Nighttime Visibility of Traffic Signs: Chapter 1 - Introduction

CHAPTER 1 - INTRODUCTION

Traffic control devices provide one of the primary means of communicating important information to street and highway users. Traffic signs are one of the three basic types of traffic control devices, the others being markings and signals. Signs serve several important purposes, including providing regulatory, warning, and guidance information to road users. Signs utilize shape, color, and text or symbols to communicate various messages to road users. The basic principles that govern how shape, color, and legend are used to convey messages are presented in the Manual on Uniform Traffic Control Devices, or MUTCD (1). The MUTCD addresses many different aspects of traffic control devices, including design, placement, operation, maintenance, and uniformity. The visibility of traffic control devices is a key element that is addressed by several of these aspects.

Traffic control devices must be visible to be effective. There are numerous methods that can be used to provide daytime and nighttime visibility of traffic control devices. Retroreflective sheeting is one of the methods that has been used for many years to provide nighttime sign visibility. Retroreflectivity has been the subject of significant federal activity in recent years and includes the development of research recommendations for minimum levels of retroreflectivity for signs.

This report describes a series of workshops that were conducted to solicit public agency input regarding the development of minimum levels of retroreflectivity for signs. The findings and recommendations obtained from these workshops will be used to develop a proposed rule regarding nighttime sign visibility and retroreflectivity.

RETROREFLECTIVITY

Retroreflectivity is a type of reflection that directs light back toward the source from which it came. If an observer is located close to the light source (such as a driver located behind a vehicle's headlights), the retroreflective target appears brighter than a non-retroreflective target. Retroreflectorization of signs is accomplished through the use of sheeting that contains beads or microprismatic mirror elements. The beads or prisms return illumination from a headlamp back towards a driver. Sign retroreflectivity is represented by the coefficient of retroreflection (R_A) and the unit of measure is candela per lux per meter squared (cd/lux/m²). Retroreflectivity, however, is a material property and is only one element that defines the luminance (the apparent "brightness") of a sign. The luminance of a sign depends upon the amount of light reaching the sign; the retroreflectivity of the sign (which is influenced by the condition of the sheeting, the presence of frost or dew on the face of the sheeting, and the orientation of the sign relative to the vehicle); the relative position of the headlamps, sign, and driver (collectively referred to as the viewing geometry); and other factors such as atmospheric transmissivity (which is influenced by conditions such as rain, fog, and snow) and windshield transmissivity.

The viewing geometry is defined by various angles between the headlamps, sign, and driver, the most common of which are the entrance angle and observation angle. The entrance angle is the angle between the light source (a headlamp on a vehicle) and the perpendicular to the target (a sign). The observation angle is the angle between the light source (a headlamp) and the observer (a driver). Figures 1 and 2 illustrate the concept of entrance and observation angle for laboratory measurement and real-world conditions. In the typical roadway scenario, there are different entrance and observation angles for each headlamp on the vehicle. For viewing geometries that are the most common in the roadway environment, the observation angle is the more significant of the two angles for sign retroreflectivity.

The retroreflectivity level of a sign changes as the angles change. A sign that has a retroreflectivity level of 100 cd/lux/m² at one set of measurement angles will have a different retroreflectivity level at a different set of measurement angles. Figure 3 illustrates how the retroreflectivity of a Type III (high intensity) material changes as a function of the entrance and observation angle. Therefore, retroreflectivity is often defined by standard measurement angles, the most common of which are established by the American Society for Testing and Materials (ASTM). The standard ASTM angles for measuring sign retroreflectivity are -4 and +30 degrees for the entrance angle and 0.2 and 0.5 degrees for the observation angle. The combinations of these angles used to define sign performance (0.2, -4; 0.2, +30; 0.5, -4; 0.5, +30) for material testing are not intended to the geometries associated with a vehicle on a typical road casting light on a roadside sign.

