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11Â Â Performance Criteria for Isolated Rural Intersection Lighting To develop specifications for isolated rural intersection lighting, it is necessary to have quanti- tative criteria that characterize the performance of the lighting system. Survey participants were asked what criteria were used by their agencies to define lighting system performance (FigureÂ 8). More than one-half of the respondents indicated that illuminance criteria as defined by the Illu- minating Engineering Society (IES 2018) and by AASHTO (2018) were the primary metrics used for specifying and characterizing isolated rural intersection lighting. Luminance criteria were less frequently used (less than 25% of the time). Aside from illuminance and luminance criteria, which define the amount of light in the intersection, uniformity criteria were also used (38% use AASHTO and 55% use IES). Glare criteria were mentioned less frequently than uniformity; the veiling lumi- nance metric used by IES (2018) and AASHTO (2018) to characterize disability glare from roadway lighting is applicable only to continuously lighted, straight roadway segments. Outside of the afore- mentioned criteria, which are all included in IES and AASHTO recommendations, other commonly mentioned criteria include â¢ Light pollution criteria: 60% of respondents reported using criteria to limit light pollution. Ten respondents reported using the IES (2011) âBUGâ luminaire classification system, which includes numerical categories for geometric zones of light denoted âBâ (for backlight, which is emitted behind a luminaire where it can reach windows of nearby residences), âUâ (for uplight, which can contribute to sky glow), and âGâ (for glare, which is caused disproportionately by high-angle light from the luminaire). A slightly smaller number of respondents (8) mentioned the use of cutoff luminaires which have limited uplight and high-angle light. C H A P T E R 3 Design Factors and Criteria Chapter Summary â¢ Most transportation agencies use illuminance as the primary criterion for specifying and measuring isolated rural intersection lighting systems. â¢ Light-emitting diode (LED) technologies are increasingly (and now almost exclu- sively) used for these applications. Correlated color temperatures (CCT) for these systems range from 3,000Â K to 4,000Â K with trends toward the lower tempera- tures. Luminaires are selected with optical distributions to reduce light pollution. â¢ Approximately one-half of the agencies surveyed install âbeaconâ or âdestinationâ lighting configurations at some rural intersections. â¢ Almost all rural intersection lighting systems use dusk-to-dawn photocell control.
12 Lighting Practices for Isolated Rural Intersections â¢ Lateral distribution types: Roadway luminaires are categorized into different lateral types denoted by Roman numerals, expressing (very roughly) how far âacrossâ the road a luminaire emits light. Type I luminaires have limited lateral throw and are suitable for two-lane roads without sidewalks; Types II and III are more appropriate for wider roads with sidewalks on one or both sides of the road (IES 2018). Of the survey respondents, 48% reported using lateral types in their specifications for isolated rural intersection lighting. â¢ Spectral or color characteristics: Most commonly, respondents referred to the correlated color temperature (CCT) of a light source, which is a general measure of how âwarmâ (yellowish) or âcoolâ (bluish) the illumination from a light source is. A CCT of 3,000Â K is âwarm white,â a CCT of 4,000Â K is âneutralâ white, and a CCT of 5,000Â K or higher is âcool white.â âYellowishâ high-pressure sodium (HPS) lamps have a CCT of 2,200Â K. The most common CCT values mentioned by survey respondents were 3,000Â K (by three agencies) and 4,000Â K (by six agen- cies). Recently, the American Medical Association (AMA 2016) recommended limiting the CCT of outdoor lighting to 3,000Â K to reduce glare, impacts on fauna, and possible circadian disruption in humans at night. IES (2017a), the U.S. Department of Energy (DOE 2017), and the Lighting Research Center (LRC 2016) have all questioned the use of CCT as a metric to limit the effects described by the AMA in its recommendation. Light level recommendations from earlier editions of the guidance from IES and AASHTO were not developed with isolated rural locations in mind, but rather for continuous lighting installations that are much more common in urban locations. As a consequence, some state and local rural intersection guidelines (e.g., Wisconsin DOT 2005; City of Lewiston [ME] 2011) published before 2014 do not rely on IES and AASHTO recommendations. Coincident with IES (2018) and AASHTO (2018) acknowledgment that rural intersections have different lighting needs than urban ones, state DOT guidance (e.g., Vermont AT 2015; Texas DOT 2018; Utah DOT 2020) more commonly adheres to these recommendations, which specify lower illuminances (less than 1 footcandle, equivalent to approximately 10 lux) for isolated rural intersections. There is also a recognition that even lower illuminances could be used while Figure 8. Responses to âWhat lighting performance criteria are used by your agency in the design of isolated rural intersection lighting?â
