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Publication Number: FHWA-RD-03-082
Date: December 2003

Minimum Retroreflectivity Levels for Overhead Guide Signs and Street-Name Signs

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CHAPTER 1. INTRODUCTION

The development of minimum inservice levels of retroreflectivity (end-of-service-life values) for signs is a critical step in the evolution of providing a safe and efficient road transportation system. Recent activity in this arena began in 1984, when the Center for Auto Safety petitioned the Federal Highway Administration (FHWA) to establish retroreflectivity standards for signs and markings. In 1993, Congress required the Secretary of Transportation to revise the Manual on Uniform Traffic Control Devices (MUTCD) to include "a standard for a minimum level of retroreflectivity that must be maintained for pavement markings and signs which apply to all roads open to public travel."(1) Because of the work in progress, FHWA was able to develop suggested minimum retroreflectivity (MR) levels for signs in a relatively short time. Initial recommendations included overhead signs, but were later removed because of many unresolved issues with vehicle headlamp performance specifications and the difficulty of measuring overhead sign retroreflectivity.(2-3) Since the initial recommendations were made, vehicle headlamp performance specifications have been revised.(4) This research project was conducted to determine MR levels for overhead guide signs and street-name signs.

PROJECT OVERVIEW

As a direct result of the congressional mandate for minimum levels of retroreflectivity and the recently revised vehicle headlamp performance specifications, FHWA identified the need to conduct research to determine MR levels for overhead guide signs and street-name signs. The research project was awarded to the Texas Transportation Institute (TTI) in late 1999 and was started in mid-February 2000.

Goal

The purpose of the research was to develop scientifically based minimum levels of retroreflectivity for overhead guide signs and street-name signs.

Research Activities

The research project was a 15-month effort. The research activities are described below:

  • First Panel Meeting: The initial meeting between the researchers and the Contracting Officer's Technical Representative (COTR) took place on January 12, 2000, during the Transportation Research Board's 79th Annual Meeting in Washington, DC. This meeting was held approximately 1 month before the project was officially started. In this meeting, the researchers and the COTR discussed:
    • Project objectives and the general plan for meeting the objectives.
    • Key findings from previous research.
    • FHWA's concerns and experiences.
    • Activities in which the researchers would require FHWA assistance.
    • Issues and/or factors that needed to be addressed in the research, including minimum luminance, implementation of MR levels, and headlamps.
  • Literature Review: The research team reviewed a significant amount of previous research to assess the state-of-the-art in sign legibility and to identify experimental procedures that might have application to the research. Chapter 2 describes the results of the literature review.
  • Current Practices Survey: One of the initial efforts of the project was a review of traffic engineering manuals and a survey of State and local practices regarding overhead guide signs and street-name signs. Chapter 3 describes these activities and summarizes the results. Appendix A shows the survey and the detailed results.
  • Second Panel Meeting: The second meeting took place on May 26, 2000, in College Station, TX. The meeting was held after the literature review and the current practices review were completed. This meeting included the researchers, the COTR, and FHWA engineer Greg Schertz. In this meeting, the group discussed:
    • How the findings of the literature review and current practices could be combined to develop initial recommendations for MR of overhead guide signs and street-name signs.
    • Advantages and disadvantages of using the photometric models available at that time.
    • Voids in the research that need to be addressed to complete the research.
    • Future research activities needed to satisfy the research objectives.
  • Development of TTI MR Model: After the second panel meeting, the COTR and the researchers identified the need to develop an analytical model that can be used to determine MR levels. An overview of this model is explained in chapter 4. The details of the model are provided in appendix B.
  • Third Panel Meeting: The third meeting took place in September 2000 at the Turner-Fairbank Highway Research Center (TFHRC). This meeting included the researchers, the COTR, and FHWA researcher Carl Andersen. The meeting was held after the researchers completed the work on the development of the analytical model. The results of the literature review and current practices review were used to develop initial MR recommendations for overhead and street-name signs. In this meeting, the group discussed:
    • Sensitivity of key modeling factors, such as headlamp luminous intensity profiles, distance, and speed.
    • Implications of the initial MR levels for overhead guide signs and street-name signs.
    • Initial recommendations for a field study to address the shortcomings of the data available through the literature review and current practices review.
  • Fourth Panel Meeting: The fourth meeting between the researchers and the COTR took place in January 2001 at the National Committee on Uniform Traffic Control Devices (NCUTCD) meeting in Washington, DC. Like the previous meeting, Carl Andersen attended this meeting. The meeting was held after the researchers submitted their experimental design for the nighttime data collection to determine minimum luminance. In this meeting, the group discussed:
    • Dependent and independent factors to be considered in the study, including their limits.
    • Anticipated timeframe for conducting the study.
    • Number and age of the subjects.
    • Procedure to be used.
    • Expected results, including how they will be used to enhance the initial recommendations (developed using findings from the literature review and current practices review).
  • Field Evaluation: During March 2001, the researchers conducted a nighttime field study to determine the minimum luminance needed to read overhead guide signs and street-name signs. The study was designed to fill the voids found through the literature review. The signs were designed based on the current practices findings. Chapter 5 describes the field evaluation and subsequent findings.
  • Data Analysis: Once the field studies were completed, the researchers conducted sensitivity analyses of key factors to be used for the final model runs. Factors included in the analyses were minimum luminance as a function of distance, headlamp luminous intensity profiles, driver accommodation level, sign position, retroreflective sheeting type, speed, and vehicle type. With the sensitivity analyses completed, the researchers developed their recommendations for MR levels for overhead and street-name signs. Chapter 6 describes the analyses and findings.
  • Fifth Panel Meeting: In May 2001, the researchers presented their findings to the COTR and other FHWA personnel at TFHRC. The presentation included a summary of the research activities and findings, including final recommendations and identified areas for future research.

