Lighting Practices for Isolated Rural Intersections (2022)

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30 The information-gathering activities (literature review and transportation agency survey) undertaken for this project have led to the following conclusions about the practice of lighting for isolated rural intersections in the United States: • The literature review and statements of the DOTs participating in the survey suggest that isolated rural intersection lighting has a meaningful safety benefit overall, but specifics on what types of intersections most benefit from lighting and what types of crashes are most impacted by lighting are not understood well. • As a possible consequence, many state transportation departments currently cannot quantify the economic benefit of rural intersection lighting for specific locations, as it compares to the costs, which are relatively easier to estimate. • Different agencies have different specific warranting criteria that contain different thresholds for common factors such as nighttime crash frequency, traffic volume, and geometric configu- ration. It is not clear whether these differences are consequential for safety at isolated rural intersections. • Most transportation departments use IES or AASHTO criteria for specifying and measuring lighting performance, and most DOTs use luminaires that produce little upward light. • Approximately one-half of the agencies have used delineation lighting configurations, and almost none have implemented adaptive lighting control systems in rural intersection lighting. • Maintenance of isolated rural intersection lighting systems is largely performed on an ad hoc basis after lights burn out or equipment breaks; it is not understood whether regular mainte- nance schedules could have a benefit. Knowledge Gaps and Research Needs Transportation agencies participating in the survey were asked what information they needed most that they did not currently have to make better-informed decisions about isolated rural intersection lighting (Figure 27). Six percent of responding agencies selected information about lighting technologies; 22% selected the criteria (e.g., illuminance, CCT) needed to characterize lighting performance. The areas where respondents stated that knowledge was most lacking were funding sources for rural intersection lighting (59%), warranting criteria for deciding when to install lighting (50%), and evidence about the specific benefits of rural intersection lighting (38%). “Other” responses (totaling 38% of respondents) focused heavily on how to determine who is respon- sible for maintaining (and paying for) these systems (whether the state, the local government, or a combination of these) and how rural lighting systems should be maintained. One agency stated that an improved understanding of the benefit of delineation lighting would be helpful. C H A P T E R 7 Conclusions, Knowledge Gaps, and Research Needs

Conclusions, Knowledge Gaps, and Research Needs 31   Given the lack of implementation of adaptive lighting strategies at isolated rural intersections, further research to better understand the potential benefits of adaptive lighting could be useful to agencies. There is consistency among the responses in Figure 27 about warranting criteria and under- standing the benefits of isolated rural intersection lighting and the responses to previous ques- tions in Figures 23 through 26 about state DOT understanding of the expected economic impact from improved safety at these locations. Overall, these responses suggest that a knowledge gap for transportation agencies is the ability to make location-specific predictions about the net benefit/ cost ratio that might be achieved if the lighting is installed at a specific rural intersection (or at multiple intersections along the same rural highway). Although some data are available on a statewide basis that probably justify the use of rural intersection lighting overall, it is also clear that the effects of location-specific factors such as intersection geometry, the number of lumi- naires, and the location’s nighttime crash history can affect the ability of lighting to improve safety at rural intersections. Finally, there is a need for consistent guidance for DOTs and their contractors to help with the entire process of considering, warranting, designing, and evaluating lighting at isolated rural intersections. This synthesis effort has gathered much of the existing knowledge and expertise, but by addressing the knowledge gaps identified in this report, future research can lead to an objective and streamlined process for determining when and where isolated rural intersection lighting can have the greatest return on investment. Figure 27. Responses to “What information is your agency lacking to help make appropriate decisions about isolated rural intersection lighting?”