Guidelines for Nighttime Visibility of Overhead Signs (2016)

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3 Introduction Effective highway signing is an important component to driver decision making, comfort, and safety. Signs must be visible and at the right location for drivers to have adequate time to properly respond to them. Sign visibility is made pos- sible when at least one light source provides illumination. While daytime sign visibility is generally provided by natural light from the sun, the principal source of light for nighttime sign visibility is the vehicle’s headlamps. Retroreflective sign sheeting materials are used on practically all traffic signs to redirect the vehicle headlamp illumination back toward the driver, which enhances the nighttime visibility of the signs. There are also other sources of light that can help illumi- nate signs at night. Sign lighting, which can be external or internal to the sign, can be used to supplement vehicle head- lamp illumination. Sign lighting has been used throughout the United States to enhance the nighttime visibility of signs. However, as retroreflective sheeting materials have become more efficient at returning vehicle headlamp illumination back toward the driver, the use of sign lighting has decreased. Other light sources may also contribute to sign lighting, such as overhead roadway lighting and lights from nearby busi- nesses. On the other hand, when these additional light sources grow to a certain point or are poorly designed or controlled, they can create a complex nighttime viewing environment that can negatively impact sign visibility to the point that sign lighting may be needed. The available nighttime sign visibility guidelines have gen- erally been developed from data collected in conditions rep- resenting rural environments that are not visually demanding (1). Organizations such as the Illuminating Engineering Soci- ety of North America (IESNA) and AASHTO indicate that signs should be brighter in areas of increased ambient light (2, 3). There are multiple shortcomings of those guidelines, and they are in need of an update. The purpose of this research was to develop new information about the signing needs of nighttime drivers to help revise nighttime sign visibility guide- lines for highway agencies across the United States. Research Objectives The objective of this research was to develop data-supported guidelines adapted to site-specific situations for providing effective nighttime visibility of guide and street name signs. The need for this work is a result of a transition from a time when, by policy and need, most overhead guide signs were lit, to the present time when only certain signs are lit. Research has repeatedly shown that when signs are constructed with the newest retroreflective sheeting materials and highway font, and installed in rural areas with little to no ambient back- ground visual clutter, sign lighting is not needed, except for conditions of unusual highway geometries or disadvantaged sign locations. On the other hand, in highly developed areas, there can be so much visual background clutter that sign light- ing may be needed to meet the needs of nighttime drivers. While the overhead guide sign evolution has been trend- ing toward fewer lit signs, the trend for overhead street name signs has been somewhat the opposite, especially at signal- controlled intersections, which happen to generally coincide with relatively higher visually complex background scenes compared with stop-controlled intersections. The newest internally illuminated overhead street name signs are gener- ally made with LEDs, which require less maintenance and use less power than previous generations of internally illuminated light sources. Research Approach The research was divided into 11 tasks across 3 phases. The first phase (Tasks 1–4) involved a literature review, agency interviews, a pilot study to test a study concept, and an interim report in preparation for a meeting with the NCHRP Proj- ect 05-20 panel. The second phase (Tasks 5–8) comprised the principal research tasks, including the closed- and open-road testing with study participants and an analysis of the study data. The third phase (Tasks 9–11) involved the development C H A P T E R 1

4of the final project deliverables. The research tasks were as follows: • Task 1—Review Relevant Literature. This task involved searching for and reporting on literature relevant to the visibility of signs at night, with an emphasis on overhead guide signs. This literature included research on not only the factors that impact visibility but also the national and inter- national standards and guidelines that address it. Chapter 2 contains the findings from the literature review. • Task 2—Conduct Focused Interviews. The second task involved phone interviews with state agencies to deter- mine the state of knowledge and practice, emerging tech- nologies, and policies and practices of lighting overhead guide signs and street name signs. The survey also inquired about the sheeting used on the signs because most agencies tend to use retroreflective sheeting in place of sign lighting. Chapter 2 also contains the findings from the interviews with state agencies. • Task 3—Conduct Proof-of-Concept Research on Mea- suring Background Complexity. The objective of this task was to create a method to measure the roadway environ- ment complexity. Study participants not used in the other tasks rated the roadway complexity from several images of guide signs at night. Statistical modeling matched the ratings given by the study participants with measures obtained by image processing software. The resulting model was used to identify how the components of an image (representing the driver’s view) as evaluated by the software influence the driver’s perception of complexity. The research included an assessment of environmental (background) complexity because it was hypothesized that complex environments inhibit a driver’s ability to see and read overhead guide signs. The specific results of the proof- of-concept work are not detailed in this report because they led to the work completed in Task 5. • Task 4—Prepare Interim Report and Meet with NCHRP Project 05-20 Panel. An interim report summarizing the work and findings of the first three tasks was produced before meeting with the NCHRP Project 05-20 panel. Input from the study panel at the meeting was used to direct the work in the second phase of the research. • Task 5—Develop a Technique to Assess Ambient Lumi- nance and Background Complexity. This task was an extension of the proof-of-concept work in Task 3 that pro- duced a systematic method for processing images based on the model developed in Task 3. The technique used to assess the ambient luminance and background complexity is presented in Appendix B. • Task 6—Conduct Closed-Course Study. The closed-course study was designed as a factorial experiment that tested the effects of sign lighting, retroreflective sheeting, and street lighting on the legibility of overhead guide signs. The mea- sure of interest was the distance from the sign at which study participants read the sign legend. The study design and findings from the closed-course study are presented in Chapter 3. Additional information about how the light sources and sheeting affect measured luminance and con- trast is provided in Appendix A. The findings suggest that luminance (within levels of the study) has little impact on guide sign legibility in rural environments (i.e., low visual complexity and no sources of glare). The type of sign light- ing and the use of street lighting had minor impacts on the guide sign legibility distance. As expected, younger-aged drivers correctly read signs earlier than older-aged drivers. • Task 7—Conduct Open-Road Study. The open-road study was designed to evaluate the factors that influence the rec- ognition of overhead and shoulder-mounted guide and street name signs in real driving environments. At each of three locations across the United States, the research team identified a study corridor and recruited participants to drive through it in an instrumented vehicle. Images of the roadway scene approaching each study sign were evaluated with the image processing tool developed in Tasks 3 and 5 to determine the level of visual background complexity. Research participants drove the study corridor at night and identified signs of interest as soon as they could. The study design and findings from the open-road study are presented in Chapter 4. The findings indicate that visual complexity negatively affects the distance at which a driver recognizes a sign’s message. Increased sign luminance was found to result in increased recognition distance. Details of this task are included in Appendix C. • Task 8—Analyze Study Data. This task involved the analy- sis and synthesis of the findings from the closed-course research and the open-road research. The key findings that were identified were used to develop data-supported guidelines for nighttime sign visibility. • Task 9—Prepare Phase II Deliverables. This task involved reporting on the findings from Tasks 5–8 and producing data- supported guidelines for nighttime sign visibility. Chapter 5 contains a description of the key findings and assumptions used to develop the guidelines. The guidelines are included in Appendix D as a revised Chapter 10 of the 2005 AASHTO Roadway Lighting Design Guide (Chapter 10 is Roadway Sign Lighting). • Task 10—Prepare Draft Final Report. In November 2015, a draft final report was submitted to the NCHRP Project 05-20 panel. • Task 11—Review and Revise Final Report. This document represents the final product of the research, having been revised based on input from the NCHRP Project 05-20 panel.