effects of license plate attributes on automatic …...• automatic number plate recognition •...

http://www.itre.ncsu.edu

Institute for Transportation Research and Education – N.C. State University

Effects of License Plate Attributes on Automatic

License Plate Recognition

Daniel J. Findley, PhD, PE Christopher M. Cunningham, PE

Jeffrey C. Chang, EI Kyle A. Hovey

Michael A. Corwin


• Primary purpose of license plates is to identify vehicles

• NC DMV standard issue plate is embossed with 3 letters and 4 numbers (red ink 2007-2009)

• ~100 specialized license plates – Full background – Standard issue background

• Personalization also an option on standard plates and most specialized plates

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Background

Presenter

Presentation Notes

Each state can have hundreds or thousands of different license plates, traveling through neighboring states


• Automatic License Plate Recognition (ALPR) – A tool to identify vehicles – Also known as:

• Automatic number plate recognition • Automatic vehicle identification • Car plate recognition • License-plate recognition

– Operates at up to 160 mph – 1,000’s of license plate checks per shift (50-100 manually)

Background

Presenter

Presentation Notes

Law officers are using this innovative technology with much success to identify vehicles of interest, such as: Stolen vehicle identification or revoked drivers Amber Alerts Wanted felons Monitor schools and campuses Delinquent citations for revenue enforcement Crime scene intelligence and surveillance Assist in drug enforcement Made up of two primary components – camera system and processing system


Objective

• To determine the readability of North Carolina’s license plates with an ALPR system

• Focus on law enforcement applications using two ALPRs – City of Raleigh Police Department – Federal Signal / PIPS Research and Development

• Controlled environment

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ALPR Predicted Characters

ALPR Captured Image

Actual License Plate


Test Location – NC SHP Track

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Experiment Design

• Collect plates (902 total) • Catalog

– Type – Plate number – Condition – Reflectivity

• Sort for field test


Experiment Design – 6 Plate Types

Presenter

Presentation Notes

Standard issue plates Standard issue plates (pre-2007 and current) with blue characters (240 plates – Example plate LRV-7900) Standard issue plates (2007 to 2009) with red characters(274 plates – Example plate WXF-7388) Specialized and personalized plates Specialized plates in the standard issue format with special artwork on the left and a stacked suffix (95 plates – Example plate 0325 KF) Specialized plates in the standard issue format with special artwork on the left and no stacked suffix (47 plates – Example plate S4486) Existing specialty plates with the previous full-background design (6 plates – Example plate 0409 BP) New specialty plates with the new full-background standard template format (240 plates – Example plate 6C99 SM)


Experiment Design

9 feet

Presenter

Presentation Notes

30 license plates (25 replaced each run, 5 for calibration) 36 runs Lateral offset = 9 feet Distance between plates = 40 feet 25 mph vehicle 24” above ground 2 hours with 5 people changing license plates at 5 stations each


Measures of Effectiveness

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Read Rate and Capture Rate

• Three options for evaluating an ALPR: – No capture:

– Capture only:

– Capture and read:

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ABC-1234

ABC-1234

ABC-1234


Overall Results

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PIPS R&D ALPR Match Type and Quantity

Raleigh PD ALPR

Total Capture and Read

Capture Only

No Capture

Raleigh PD ALPR Match

Type and Quantity

Capture and Read 271 16 6 293 Capture Only 81 297 23 401

No Capture 15 70 123 208 PIPS R&D ALPR Total 367 383 152 902

Presenter

Presentation Notes

Explain this well! 293 plates captured and read by Raleigh PD, 367 captured and read by PIPS, among those 271 were captured and read by both Etc, etc.


Results by Plate Type

Plate Type Syntax Type

# Plates Raleigh PD

Capture Rate % (Number)

Read Rate % (Number)

Standard Issue - Blue Ink Std 154 96% (148) 96% (142) Person 86 84% (72) 40% (29)

Standard Issue - Red Ink Std 249 66% (164) 45% (74) Person 25 56% (14) 7% (1)

Specialty FIF - No Stacked Character Std 43 86% (37) 57% (21) Person 4 50% (2) 50% (1)

Specialty FIF - Stacked Character Std 83 80% (66) 20% (13) Person 12 33% (4) 0% (0)

Specialty Non-FIF - New Style Std 213 80% (171) 7% (12) Person 27 52% (14) 0% (0)

Specialty Non-FIF - Old Style Std 5 20% (1) 0% (0) Person 1 100% (1) 0% (0)

Total 902 77% (694) 42% (293)

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Note: Std = Standard, Person = Personalized


Stacked Characters

• 341 stacked characters on the 902 plates • Raleigh PD ALPR system was able to capture

75% and able to accurately read 60% of the captured plates

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Presenter

Presentation Notes

Full tables of alphanumeric characters and stacked characters are available, please see me for more details


