 2010, the Sacramento County (Calif.) Sheriff's Department investigated the home invasion of a 68-year-old woman. The victim had been tied up and forced to drink alcohol until she was almost unconscious, while the suspect ransacked her home. She was able to see and remember the license plate of the suspect's car.

When deputies entered the plate into the Vigilant Video database, they found the car had been recorded three times in recent weeks. Given the location information and a color photo of the car from the database capture, a K-9 deputy located the car in three minutes. The suspect foolishly tried to make a run for it and became a chew toy for the deputy's German shepherd. When arrested, the suspect still had the victim's driver's license in his pocket.

What's to Come?

Most ALPR vendors are tight-lipped about planned upgrades and new products that will be rolled out in the coming year. Matt Brady with Federal Signal said that we can expect smaller form factors for much of the hardware, so that future configurations may fit under light bars, rather than having to be mounted outboard of them or on the bumpers. As computing power grows, the systems will handle more lanes of traffic, have better accuracy rates, and will recognize more types of license plates.

Although there is some speculation that we'll see an ALPR unit that uses existing in-car video cameras, the vendors who were interviewed for this article weren't warm to that idea. ALPR cameras are generally of higher quality than those in in-car video systems, and most use IR emitters to reflect off of license plates, especially in low-light conditions. For the foreseeable future, ALPR will use dedicated cameras engineered for that purpose. 

Tim Dees is a retired police officer and the former editor of two major law enforcement websites who writes and consults on technology applications in criminal justice. He can be reached at

ALPR Manufacturers:

Digital Ally Inc.

ELSAG North America

Federal Signal PIPS


Group Techna

Hub-Data 911


Laser Technology

NDI Recognition Systems



Transport Data Systems

Vigilant Video


PHOTOS: Plate Hunters

New Uses for ALPR Systems

About the Author
Tim Dees Headshot
Officer (Ret.)
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