Chapter 7 provides the initial recommendations made as a result of the research described in the report. However, additional research was conducted that resulted in revised recommendations. This additional research was not part of this project; however, it directly affects the results. Therefore, chapter 8 was included to describe the revisions and the subsequent recommendations for overhead guide signs and street-name signs. Chapter 8 also provides a list of future research topics.

STUDY ISSUES

This section briefly describes the major study issues that impact MR levels.

Visibility Factors

The number of factors related to highway sign visibility can be overwhelming. The factors identified through the literature review can be categorized into four main headings as shown in table 1. Under each category are the corresponding design elements.

Table 1. Legibility Factors

Sign

Vehicle

Driver

Environment/Road

  • Position
    • Ground-mounted
      • Right
      • Left
      • Lateral offset
  • Overhead
    • Height
    • Lane positioning
    • Tilt
  • Size
  • Shape
  • Color
    • Background
    • Legend
  • Legend
    • Symbol
    • Alphabet
      • Font
      • Size
      • Stroke width
      • Letter spacing
      • Line spacing
  • Lighting
  • Retroreflective material
  • Type
    • Sports car
    • Passenger car
    • Pickup truck/SUV
    • 18-wheeler
  • Headlamp
    • Type
      • Halogen-tungsten
      • High-intensity discharge
    • Illumination distr.
    • Aim
    • Cleanliness
  • Windshield
    • Transmissivity
    • Cleanliness
  • Constant voltage
  • Visual characteristics
    • Acuity
    • Contrast sensitivity
    • Color deficiency
    • Other
  • Awareness
  • Mental load
  • Alcohol/drugs
  • Atmospheric conditions
    • Rain
    • Fog
    • Haze
    • Other
  • Background complexity
    • Urban
      • Residential
      • School
      • Commercial
      • Industrial
    • Rural
  • Time of day
    • Day
    • Dusk
    • Night
  • Horizontal alignment
  • Vertical alignment
  • Sight distance
  • Pavement reflectance

While each of the design elements listed above affect visibility on some level, not every element has the same effect and not all factors act independently. Given the limited time and resources associated with this project, it was not reasonable to explore each of the elements listed above. Furthermore, all of these elements can be reduced to three main components that impact visibility: the amount of light reaching the sign (illuminance), the efficiency of the retroreflective material (retroreflectivity), and the returned light that makes the sign appear bright (luminance). These three main components can be combined with a variety of other issues, such as the visual ability of the driver and the vehicle type, to determine the required luminance for the traffic signs. The luminance and contrast determine the legibility and recognition of highway signs. Therefore, these issues were explored using past research findings to help define and quantify those factors that are most influential in overhead and street-name sign visibility.