Design Factors and Criteria 13Â Â maintaining equivalent perceived brightness when a âwhiteâ light source like LED is used to replace a âyellowishâ one like HPS. In practice, uniformity criteria are probably less likely to be met or even checked following the installation of lighting for isolated rural intersections, because such lighting is frequently installed with one or two luminaires (Bullough etÂ al. 2013; Weixel 2017; City and County of Denver [CO] 2019), rendering uniformity a moot point. Delineation, Destination, or Beacon Lighting Current IES (2018) and AASHTO (2018) recommendations for isolated intersection lighting include guidelines for a type of lighting known alternatively as delineation lighting, destination lighting, or âbeaconâ lighting. (Here, âbeaconâ does not refer to a flashing or steady light signal but to the function served by a conventional roadway luminaire; to minimize confusion, the term delineation lighting will be used throughout the remainder of this synthesis.) Delineation lighting uses the presence of a roadway luminaire that can be seen by approaching drivers, and which serves to notify them about the location of a potential traffic conflict caused by the nearby inter- section. It is not the intended function of delineation lighting to illuminate potential hazards such as vehicles or pedestrians, but to simply provide a visual cue to the location of an intersection. Nearly one-half (42%) of the state DOTs participating in the survey for this synthesis (FigureÂ 9) have installed luminaires at isolated rural intersections for delineation lighting. While this technique appears to be familiar to transportation agencies, almost all agencies reported only using this technique for a small proportion (approximately 25%) of their isolated rural inter- sections. One agency reported using delineation lighting for over one-half of its rural intersection locations. The notion of delineation lighting, using configurations similar to that shown in FigureÂ 10, also has some support in the research literature (Torbic etÂ al. 2015). Maze etÂ al. (2010) suggested locating a roadway luminaire at the edge of the hazardous approach zone so that drivers might learn to be careful once they drove past the luminaire. Agencies such as North Dakota DOT (2020) have been replacing flashing signal lights at some isolated rural intersections with roadway luminaires serving as delineation lighting, and at least one other agency has explicit recommenda- tions for this type of lighting (Vermont AT 2015). Figure 9. Responses to âDoes your agency ever install isolated rural intersection lighting as âbeaconâ (or delineation) lighting primarily to create a visible element to approaching traffic rather than to illuminate the road and potential hazards?â
14 Lighting Practices for Isolated Rural Intersections Equipment and Hardware Configurations Survey participants were asked to identify the types and configurations of lighting equipment they use at installations of lighting for isolated rural intersections (FigureÂ 11). Among the most commonly selected responses were the pole configurations, the type of light source used in the lighting system, and the mounting height of the luminaires. Specifically, the agencies responding to this question reported slightly more use of new poles installed specifically for the lighting than the use of existing utility poles at these locations. One respondent stated that luminaires were attached to an existing signal arm for a traffic signal at the location. In comparison, Isebrands etÂ al. (2010) found that 75% of isolated rural intersection lighting installations, investigated as part of a study in Minnesota, had lights mounted on existing utility poles. Some statements about pole location (in the written guidelines and policies for rural inter- section lighting) were identified during the literature review. Wisconsin DOT (2005) requires poles in rural locations to be at least 12Â feet from the edge of the roadway to reduce the potential Figure 10. A single luminaire mounted at a rural intersection serves as a form of delineation lighting. Courtesy of FHWA. Figure 11. Responses to âWhat lighting equipment and/or hardware configurations are considered in the design of isolated rural intersection lighting?â
