Solid-State Roadway Lighting Design Guide: Volume 1: Guidance (2020)

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CHAPTER 3 Techniques of Lighting Design Current Guide The AASHTO Roadway Lighting Design Guide (AASHTO 2018) 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 of lighting systems for various roadway types and operating conditions. Application of SSL does not significantly change any of the requirements for lighting design techniques in the current AASHTO guide, 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 of SSL is the ability to easily control the output of the fixture and vary light levels on the basis of several factors, including changing conditions and correction of the light loss factor. When an adaptive lighting system is being applied 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 light- ing levels and how systems may be dimmed, documents prepared by IES (IES RP-8-18) as well as the FHWA Lighting Handbook (Lutkevich et al. 2012) do attempt to address these questions. The lighting level used on a roadway is based on several factors, including the number of pedestrians, intersections, traffic volumes, The current AASHTO Roadway Lighting and others, that consider the risks to motorists and pedestrians relative Design Guide gives minimum lighting to operating conditions. For streets where pedestrians are present, levels but does not address lighting IES RP-8-18, Recommended Practice for Design and Maintenance of levels at times of reduced activity. IES Roadway and Parking Facility Lighting (IES 2018) uses pedestrian does offer a dimming methodology volumes to determine the required light levels (Figure 11). As the based on pedestrian volumes, and pedestrian volumes on a street decrease later at night, the lighting FHWA offers one that uses a different level on the street can also decrease. IES RP-8-18 does not include a roadway classification method. This methodology for the reduction of lighting on freeways. chapter includes these methods as an Prior versions of IES RP-8 used the same land use classifications of option for those looking for assistance commercial, intermediate, and residential that AASHTO currently uses in determining levels for periods of but switched to high, medium, and low pedestrian volumes, which reduced activity. allowed it to better address street operating variables. AASHTO also 17   

18   Solid-State Roadway Lighting Design Lavg: Maintained average pavement luminance L min: Minimum pavement luminance Lmax: Maximum pavement luminance LV,max: Maximum veiling luminance Lavg: Maintained average pavement luminance L min: Minimum pavement luminance Lmax: Maximum pavement luminance LV,max: Maximum veiling luminance Source: IES RP-8-18 (IES 2018). Figure 11.   Roadway and street lighting levels. uses both illuminance and luminance methodology, so illuminance- If using the illuminance method in the based modifiers also could be considered in the same manner. AASHTO Roadway Lighting Design Guide FHWA’s Guidelines for the Implementation of Reduced Lighting on and looking for an approximation to Roadways (Gibbons et al. 2014) offers a method for classifying streets and convert the IES and FHWA levels to illu- highways that allows for the dimming of lighting on the basis of various minance, use the ratio of approximately factors, such as traffic volumes, and can be reviewed and considered 1 cd/m2 = 1 fc for R1 pavement and as part of the guidelines, as shown in Figure 12 and Figure 13. 1 cd/m2 = 1.5 fc for R3 pavement. Use these other references and methodologies for classifying streets to determine appropriate light levels for the dimmed condition when an adaptive lighting system is being used. Source Color Selection SSL products offer a range of colors of varying spectral content that can be applied to a roadway, as shown in Figure 14. Each of these spectral content sources has properties that can affect subjective preference, visibility of objects, and light scatter and physiological influences, which are further discussed in Chapter 11. 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 “brighter” 

Techniques of Lighting Design   19   Source: Gibbons et al. (2014). Figure 12.   Classification method for streets (S class = 6 weighting factors). or more like daylight. Others dislike the higher CCT luminaires and note the “warmness” of the lower CCT sources. Change in color from Research does not show a strong HID to LED evokes reaction from some and goes completely unnoticed subjective preference to LED source color. by others. Limited study has been done on the subjective preference of LED color. The Northwest Energy Efficiency Alliance Seattle Adaptive Lighting Study (Clanton and Associates Inc. 2014) shows some trends associated with subjec- tive source preference, but no statistically signi­ficant differences emerged about the preferred CCT of LED. The difference in detection distance was also investigated as part of NCHRP 05-22 and was part of the testing performed on the Virginia Smart Road (Figure 15 and Figure 16). 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. 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 the driver detection distance of large and small objects is being evaluated. Chapter 11 includes additional considerations relating to the CCT of sources. 

Source: Gibbons et al. (2014, Tables 10 and 11, p. 16). Figure 13.   Classification method for highways (H class = 5 weighting factors). Figure 14.   Comparison of streets with different CCTs. 

Techniques of Lighting Design   21   700 600 Detection Distance (ft) 500 400 300 200 100 0 LED 3000K LED 4000K LED 5000K Light Source Type Figure 15.   Effect of type of light on the driver detection of pedestrians or wildlife (NCHRP Project 05-22). Differences in Lighting Highways and Streets From a lighting perspective, the major differences between highways and streets are speed, the number of conflict areas, and the presence of pedestrians and cyclists. Traditional roadway lighting has a strong relationship with crash reduction on both kinds of roadways. Changing 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 if traditional design methods are used with LEDs. 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 250 200 Detection Distance (ft) 150 100 50 0 LED 3000K LED 4000K LED 5000K Light Source Type Figure 16.   Effect of type of light on the driver detection of objects in the roadway (NCHRP Project 05-22). 

