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17 Chapter 3 â Techniques of Lighting Design Current Guide The AASHTO Roadway Lighting Design Guide discusses methodologies lighting system design, including illuminance and luminance criteria, uniformity requirements, and glare limits. It offers pole placement guidance, calculation methods, and a warranting system to assist in the prioritization lighting systems for various roadway types and operating conditions. Application of SSL does not significantly change any of the requirements in the current AASHTO guide for lighting design techniques, but installations may benefit from additions to the current requirements and recommendations. Additional Considerations for Solid State Lighting Determining Levels for Use of Adaptive Lighting One of the greatest advantages to SSL is the ability to easily control the output of the fixture and vary light levels based on several factors including changing conditions and light loss factor correction. When applying an adaptive lighting system to a roadway or a street, one key concern is whether dimming the lighting system is safe and, if so, to what levels the lighting can be dimmed. Although the current AASHTO guide does not address adaptive lighting levels and how systems may be dimmed, documents prepared by the (IES RP-8-19) as well as the Federal Highway Administration [FHWA] Roadway Lighting Handbook) do attempt to address these questions. The lighting level used on a roadway is based on several factors including number of pedestrians, intersections, traffic volumes, etc., which consider the risks to motorists and pedestrians relative to operating conditions. For streets where pedestrians are present, IES RP-8-18 uses pedestrian volumes to determine the required light levels (Figure 11). As the pedestrian volumes on street decrease later at night, then the lighting level on the street can also decrease. It does not include a methodology for the reduction of lighting on freeways. The current AASHTO guide gives minimum lighting levels but does not address lighting levels at times of reduced activity. IES does offer a dimming methodology based on pedestrian volumes, and FHWA offers one by using a different roadway classification method. This chapter includes these methods as an option for those looking for assistance in determining levels for reduced activity periods.
18 Figure 9. Excerpt from RP-8-18 showing Roadway and Street Lighting Levels Prior versions of the IES RP-8(Illuminating Engineering Society, 2018) used the same land use classifications of commercial, intermediate, and residential that AASHTO currently uses but switched to high, medium, and low pedestrian volumes allowing them to better address street operating variables. AASHTO also uses both illuminance and luminance methodology so illuminance-based modifiers also could be considered in the same manner. FHWAâs Adaptive Lighting Guideline (Gibbons, Guo, Median, Terry, Du, Lukevich, Corkum, & Vetere, 2014) offers a method to classify streets and highways, which allows for the dimming of lighting based on various factors like traffic volumes and can be reviewed and considered as part of the guidelines as shown in Figures 12 and 13. If using the illuminance method in the AASHTO lighting guide and looking for an approximation to convert the IES and FHWA levels to illuminance, use the ratio of approx. 1 cd/m2=1fc for R1 pavement and 1 cd/m2=1.5fc for R3 pavement.
19 Figure 10. Excerpt from FHWA Adaptive Lighting Report - Classification Method for Streets (S class = 6- weighting factors)
20 Figure 11. Excerpt from FHWA Adaptive Lighting Report - Classification Method for Highways (S class = 5- weighting factors) Use these other references and methodologies for classifying streets to determine appropriate light levels for the dimmed condition when using an adaptive lighting system. Source Color Selection SSL products offer a range of colors of varying spectral content that can be applied to a roadway as seen in Figure 12. Each of these various spectral content sources has properties that can affect the subjective preference, visibility of objects, and light scatter and physiological influences, which are further discussed in Chapter 11.
21 Figure 12. Comparison of Different CCT Streets Subjective preference of source color can vary greatly. Some individuals like the higher CCT sources with completed retrofits to LED luminaires noting that they appeared âbrighterâ or more like daylight. Others dislike the higher CCT luminaires and note the âwarmnessâ of the lower CCT sources. Change in color from HID to LED evokes reaction from some and goes completely unnoticed by others. Limited study has been done on the subjective preference of LED color. The NEEA Seattle Adaptive Lighting Study(Clanton & Associates Inc., 2014) shows some trends associated with subjective source preference, but no statistically significant differences emerge about preferred CCT LED. The difference in detection distance was also investigated as part of this study and was part of the testing performed on the Smart Road (Figure 13 and Figure 14. The results indicate that there is a spectral component to detecting objects, and certain CCT LED sources (using current conventional technology of blue LED with yellow phosphor coatings) show benefits in detection distance. This difference also seems to vary depending on speed. Research does not show a strong subjective preference to LED source color.