Design Factors and Criteria 15Â Â for collisions, and even farther for high-speed roads. New Hampshire DOT (2010) suggests that when possible, existing signal poles and masts be used to mount luminaires to minimize the number of additional poles at rural intersections. Example pole layouts for isolated T-junctions and four-way intersections are illustrated in FigureÂ 12 (adapted from Frering etÂ al. 2018). Regarding the type of light sources, over 90% of the respondents who selected the light source configuration reported that LED luminaires were used for rural intersection lighting, while just 21% used HPS luminaires (the total exceeds 100% because some agencies use LED and HPS). This is consistent with the evolution in lighting technology and the improved efficacy of LED compared to HPS (LRC 2012; Edwards 2015). LEDs can easily result in 40% or greater energy savings compared to HPS roadway lighting systems (LRC 2012). Few written policies specifically dealing with isolated rural intersection lighting address the light source technology, suggesting that many of them carry language from when HPS was the primary roadway lighting source. In terms of mounting height, written policies and guidelines for rural intersection lighting did not specifically address this, and, pole heights for general road lighting practice are also likely followed at rural isolated intersections (or, existing utility poles are used). Responses to the survey question about pole height generally varied in heights from 25 to 45Â feet for the mounting height with 30Â feet being most commonly included in the range of heights (by approximately half of those responding to this question). Regarding the number of luminaires used for isolated rural intersection lighting, 55% of respondents who answered this question reported using between one and four luminaires per intersection (four of these respondents use a single luminaire); 25% stated that they used the minimum needed number of luminaires and poles. The most common luminaire configurations reported were those with a flat lens (59% of respondents to this question), cutoff optics (24% of respondents), or dark-sky friendly luminaires (6%). The most common wattage for use in rural intersection lighting among the survey respondents was 250 W HPS (or the equivalent wattage LED to achieve the same light output) followed by 150 W HPS. Pierce County [WA] (2011) describes using 150 W HPS or its equivalent in rural intersection lighting, and the Lighting Research Center (LRC 2012) used a 150 W HPS as the base case for rural intersections. A few Figure 12. Example pole layouts for isolated intersections at a T-junction and a four-way intersection.
16 Lighting Practices for Isolated Rural Intersections survey respondents stated that the wattage needed would be minimized or dependent on the requirements of the specific location. Regarding lighting controls, almost every survey respondent selecting this option reported that they used photocells to control the lighting system on a dusk-to-dawn schedule. One respondent mentioned that an astronomical time clock was used in some installations. None of the written policies for rural intersection lighting specifically addressed the type of controls used. In a separate question, agencies participating in the survey were asked if they had used solar or any other alternative form of power in any of their isolated rural intersection lighting instal- lations (FigureÂ 13). Out of the 29% of participants responding âyesâ to this question, many of them stated that their agencies had only used solar power in a few test or pilot installations or at a relatively small number of locations. It was mentioned by one respondent that having access to electrical power at some rural locations is a challenge, and this challenge is also mentioned in the literature (Hallmark and Hawkins 2014; Torbic etÂ al. 2015). Solar power may be one avenue for overcoming this obstacle (DX3 Head Office 2011). As described in the case examples chapter of this synthesis, Utah DOT has installed multiple solar-powered lighting installations at rural intersections since 2016 and has not experienced any maintenance issues or other problems with these installations. No other forms of alternative power were identified in the survey or the literature. Figure 13. Responses to âHas your agency used solar/photovoltaic or other alternative power source(s) for isolated rural intersection lighting?â