Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 S U M M A R Y Effective highway signing is an important component to driver decision making, comfort, and safety. The objective of this research was to develop guidelines for providing effective nighttime performance of overhead signs. The need for this work has stemmed from a tran- sition covering a period from a time when, by policy and need, all overhead guide signs were lighted, to the current time when only certain overhead signs are lighted. At the same time, overhead street name signs have seen a reverse trend and are now more commonly lit (usu- ally with internal illumination), especially at signalized intersections. Research has repeatedly shown that when overhead guide signs are constructed with the newest retroreflective sign sheeting materials and highway font, and installed in rural areas with little to no visual clutter, sign lighting is not needed. However, there has been little research to address or identify the conditions in which sign lighting is needed or what type of retroreflective material for overhead signs will meet the needs of nighttime motorists. Going into this research, it was generally thought that sign lighting may be needed in areas with high levels of visual clutter or areas where the geometrics of the highway are such that inadequate headlamp illumination is directed to overhead signs. The existing relevant policies and guidelines regarding sign lighting provided little useful information to determine when sign lighting is needed, and the reference material available was out of date. The research conducted and described in this report was designed to provide new discoveries related to the needs of nighttime motorists with a focus on overhead sign visibility. Two complementary nighttime visibility studies were conductedâboth of which were specifically designed to pro- duce results that can be used to develop guidelines for overhead sign visibility. The first study was conducted on a closed course and investigated the legibility distances of three different sign legend and background configurations under different sign lighting treat- ments. The signs were lit by either high-pressure sodium (HPS) or light-emitting diode (LED) systems or they were not lit at all. Additionally, roadway lighting was added for some trials to evaluate its effect. The sheeting materials that made up the sign configurations are com- monly used and are specified in the Manual on Uniform Traffic Control Devices (MUTCD). The sheeting types (American Society for Testing and Materials [ASTM] Types III, IV, and XI) represent a variety of retroreflective properties. Findings from the closed-course study indicate that sign lighting does not significantly impact legibility distance of signs in rural and dark areas, suggesting that headlamps alone provide sufficient illumination for visibility. Photometric analyses determined the extent of the lighting impact in terms of luminance and contrast on the signâs legend and background. Measurements of luminance and contrast were found to have no impact on legibility distance under the controlled conditions of the closed-course study. The second study was conducted on the open road. The study investigated the effects of both sign luminance and visual complexity on the distance at which a driver can read Guidelines for Nighttime Visibility of Overhead Signs
2overhead signs during a recognition task. Visual complexity was measured with a software tool developed specifically for this research. The tool identifies elements of the visual scene from a photometrically calibrated digital image and quantifies their effects into a measure of visual complexity. The tool was developed using nighttime images and ratings from a dif- ferent set of study participants. Findings from the open-road study indicate that the visual complexity of sign surroundings reduces the distance at which drivers correctly recognize information from signs, but this is countered by increases in legend luminance. The combined findings from the closed-course and open-road studies provided a number of results that were used to develop revised guidelines designed to provide adequate night- time visibility of overhead signs. One of the most useful findings was the development of an empirically derived relationship that describes the connection between the needs of nighttime motorists and the visual complexity surrounding an overhead sign. With a visual complexity scale of 1 through 5, the research findings show that the negative effects of increasing visual complexity can be countered with an increase in legend luminance of 5.6 cd/m2 (using the FHWAâs base level of 2.3 cd/m2 as the absolute minimum needed for nighttime drivers in rural conditions as the baseline visual complexity level of 1). The proposed guidelines resulting from this study are based on needs of nighttime motor- ists and have been formatted specifically for the AASHTO Roadway Lighting Design Guide, which is currently being updated. The proposed guidelines are flexible and should provide performance targets for innovations in sign lighting and sign sheeting technologies for years to come. The revised chapter on roadway sign lighting (shown in Appendix D of this report) was provided to the AASHTO Task Force responsible for revisions. This approach allows for a quick review by state agencies, with eventual adoption in the most appropriate pro- fessional reference document for sign lighting. The guidelines also include recommended retroreflective sheeting materials that can be used to meet nighttime driver needs for specific complexity levels. This information was added to assist state agencies when they are updating their policies and specifications specific to overhead signing.