22 Figure 13. Effect of Light Type on the Driver Detection of Pedestrians or Wildlife (based on the results of this study NCHRP 5-22) Figure 14. Effect of Light Type on the Driver Detection of Objects in the Roadway (based on the results of this study NCHRP 5-22) On roadways, there does appear to be an advantage to using 4000K sources rather than 3000K or 5000K. Differences are not always significant but are consistent when evaluating driver detection distance of large and small objects. Chapter 12 includes additional considerations relating to CCT of sources.
23 Differences in Lighting Highways and Streets The major differences from a lighting perspective between highways and streets are speed, number of conflict areas, and presence of pedestrians and cyclists. Lighting has a strong relationship in crash reduction for both kinds of roadways using traditional roadway lighting methodology. Change to LED systems may affect this relationship. Older HID lighting technologies generally had optical systems that could not precisely control the spread of light so areas adjacent to the roadway generally were illuminated (Figure 17). Newer LED optical systems offer very controlled light distribution so areas adjacent to the roadway, including bike lanes and sidewalks, may not receive adequate lighting using traditional design methods. The current AASHTO guide offers recommendations for streets, sidewalks, and bicycle ways. When these types of facilities are within the right-of-way, all these areas should meet AASHTO recommendations. Even if this methodology was not used before, implementation of LED technology makes the exercise more relevant. For highways and freeways, lighting is generally prescribed for the travel way, but adjacent areas do not require lighting because these areas have historically had spill lighting from the roadway lighting system. With increased LED lighting control, however, they will likely have less light. Other lighting recommendations including from CIE have developed criteria for lighting areas adjacent to the travelled way called Surround Ratio (SR) (Figure 18 and Figure 19). Figure 15. Street and Roadway Differences Figure 18. High and Low Surround Ratio
24 Figure 19. Excerpt from CIE 115 Table 2 Research performed for this project shows that detection distances are increased when lighting is used in areas adjacent to the roadway. Therefore, consider SR as a criterion. The criteria for SR should be 0.8, higher than that used by CIE, and be the ratio of the average illuminance of an area adjacent to the travel way of 12 feet (ft) (3.6 meters [m]) and average illuminance of the lane of the travel way adjacent to it. For example, using the configuration shown in Figure 20, the average illumination of the area shown as shoulder divided by the average illumination of the adjacent lane should be 0.8 or greater. Lighting class Road surface luminance Threshold increment Surround ratio Dry Wet * Lav in cd/m2 Uo Ul Uo TI in % SR M1 2.0 0.40 0.70 0.15 10 0.5 M2 1.5 0.40 0.70 0.15 10 0.5 M3 1.0 0.40 0.60 0.15 15 0.5 M4 0.75 0.40 0.60 0.15 15 0.5 M5 0.50 0.35 0.40 0.15 15 0.5 M6 0.30 0.35 0.40 0.15 20 0.5 In general, highways (limited access roadways) have only requirements for lighting the travel lanes. Streets (which often have pedestrian areas or sidewalks) have requirements for travel lanes and sometimes sidewalk areas. Research for this project shows the advantage to providing lighting outside of the limits of the travel lanes. It is recommended that an SR of 0.8 be used on both highways and streets. Some highways may require lighting more area adjacent to the active lanes than others.
25 Figure 20. Example Diagram for Applying Surround Ratio (SR= Average Illumination in Shoulder Area S / Average illumination in Travel Way T) Warranting Considerations The AASHTO guide establishes warranting conditions for highways and freeways. It does not, however, establish warrants for streets and other types of roadway facilities. Noting that many roadway improvements provide additional safety benefits to users and that warrants do not establish a requirement to light a roadway but merely assist in prioritizing projects and funding of those roadway improvements, other resources are available for use in those evaluations. The AASHTO Highway Safety Manual â 2010 (HSM) shows currently available information on crash frequency and severity so that various improvements to roadways can be quantified and evaluated for effectiveness. Some of the effects of various treatments, such as geometric improvements or operational changes on roadways, are quantified as crash modification factors (CMFs). CMFs represent the change expected in crash frequency from a specific change in conditions. For example, in looking at the impact of highway lighting on all roadway types that previously had no lighting, for nighttime injury crashes, HSM reports (information from the Crash Modification Clearinghouse shown in Figure 21) that research has shown a resultant CMF of 0.72 (a reduction of 28% in nighttime injury crash types). If the expected average crash frequency is 10 injury crashes/year for a no-lighting condition, after implementation of a highway lighting system, one would expect 10 X 0.72 CMF = 7.2 injury crashes/year. AASHTO provides warrants for highways and interchanges. For other types of roadways, to consider CMFs or other warranting methodologies described in this section as options. Warrants are however not required to light a roadway. They are only a way to prioritize funding and improvements.
