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35 Research Findings The principal objective of this research was to develop data-supported visibility guidelines for overhead guide and overhead street name signs. The research team reviewed the available literature, surveyed state DOT signing and light- ing practices, and conducted a closed-course visibility study and an open-road visibility study involving nighttime drivers reading signs under diverse conditions. In the closed-course study described in Chapter 3, drivers participated in a factorial study that included different types of sign lighting at multiple illumination levels, different types of sign sheeting, and use of overhead street lighting. The open-road study described in Chapter 4 involved having drivers navigate a designated course while searching for specific signs that researchers directed them to find. Because the study involved signs pres- ently in service, there was little control over elements such as sign sheeting type/condition or sign lighting. The results from these research activities were synthesized and combined to revise Chapter 10 of the AASHTO Roadway Lighting Design Guide (2005 edition). This chapter presents a description of the key findings that were implemented in the revisions to Chapter 10 of the AASHTO Roadway Lighting Design Guide. The recommended guidelines and revised Chapter 10 are included in Appen- dix D of this report. Conclusions and future research rec- ommendations are also described in this chapter. Key Findings The following points describe specific findings from the research that were used to revise the Chapter 10 materials from AASHTOâs Roadway Lighting Design Guide, 2005 edition. ⢠Research has consistently demonstrated that visual com- plexity has a direct effect on nighttime sign visibility (mea- sured in terms of detection, recognition, and legibility). As the visual complexity increases, signs become harder to see, recognize, and read. While this relationship has been studied and demonstrated, it had not been quantified in a way that could be used to develop guidelines. In this research, the research team quantified the need for higher levels of sign luminance as visual complexity increases. ⢠FHWA has researched, developed, and published (in the MUTCD) minimum maintained sign retroreflectivity levels. The minimum retroreflectivity levels are derived from minimum luminance thresholds representing dark rural conditions (i.e., low visual complexity). For overhead guide signs, FHWA used a minimum luminance level of 2.3 cd/m2 to establish the corresponding minimum retroreflectivity levels. ⢠Guide sign illuminance measurements provide no indication of the luminance performance from the driverâs perspective (see Figure 6). Before the use of guide signs fabricated with retroreflective materials, illuminance thresholds were rea- sonable design criteria. For decades, guide signs have been fabricated with retroreflective sheeting materials, so there is no longer a need for illuminance thresholds. ⢠In the closed-course study, which had low visual complex- ity, guide sign legend luminance ranging from 8 to 47 cd/m2 (measured at a distance of 640 ft, representing a legibility index of 40 ft/in. of uppercase letter height) performed sta- tistically similarly using legibility as the dependent variable. These levels of legend luminance were higher than the level used by FHWA as an absolute minimum for nighttime per- formance (2.3 cd/m2). ⢠A method was developed to quantify visual complexity on a scale of 1 through 5 (where 1 = very rural areas and 5 = the most visually complex sign surroundings). The method involves a high degree of expertise, sophisticated equipment, and image processing. While this method was used in the study (and is described in Appendix B), an easier way to estimate visual complexity was needed for the guidelines. Nighttime images representing visual complexity levels of 1 through 5 have been included in the guidelines to help practitioners estimate the appropriate visual complexity level. C H A P T E R 5
36 ⢠In the open-road study, sign legend recognition decreased with increasing levels of visual complexity. However, the negative impacts of visual complexity were offset with increased levels of legend luminance. ⢠A statistical relationship that links visual complexity and legend luminance was derived from the open-road study. The linear relationship shows that a 5.6 cd/m2 increase in legend luminance counteracts a unit increase in visual complexity. Conclusion The proposed guidelines have been based on the needs of nighttime motorists and have been formatted specifically for the AASHTO Roadway Lighting Design Guide, which is cur- rently 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 overhead sign lighting (as shown in Appen- dix D) was provided to the AASHTO task force responsible for the revisions. This approach allows for a quick review by state agencies, with eventual adoption in the most appropri- ate professional reference document for sign lighting. The guidelines also include a link to a list of retroreflective sheet- ing materials that can be used to meet nighttime driver needs for specific complexity levels. This information was added to assist the state agencies when they are updating their policies and specifications specific to overhead signing. Future Research Needs This research produced multiple discoveries that provide for a better understanding of the needs of nighttime motorists. One of the most useful discoveries not fully implemented in the pro- posed guidelines is the method for quantifying the visual com- plexity surrounding a sign. The research team developed and used a methodology for the research, but it required special- ized equipment and custom-built software. During the research project, unsuccessful attempts were made at simplifying the method so that it could be used by state agencies. A follow-up effort would be to develop a system that could be used to drive a roadway at night, process the surrounding environment around a sign, and provide a visual complexity level. A mobile technol- ogy like this would remove the subjectivity associated with the use of the calibrated images used in the proposed guidelines and provide a safer way to implement the guidelines.