Materials

Traffic signs use retroreflective sheeting to help ensure that the signs communicate the same message day and night. Retroreflectivity redirects vehicle headlamp illuminance back toward the driver. There have been substantial improvements in retroreflective technology since it was first introduced using large glass beads called "cat's eyes." The currently available retroreflective technology is defined and described in American Society for Testing and Materials (ASTM) D4956.(5) As of 2001, ASTM has defined seven types of retroreflective sheeting approved for traffic signs. These types of sheeting can be broadly classified into two groups: one that uses microsized glass beads to retroreflect headlamp illuminance and another that uses microsized prisms to retroreflect the light. Table 2 includes a list of the currently defined retroreflective sheeting available for permanent traffic signs (according to ASTM D4956). This report uses the ASTM-type designation when referring to specific sheeting types.

Table 2. Types of Retroreflective Sheeting

Type Designation Description

I

Medium-high-intensity retroreflective sheeting, sometimes referred to as "engineering grade," and typically enclosed-lens glass-bead sheeting. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

II

Medium-high-intensity retroreflective sheeting, sometimes referred to as "super engineer grade," and typically enclosed-lens glass-bead sheeting. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

III

High-intensity retroreflective sheeting that is typically encapsulated glass-bead retroreflective material. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

IV

High-intensity retroreflective sheeting. This sheeting is typically an unmetallized, microprismatic, retroreflective-element material. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

VII

Super-high-intensity retroreflective sheeting having the highest retroreflectivity characteristics at long and medium road distances as determined by the RA values at 0.1° and 0.2 ° observation angles. This sheeting is typically an unmetallized, microprismatic, retroreflective-element material. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

VIII

Super-high-intensity retroreflective sheeting having the highest retroreflectivity characteristics at long and medium road distances as determined by the RA values at 0.1° and 0.2° observation angles. This sheeting is typically an unmetallized, microprismatic, retroreflective-element material. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

IX

Very-high-intensity retroreflective sheeting having the highest retroreflectivity characteristics at short road distances as determined by the RA values at a 1.0° observation angle. This sheeting is typically an unmetallized, microprismatic, retroreflective-element material. Typical applications for this material are permanent highway signing, construction-zone devices, and delineators.

Vehicle Headlamps

As mentioned, in the mid-1990s, one of FHWA's greatest concerns regarding the initially proposed MR levels for overhead signs was the global harmonization efforts related to headlamp specifications. In 1997, the Federal Motor Vehicle Safety Standards (FMVSS) related to headlamp specifications for vehicles sold in the United States were revised to include harmonized headlamp specifications. The research effort used currently available headlamp profiles as identified in the literature review and recently obtained illuminance data from the roadway to identify the headlamp profile that best replicates those currently found on the roadway. The advantage of this approach is that real-world factors, such as headlamp misalignment, headlamp cleanliness, and variations in available voltage, are considered, rather than using an exclusively theoretically based headlamp profile.

Driver

In recent years, there has been a concentrated effort to accommodate the needs of older drivers. This is especially critical for the establishment of MR levels since a driver's vision generally degrades with age, thus requiring brighter signs. The research conducted as part of this study focused on accommodating the needs of older nighttime drivers.

Measuring Retroreflectivity

The establishment of minimum levels of retroreflectivity for overhead and street-name signs is only one part of the process of ensuring that these signs have adequate nighttime visibility. Once minimum levels are developed, agencies need to be able to measure their signs and compare the measurements to the minimums. This is a challenge for both types of signs as discussed below.