MUTCD VISIBILITY CRITERIA

The current MUTCD specifically addresses the visibility of signs in several sections of the Manual. Portions of some of the most significant sections from Parts 1 and 2 are presented in Figures 4 and 5, respectively. Sign retroreflectivity is specifically addressed in Section 2A.08, which states that "Regulatory, warning, and guide signs shall be retroreflective or illuminated to show the same shape and similar color by both day and night, unless specifically stated otherwise in the text discussion in this Manual of a particular sign or group of signs" (1). In publishing the 2000 MUTCD, the FHWA added Section 2A.09 to reserve a section for future guidelines on minimum retroreflectivity levels.

The MUTCD has had various requirements for retroreflectorization of signs through the various editions. Appendix A presents the language from each MUTCD edition relative to sign retroreflectivity and illumination and the maintenance of sign retroreflectivity. While there were numerous refinements in the early editions of the MUTCD, the language for retroreflection has been essentially the same since the publication of the 1971 MUTCD. That language states:

1971 MUTCD - Regulatory and warning signs, unless excepted in the standards covering a particular sign or group of signs, shall be reflectorized or illuminated to show the same shape and color both by day and night.

While there was not a major change with the 2000 MUTCD in the impact of the MUTCD language related to retroreflectivity, there was a significant change in the language related to the maintenance of signs. The following compares the language of the 1988 and 2000 MUTCD relative to maintaining sign retroreflectivity:

Figure 1. Entrance and Observation Angles for Laboratory Measurement

Figure 2. Entrance and Observation Angles in Real-World Conditions

Figure 3. Retroreflectivity Variability

1988 MUTCD - "All traffic signs should be kept in proper position, clean and legible at all times. To assure adequate maintenance, a suitable schedule for inspection, cleaning and replacement of signs should be established. Special attention and necessary action should be taken to see that weeds, trees, shrubbery and construction materials do not obscure the face of any sign." [GH3]

2000 MUTCD - "All traffic signs should be kept properly positioned, clean, and legible, and should have adequate retroreflectivity. To assure adequate maintenance, a schedule for inspecting (both day and night), cleaning, and replacing signs should be established. Steps should be taken to see that weeds, trees, shrubbery, and construction, maintenance, and utility materials and equipment do not obscure the face of any sign."

The 2000 MUTCD also contains other sections that address the visibility of signs, both in daytime and nighttime conditions. Appendix A also includes selected pertinent language from the 2000 MUTCD related to traffic control device visibility.

NATIONAL RETROREFLECTIVITY EFFORTS

Although there has been a requirement for retroreflective signs in the MUTCD for many years, this requirement has no specific values of retroreflectivity. In 1985, the Center for Auto Safety (CAS) petitioned the FHWA to initiate rulemaking on the issue of minimum standards of retroreflectivity for traffic control devices. That petition contended that the range of drivers was not being accommodated by the traffic control devices allowed in the MUTCD with respect to nighttime conspicuity dependent upon retroreflective illumination. In April 1985, the FHWA published an Advance Notice of Proposed Rulemaking in the Federal Register to solicit comments on (1) proposed amendments to the MUTCD addressing retroreflectivity standards for signs and markings, and (2) ten questions related to sign and pavement marking retroreflectivity. The text of the notice is provided in Appendix B.

The FHWA had already begun a program of research on nighttime visibility of traffic control devices before the CAS petition for minimum levels of sign and marking retroreflectivity. This research program continued through the 1980s and into the 1990s, resulting in many research reports (2, 3, 4, 5, 6, 7), and culminating in 1993 with initial research recommendations for minimum levels of in-service sign retroreflectivity (8, 9).

Shortly before the FHWA published the initial research recommendations for minimum sign retroreflectivity, Congress included the following requirement in the 1993 Department of Transportation Appropriations Act. This Congressional mandate formally directed the FHWA to develop and implement minimum retroreflectivity standards for signs and markings.