26 The amount of information for crash analysis and evaluation is actively growing and can be found at the Crash Modification Factors Clearinghouse at www.cmfclearinghouse.org provided by FHWA. In this clearinghouse, the viewer can sort through data by type of countermeasure, crash type, crash severity, and roadway type. The viewer can also see a measure of accuracy and precision of the data, as well as applicability, as judged and rated by a panel of reviewers. Figure 16. Crash Modification Factors For lighting of arterial, local, and collector roads, the warrant system from the Transportation Association of Canada (TAC) Guide for the Design of Roadway Lighting(Transport Association of Canada, 2006) which was based on the U.S. Department of Transportation 1978 Roadway Lighting Handbook may be useful (Figure 22). The warrant system is based on geometric, operational, environmental, and crash factors. For each factor, a numeric rating (R) from 1 to 5 corresponding to the defined criterion is defined. Each criterion is assigned a weight (W) to indicate its relative importance. The rating value (R) is multiplied by the weight (W) to obtain a point-score (R x W) for each criterion characteristic, indicating its relative significance. The overall point-score for all items indicates the need for lighting, as well as the relative risk on that road compared with other roadways. When undertaking a warrant analysis, the length of roadway segment analyzed should be as long as possible and take into account future development. Where the roadway classification or roadway land use classification changes, conduct a separate warrant analysis for each roadway section. Use a single warrant analysis where classifications are relatively constant along the segment of roadway under consideration.
27 Figure 22. Transportation Association of Canadaâs Warrants for Arterial, Collector and Local Roads The Transportation Association of Canada Guide for the Design of Roadway Lighting includes a warranting system for intersection lighting based on geometric, operational, environmental, and crash factors (Figure 23). The critical factors determining the need for illumination are traffic volumes and night-time crashes, and the warrant point score indicates whether full intersection lighting, partial lighting, or delineation lighting is needed. Full intersection lighting denotes illumination covering an intersection in a uniform manner over the traveled portion of the roadway. Partial lighting is the
28 illumination of key decision areas, potential conflict points, and/or hazards in and on the approach to an intersection. Illumination of vehicles on a cross street or median crossing, or lighting that marks an intersection location for approaching traffic, is referred to as sentry or delineation lighting. Figure 23. Transportation Association of Canadaâs Warrants for Freeways
29 Consideration for High-Mast Lighting The primary consideration for using LED technology as part of a high-mast system is the control of glare and light trespass (Figure 24). See Chapter 11 for further details on light trespass limits and Chapter 3 for discussion of control of veiling luminance ratios relative to glare for the driver. Perceived glare is something generally sensed by abutters to the high-mast installation and difficult to quantify. Even if the light trespass illuminance levels are well below recommended limits, the inherent brightness of some LED sources against a dark sky or background can cause complaints from some residents abutting the roadway. Figure 17. High-Mast Lighting Light Trespass The only effective way to address perceived glare issues is to provide shielding for the luminaire. HID high-mast luminaires generally use shields mounted to the side of the luminaire to obscure the source and some of the reflector, limiting the amount of side or backlight. LED high-mast luminaires, however, often use the LED source spread across the entire luminaire emitting surface making it difficult to fully obscure all of bright LED chip assemblies. Therefore, shielding individual LEDs seems the most effective way of controlling brightness from LED arrays and larger chip-on-board assemblies. Other methods where shields are distributed across the luminaire emitting surface also could be effective. Other considerations when using SSL for high-mast installations are potential savings in pole and foundation requirements because of fewer luminaires, less weight, lower luminaire effective projected area (EPA). Also, consider the requirements for lowering devices to balance the expected need to access luminaires against the cost and weight of the lowering system.
30 Key Issues During Lighting Design â¢ Consider an adaptive lighting system (or at least provide a controls-ready system) and establish light levels for dimming. â¢ Select CCT for the installation based on visibility differences for certain spectral content sources as well as preferences determined by the DOT or Authority. Generally, 4000K provides consistent results but consider other factors as discussed in other sections of this guide. â¢ Include the shoulders adjacent to the roadway in lighting design. When pedestrians and/or cyclists are present, consider the AASHTO recommendations for those areas as part of the design. For highways and freeways, adjacent areas should meet the SR levels discussed in this chapter. â¢ Consider warrants and other factors as part of the decision process for roadway safety improvements. â¢ Consider high-mast lighting, which can create more light trespass and perceived brightness issues from abutters, only for large area lighting or very wide roadway cross sections. â¢ Identify sensitive receptors and housing areas close to the roadway right-of-way early in design. â¢ Consider shielding in lighting system design. â¢ Because individual LED shields perform better than luminaire shields for high-mast lighting, carefully select shielding types. â¢ LED luminaires may help reduce structural requirements (fewer fixtures/weight/EPA) for poles. â¢ Consider longer periods between needed access to luminaires.