Calibration • For repeatability testing, 5 stations did not change during

testing • Four of the calibration stations had consistent readings on

all but 1 of the possible 288 readings – Correct & Consistent: All 36 runs for the 2 standard syntax blue

character plates and specialized plate in the standard issue format resulted

– Incorrect & Consistent: Personalized standard issue blue character license plate, the Raleigh PD ALPR system reported an “8” instead of a “B”

• One inconsistency: PIPS R&D ALPR same error as the Raleigh PD ALPR system for 35 of the runs and a different error on one of the runs in which the number string “15” was reported as “IS”

• The specialty plate with the full-background design created more difficulty in terms of repeatability for the two systems – Approximately two-thirds of the readings were consistent

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Presenter

Presentation Notes

To test for repeatability of the ALPR systems, 5 calibration stations contained license plates which did not change throughout the 36 runs of testing The five license plate types were: standard issue plates with blue characters (one plate with personalized syntax and two plates with standard syntax) specialized plate in the standard issue format with special artwork on the left and no stacked suffix, specialty plate with the previous full-background design and a stacked character The most common error for each ALPR system resulted from adding an extra number “3” at the beginning of the license plate number, likely due to the artwork on the left side of the plate, and not reporting the stacked characters on the right side of the plate.


Modeling Readability • A logistic regression model was developed to model the

probability of capture and read for standard issue license plates based on the Raleigh PD ALPR system

• Each of the terms were statistically significant • Equations are:

– Capture Rate = 0.05 - 1.3 * Red Ink + 0.03 * Contrast - 0.8 * Personalized – Read Rate = 0.4 - 2.1 * Red Ink + 0.01 * Contrast - 3.1 * Personalized

Where: – Red Ink = Presence of Red Ink on License Plate (0 = Blue Ink, 1 = Red Ink) – Contrast = Contrast Ratio of License Plate Characters and Background – Personalized = Presence of Personalized Syntax (0 = Standard Syntax, 1 =

Personalized Syntax

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Modeling Readability - Capture Rate

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Modeling Readability - Read Rate

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Summary • Most readable: standard issue, blue ink license

plate • Personalization resulted in a read rate ½ of the

read rate of the standard issue syntax • Red ink performed significantly worse • Specialty plates without the stacked character

were more easily captured and read • Specialty license plates with the full

background were generally captured, but difficulty accurately reading the plates

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Presenter

Presentation Notes

Standard issue, blue ink license plate has the highest capture and read rates of all the plates Personalization of the blue ink license plate resulted in a read rate approximately half of the read rate of the standard issue syntax Red ink used on plates from 2007 to 2009 performed significantly worse than blue ink in terms of both capture rate and read rate Among the specialty license plates with the First in Flight background, the plates without the stacked character were more easily captured and read Among the specialty license plates with the full background, the ALPR system was generally able to appropriately recognize the plates as license plates, but had difficulty accurately reading the plates for both the standard syntax and personalized syntax (none of the 27 personalized plates were read correctly by either system)


Conclusions and Recommendations • Many incorrect matching combinations are

matches between letters and numbers, supporting a need for syntax to read plates consistently

• Found significant difficulty with accurate readings of various specialty and personalized license plates

• Readability should be a criteria for consideration when decisions are made regarding new license plate designs – Ink color, syntax type, and contrast have a significant

impact – Stacked characters, background colors, and consistent

location of symbols are important on specialty plates

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Presenter

Presentation Notes

ALPR systems are a tool to efficiently identify thousands of vehicles per hour, if license plates are readable Ink color, syntax type, and contrast have a significant impact on the ability to recognize and accurately read a license plate using an ALPR system The inherent purpose of a license plate – to identify a specific vehicle – should be considered when designing, approving, or instituting a new license plate Although it might be possible to adjust and modify an ALPR to capture some of the more difficult to read license plate types presented in this paper, these modifications would likely impact a system’s ability to accurately read other license plate types. Therefore, law enforcement agencies should consider the likelihood of encountering each license plate type when optimizing an ALPR system to maximize the overall likely capture and read rate among the population of license plates common in the agency’s jurisdiction


Acknowledgements • This study was funded by the North Carolina

Department of Transportation. • The authors would like to acknowledge the efforts of

those who supported the research: – Dr. Moy Biswas and Heath Gore of the Research Unit – Michael Robertson and Johanna Reese of the NCDOT DMV – Major Troy Butler and the North Carolina State Highway

Patrol – Karen Brown, Charles Congleton, Jimmy Evans, Robert

Leon, Tracey Goodwin, and additional staff from Correction Enterprises

– Officer John Maultsby of the City of Raleigh Police Department

– David Bynum with Federal Signal / PIPS Technology

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Questions? Daniel J. Findley, PhD, PE

Senior Research Associate Institute for Transportation Research and Education

North Carolina State University

[email protected] (919) 515-8564

www.itre.ncsu.edu

http://www.itre.ncsu.edu/�

effects of license plate attributes on automatic …...• automatic number plate recognition •...

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