"The Secretary of Transportation shall revise the MUTCD to include a standard for a minimum level of retroreflectivity that must be maintained for traffic signs and pavement markings which apply to all roads open to public travel."

In 1995, two years after the publication of the initial research recommendations for minimum retroreflectivity levels, the FHWA sponsored a series of three workshops on minimum retroreflectivity levels. A summary of the comments and feedback from the 1995 workshops is provided in Appendix C. Based on the input received in these workshops, combined with additional analysis, the FHWA slightly revised the minimum levels in 1998 (10, 11). Appendix D presents the 1993 and 1998 research recommendations for minimum levels of retroreflectivity.

Late in 1998, the FHWA was close to issuing a proposed rule on minimum levels of sign retroreflectivity when the Board of Directors of the American Association of State Highway and Transportation Officials (AASHTO) requested the FHWA to delay any future action of minimum retroreflectivity levels until an AASHTO task force could review the issue and develop recommendations for the FHWA to consider. The AASHTO Standing Committee on Highways created the Retroreflectivity Task Force in early 1999. It included representatives from federal, state, city, and county transportation agencies, plus industry, research, and private sector representatives. The efforts of this Task Force led to a resolution that was adopted by the AASHTO Board at its December 2000 meeting. The resolution is presented in Appendix E. The key points of the resolution are:

• Traffic signs should be visible at night.
• Agencies need processes to provide signs that are visible at night.
• The processes used to provide sign visibility at night should not impose undue burdens on transportation agencies.
• Agencies should be able to choose from several different processes that can be used to provide sign visibility at night.
• Minimum visibility requirements should be simple and unambiguous so that they can be easily and properly applied.
• Minimum retroreflectivity values should not be included as part of the MUTCD.
• Agencies should have six years to implement the methods.

The FHWA continued to conduct additional research on sign retroreflectivity in the late 1990s, including research to develop minimum levels for overhead signs and street name signs. This research identified a need to update the minimum retroreflectivity levels to better reflect national trends related to vehicle types and sizes, headlamp performance, nighttime driver needs, and newer sheeting materials. As the sign retroreflectivity workshops were being conducted in the summer of 2002, the FHWA was also sponsoring research to update the minimum retroreflectivity levels. As the workshops were conducted, the researchers presented the latest findings in developing the minimum retroreflectivity levels (12). Table 1 presents the research recommendations for updated minimum retroreflectivity levels. This information was submitted to the FHWA shortly after the workshops concluded; however, the researchers presented interim versions of these levels during the workshops as "works-in-progress."  The updated levels are based on the assumptions in the list below. More detailed descriptions of these assumptions are included in the Transportation Research Board (TRB) paper (12) and the research report (13).

• Roadway - Straight and flat.
• Sign - Sign faces perpendicular to roadway.
• Demand sign luminance - Selected threshold levels equivalent to accommodating legibility or recognition for 50 percent of drivers over 55 years of age.
  • Driver needs for demand luminance are based on available data from the Texas Transportation Institute (TTI) and Mercier et al. studies (12, 14).
  • Legibility distances are based on providing 40 feet per inch of letter height. [GH4]
  • Recognition distances are based on CARTS Minimum Required Visibility Distance (MRVD) values.
  • Minimum demand luminance equals 1.0 cd/m².
• Ambient lighting - Representative of a dark rural environment.
  • No ambient lighting.
  • No glare sources.
• Headlamps - 50^th percentile 2000 market-weighted, low-beam distribution from University of Michigan Transportation Research Institute (UMTRI) (15).
• Vehicle - Vehicle dimensions represent a sport utility vehicle or pick-up truck.

