Synthesis of Sign Retroreflectivity Deterioration Rates Across the U.S.

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Synthesis of Sign Retroreflectivity Deterioration
Rates Across the U.S.

• Elizabeth A. Harris, William Rasdorf, and
  Joseph Hummer

Research Sponsored by North Carolina DOT
Technical Session 3B Roadway and Intersection
Safety
Southern District ITE Annual Meeting April 22-25,
2007, Knoxville, TN
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Why Traffic Signs?

• Essential component of roadway and intersection
  safety
• Effective sign asset management systems are
  needed in order to ensure safety
• FHWA minimum retroreflectivity standards are
  coming

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Retroreflectivity (Ra)

• Ratio of the light sign reflects to driver (cd)
  to the light that illuminates the sign (lx) per
  unit area (m2).

cd
lux
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Sheeting Types

• Type I (Engineering Grade)
• Type III (High Intensity)
• Others

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Proposed Standards
Sheeting type should not be used
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Research Background

• NCDOT Research period July 2004 to June 2006
• Research group led by Dr. William Rasdorf and Dr.
  Joe Hummer at NCSU
• Research in response to proposed FHWA minimum
  sign retroreflectivity standards

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Situation in North Carolina

• NCDOT maintains 78,800 road miles
• Over 1 million signs
• Uses visual nighttime inspection method

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Sites Visited
Siler City
Asheville
Greenville
Shelby
Fayetteville
5 NCDOT Divisions
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Data Collection Procedure

• Visited Divisions January-April 2005
• Measured white, yellow, red, and green signs
• Focused on Type I (Engineering Grade) and Type
  III (High Intensity) sheeting

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Data Collection Procedure

• Nighttime Inspection Visual Evaluation
• Ride with sign crews during inspection

• Daytime Retroreflectivity Evaluation
• Measured uncleaned retroreflectivity of signs
  from nighttime evaluation using a
  retroreflectometer

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Sign Deterioration Studies

• FHWA (1991)
• AASHTO (NTPEP Testing Program)
• Oregon (2001)
• Louisiana (2002)
• Indiana (2002)
• North Carolina (2006)

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Study Data Collection

• All used a hand-held retroreflectometer to
  measure sign retroreflectivity
• Type I and Type III signs were typically measured
• White, yellow, red, and green signs
• Signs measured cleaned and uncleaned

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Synthesis of Studies

• Deterioration with age models determined for 8
  combinations of sign color and sheeting type
• Found weak correlation between sign age and
  retroreflectivity
• However, we found that retroreflectivity
  generally declines linearly over time

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Sample Type I Red Signs
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Selection of Reasonable Models

• Highest R2 Values (less scatter)
• Deterioration trend
• Age when signs would be replaced in NC
• Based on NCDOT visual minimum of 20 Ra for white
  and yellow signs and 4 Ra for red and green signs

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Deterioration Observations

• The models selected are the best available from
  current research
• High variance in models due to data collection
  error, retroreflectometer error, and uncontrolled
  field conditions
• Room for future improvement
• Lack of older Type III signs in all data sets

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Sign Damage

• Human Damage
• Accidental
• Mowers and knockdowns
• Intentional
• Vandalism
• Natural Damage
• Tree sap, water damage

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Sign Damage

• Secondary roads tend to have the most damage,
  especially vandalism
• Yellow signs found to be more prone to vandalism
  than other colors
• Vandalism
• 66 Paintball
• 26 gunshots
• 8 eggs

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Sign Replacement
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Sign Asset Management

• Condition Assessment
• Day to Day Observations
• Nighttime Sign Inspections
• Predictions from Deterioration Model
• Management Methods
• Budget
• Replacement Priority
• Sign Inventory

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FHWA Retroreflectivity Maintenance Strategies

• Assessment Methods
• Visual Nighttime Inspection
• Measured Retroreflectivity
• Management Methods
• Expected Sign Life
• Blanket Replacement
• Control Sign

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Policy Implications

• Since Type III signs have a longer lifetime (15
  yrs), visual inspection performance is not as
  critical
• NCDOT is using only Type III for all new sign
  installations
• Ensure that signs are inspected regularly and
  that sign maintenance budgets are adequate

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Policy Implications

• Perform additional sign research
• Need more retroreflectivity vs. time data
• Need to measure older signs
• Will help to better determine sign lifetime
• Create a sign experimental facility to better
  test how signs degrade long-term in various
  geographical areas

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QUESTIONS?Elizabeth A. Harris William Rasdorf
Joseph Hummercontact liz_harris_at_ncsu.edu