Overhead Signs

Because of the position of overhead signs, the measurement of the retroreflectivity of these signs introduces a significant challenge. Except for the FHWA mobile retroreflectometer and the LaserTech Impulse® retroreflectometer, measurement of overhead sign retroreflectivity requires contact with the specific part of the sign being measured. Both of the noncontact instruments require data manipulation to provide retroreflectivity measurements representing the standard measurement geometry of 0.2° and -4.0°. As a result, current measurements of overhead sign retroreflectivity require lane closures and a worker on a sign bridge or in a bucket truck.

In addition to the difficulty of measuring overhead sign retroreflectivity, the large size of these signs requires a substantial number of measurements to provide a representative sample of the overall sign retroreflectivity. The current ASTM procedure for measuring sign retroreflectivity with a portable sign retroreflectometer (ASTM E1709) states that four measurements should be made. Assuming that this applies to a typical roadside sign, this results in a general average of about one reading for every 0.1 to 0.2 square meters (m2) of sign area. If a similar proportion were to be used on overhead signs (using an assumed sign size of 1x1 m), approximately 50 measurements of the sign background would be needed to get a reasonable representation of the overall sign retroreflectivity. Furthermore, with large guide signs, the legend also needs to be measured. There are no guidelines that indicate whether every letter in a sign needs to be measured, nor is there guidance on the number of measurements needed per sign. There are still a large number of signs in the field with button copy, and there are no field devices capable of accurately measuring the retroreflectivity of button copy. When the background and legend are both considered, the total number of retroreflectivity measurements could be 50 to 100 measurements for a typical sign.

These factors indicate that numerically based MR levels may not be an effective means of ensuring adequate retroreflectivity of overhead signs. Other procedures may also need to be developed for the minimum numbers to have any practical value. Alternative procedures should be based on the numerical minimums, but should not require actual retroreflectivity measurements. Examples of alternative procedures include minimum visibility distances or using a tracking schedule combined with sheeting-life curves and MR levels.

Street-Name Signs

Just as with overhead signs, there are some practical limitations on the ability to measure the retroreflectivity of street-name signs. Because of the height of street-name signs (i.e., above arm's reach), a pole-mounted retroreflectometer will typically be needed. However, street-name signs typically have crowded legends, leaving little open space for measuring the retroreflectivity of the background, especially if the positioning of the retroreflectometer is accomplished using a 2-meter (m) pole. The letter height and stroke width of street-name signs combine to provide a letter stroke that is too narrow for most retroreflectometers to measure without also measuring some of the background (green) retroreflectivity. Even if the legend retroreflectivity is to be measured, once again, accurately positioning the retroreflectometer on the end of a pole is a challenge. Finally, many street-name sign blanks are ribbed, with a thick section at the top and bottom of the blank to add rigidity. If the retroreflectometer is using a faceplate to help provide a flush and perpendicular position, then the unit may not be able to make proper contact with the face of the sign. Not using the faceplate may reduce the accuracy of measurements because of lack of proper alignment with the sign face.

These factors indicate that measuring the retroreflectivity of both the legend and the background of street-name signs may not be a practical undertaking. Again, alternative procedures may be needed, such as a minimum visibility distance or a maximum sign age.

From this point hereafter, the units of this report are presented using the common terminology among practicing traffic engineers and visibility experts. The photometric terms are expressed in SI units, as that is the standard in the industry. Sign size, letter height, and other sign-related dimensions (including legibility index) are expressed in English units because that is still the preferred practice by the transportation profession. Table 3 can be used to supplement the conversion table shown on page ii.

Table 3. Sign Dimension Conversions

Sign Size

Letter Height

inch (in)

millimeter (mm)

inch (in)

millimeter (mm)

18

450

4

100

24

600

6

150

30

750

8

200

36

900

10

250

48

1200

12

300

 
 

14

350

 
 

16

400

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