The initial development of minimum retroreflectivity levels consisted of many different values that represented many different conditions. From a purely scientific standpoint, each unique situation (longitudinal location, lateral location, height, sign type, sign legend, sign material, etc.) has a specific minimum retroreflectivity level associated with that situation. However, from a practical point of view, the minimum retroreflectivity levels need to be easy to manage and implement. This requires the large number of unique minimum retroreflectivity levels to be consolidated to a practical number. This was a particularly significant issue to the AASHTO Retroreflectivity Task Force. As part of the research effort, the minimum retroreflectivity levels were consolidated into an easier-to-use format. In consolidating the minimum levels, certain decisions were made regarding the resolution of the levels. The consolidation efforts ultimately resulted in some degree of compromise between the precision of the minimum levels and the brevity of the minimum levels. The research report provides a detailed description of how the values were consolidated to an implementable format.

WORKSHOP ORGANIZATION

Before developing a proposed rule for minimum levels of sign retroreflectivity, the FHWA wanted to obtain input from public agency personnel regarding the issues and alternatives associated with minimum levels of in-service sign retroreflectivity. As was done in 1995, the FHWA sponsored a series of four workshops to present the most current information on minimum sign retroreflectivity and potential alternatives that might be considered in developing a proposed rule. The purpose of the workshops was to solicit input from public agency personnel on the proposed minimum guidelines for nighttime sign visibility and the various means that agencies could use to implement the guidelines. The input received from workshop participants was considered an important element in formulating a proposed rule that will consider the needs and resources of transportation agencies.

Four workshops were conducted in the summer of 2002. Participation in the workshops was by invitation only and included representatives from city, county, and state transportation agencies, professional organizations (including an industry group), agency attorneys, and FHWA staff. During the course of the workshops, participants were presented with a variety of information on sign retroreflectivity, took part in a nighttime demonstration of sign retroreflectivity, and worked to develop recommendations regarding minimum levels of in‑service retroreflectivity for signs.

Workshop Locations

The workshops were held in four geographically dispersed locations. At some of the workshop locations, a transportation agency provided the meeting facilities and provided the facilities for the nighttime sign demonstration. The workshops were held in:

• Lakewood, Colorado - This workshop, located in the Denver metropolitan area, was held at a local hotel. The nighttime demonstration took place at the Denver Federal Center. The Central Federal Lands office of FHWA coordinated the workshop and demonstration arrangements. This workshop was held July 10-11, 2002.
• Hudson, Wisconsin - This workshop, located approximately 30 miles east of the Minneapolis/St. Paul metropolitan area, was held at a local hotel. The nighttime demonstration took place on a dead end frontage road in Minnesota. The FHWA division office coordinated the workshop arrangements and the Minnesota Department of Transportation supported the demonstration. This workshop was held August 13-14, 2002.
• College Station, Texas - This workshop, located approximately 100 miles northwest of Houston, was held at the Texas Transportation Institute (TTI). The nighttime demonstration took place at the TTI Riverside Campus Retroreflectivity Demonstration Complex. TTI staff coordinated the workshop and demonstration arrangements. This workshop was held August 21-22, 2002.
• Hanover, Maryland - This workshop, located in the Baltimore metropolitan area, was held at the Maryland State Highway Administration (SHA) facility. The nighttime demonstration took place on a road network near the workshop location. Maryland SHA staff coordinated the workshop and demonstration arrangements. This workshop was held September 4-5, 2002.

Workshop Participants

The primary purpose of the workshops was to solicit opinions and input from public transportation agency personnel. Therefore, the majority of participants represented city, county, or state transportation agencies, which would be directly affected by the implementation of the proposed new requirements for traffic signs. Other perspectives that were represented in the workshops included local transportation assistance program (LTAP) staff, professional organizations that have public agency members, a professional organization representing industry, attorneys of state transportation agencies responsible for defending tort claims, and representatives of the FHWA. Participation in the workshops was by invitation only. Potential participants were identified through contacts with the organizations listed below and other sources.

• American Association of State Highway and Transportation Officials
• American Public Works Association
• American Traffic Safety Services Association
• Federal Highway Administration
• Institute of Transportation Engineers
• National Association of County Engineers

Approximately 25 people attended each workshop, with a total of 99 individuals attending the four workshops. The participant list for each workshop is presented in Appendix F. Table 2 summarizes the representation in each workshop by type of participant. A total of 31 states and Puerto Rico were represented by the city, county, and state agency participants.

The workshops were organized and facilitated by five individuals, whose responsibilities are described below. Figure 6 presents photographs of the four workshops.

• FHWA Staff
  • Greg Schertz -Coordinator of the FHWA retroreflectivity rulemaking effort. He assisted in preparing workshop materials and facilitating some of the workshop presentations.
  • Ken Opiela -Coordinator of the FHWA retroreflectivity research program. He was responsible for recording the discussion and comments during the workshops.
  • Peter Hatzi -Contracting Officer Technical Representative for the workshop project, he had overall responsibility for the workshops.
• TTI Staff
  • Gene Hawkins -Principal Investigator for the workshop task. He prepared much of the workshop materials and facilitated some of the workshop presentations.
  • Paul Carlson -Researcher for FHWA to update minimum retroreflectivity levels. He assisted in preparing workshop materials and facilitating some of the workshop presentations.

Figure 6. Workshop Photographs

Workshop Schedule and Content

All four workshops consisted of two half-days of classroom activity, with an nighttime demonstration sandwiched between. During the afternoon discussion on the first day, the facilitators presented background information and described potential alternatives for the proposed rulemaking. That evening, the facilitators conducted a demonstration of signs at various levels of retroreflectivity. The following morning was devoted to reviewing the results of the nighttime demonstration, discussing the issues, and developing the workshop recommendations for MUTCD language. Table 3 lists the topics that were presented in the four workshops and the general schedule that was followed for the workshops. There were minor changes in content and schedule from one workshop to the next. The workshop presentations and handouts presented to the Hanover workshop participants have been placed on the World Wide Web at http://tcd.tamu.edu/Documents/MinRetro/MinRetro.cfm. These are representative of the materials used in all the workshops. Key pieces of information presented in the workshops include:

• Interpretation of standard - ASTM defines a standard as a "concept established by authority, custom, or agreement to serve as a model or rule in a measurement of quality or the establishment of a practice or procedure."  Using this interpretation, the FHWA has determined that including minimum retroreflectivity guidelines in the MUTCD is sufficient to satisfy the Congressional mandate for a standard even if the MUTCD language uses a "should" statement.
• Basis for minimum retroreflectivity levels - During each workshop, the most recent information on the research to develop updated minimum retroreflectivity levels was presented. These levels are shown in Table 1. Key elements that were presented in the workshops include:
  • Simplified explanations of the science of retroreflectivity.
  • Impacts of headlamps, drivers, and sheeting materials on sign visibility.
  • Reasons for updating the 1998 minimum retroreflectivity levels.
  • Processes used to consolidate the updated retroreflectivity levels.
  • Explanations of why different types of materials need different minimum retroreflectivity levels.
• Implementation options - The following options were presented to workshop participants as methods that were being considered for inclusion in the MUTCD as alternatives to measuring sign retroreflectivity:
  • Nighttime sign inspection - Signs are visually inspected at night under typical driving conditions.
  • Sign service life - Signs are replaced before they reach the end of the expected retroreflective life.
  • Inspection panels - Sign sheeting panels are used to judge retroreflectivity by comparing the panels to actual signs.
  • Control signs - The performance of a sample of signs is used to determine the replacement of all signs.
  • Measured sign retroreflectivity - An instrument is used to measure retroreflectivity.

Notes: Time of nighttime demonstration varied depending upon the sunset time.

• Exempted signs - It may be appropriate to exempt some signs from meeting nighttime visibility criteria due to the type of message in the sign, its intended audience, and/or the importance of the message. A number of signs were mentioned as possible exemptions.
• Nighttime sign visibility workshop – A demonstration of nighttime sign visibility was presented on the evening between the two days of the workshop to familiarize the participants with the appearance of various levels of retroreflectivity.