22   Solid-State Roadway Lighting Design Figure 17.   Street and roadway differences. 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 In general, highways (limited access system. With increased use of LED lighting control, however, they will roadways) have only requirements likely have less light. for lighting the travel lanes. Streets (which often have pedestrian areas or Other lighting recommendations, including those from CIE, have sidewalks) have requirements for travel developed criteria for lighting areas adjacent to the traveled way called lanes and sometimes sidewalk areas. surround ratio (SR) (Figure 18 and Table 1). Research for this project shows the Research performed for NCHRP 05-22 shows that detection distances advantage to providing lighting outside are increased when lighting is used in areas adjacent to the roadway. of the limits of the travel lanes. It is Therefore, the SR is suggested as a criterion. The criteria for SR should recommended that a surround ratio of be 0.8—higher than that used by CIE—and should be the ratio of the 0.8 be used on both highways and streets. average illuminance of an area of 12 ft (36 m) adjacent to the travel way Some highways may require lighting and the average illuminance of the lane of the travel way adjacent to it. more area adjacent to the active lanes For example, using the configuration shown in Figure 19, the average than others. illumination of the area shown as shoulder divided by the average illumination of the adjacent lane should be 0.8 or greater. 

Techniques of Lighting Design   23   Figure 18.   Surround ratio: (a) high and (b) low. Table 1.   Criteria for lighting areas adjacent to the traveled way. Road Surface Luminance Threshold Surround Lighting Dry Wet Increment Ratio Class Lav (cd/m2) Uo Ul Uo [TI (%)] [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 Source: CIE 115 (2010), Table 2, “Lighting of Roads for Motor and Pedestrian Traffic,” p. 18 . Note: Lav = average road surface luminance; Uo = overall uniformity of the luminance; Ul = longitudinal uniformity of the luminance. Warranting Considerations The AASHTO Roadway Lighting Design Guide establishes warranting conditions for highways and freeways; it does not, however, establish warrants for streets and other types of roadway facilities. Although it must be noted 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 light versus not-light evaluations. The AASHTO Highway Safety Manual (AASHTO 2010) shows currently available infor- mation on crash frequency and severity so that various improvements to roadways can be Figure 19.   Example diagram for applying surround ratio (SR = average illumination in shoulder area S ï average illumination in travel way T). 

24   Solid-State Roadway Lighting Design quantified and evaluated for effectiveness. Some of the effects of various AASHTO provides warrants for highways treatments, such as geometric improvements or operational changes on and interchanges. For other types of roadways, are quantified as crash modification factors (CMFs). CMFs roadways, crash modification factors represent the change expected in crash frequency from a specific change or other warranting methodologies in conditions. described in this section may be For example, in examining the impact of highway lighting on all considered as options. Warrants, however, roadway types that previously had no lighting, the AASHTO Highway are not required to light a roadway. Safety Manual reports that research has shown a CMF of 0.72 for They are only a way to prioritize funding nighttime injury crashes (i.e., a reduction of 28% in nighttime injury and improvements. crash types) (Figure 20). If the expected average crash frequency is 10 injury crashes/year for a condition of no lighting, one would expect 10 × 0.72 CMF = 7.2 injury crashes/year after implementation of a highway lighting system. The amount of information on crash analysis and evaluation is actively growing and can be found at FHWA’s Crash Modification Factors Clearinghouse (www.cmfclearinghouse.org). 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. For lighting of arterial, local, and collector roads, the warrant system from the TAC Guide for the Design of Roadway Lighting (TAC 2006), which was based on the U.S. Department of Transportation’s 1978 Roadway Lighting Handbook (Walton and Rowan 1978), may be useful (Figure 21). 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 Source: Crash Modification Factors Clearinghouse. Figure 20.   Crash modification factors. 

Techniques of Lighting Design   25   Source: TAC (2006). Figure 21.   TAC’s warrants for arterial, collector, and local roads. 

26   Solid-State Roadway Lighting Design value (R) is multiplied by the weight (W) to obtain a point score (R × W) for each criterion characteristic that indicates 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 a warrant analysis is being undertaken, 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, a separate warrant analysis should be conducted for each roadway section. A single warrant analysis may be used where classifications are relatively constant along the segment of roadway under consideration. TAC’s Guide for the Design of Roadway Lighting includes a warranting system for intersection lighting that is based on geometric, operational, environmental, and crash factors (Figure 22). Source: TAC (2006). Figure 22.   TAC’s warrants for freeways. 

Techniques of Lighting Design   27   The critical factors in determining the need for illumination are traffic volumes and nighttime crashes, and the warrant point score indicates whether full intersection lighting, partial lighting, or delineation lighting is needed. Full intersection lighting denotes illumination that covers an intersection in a uniform manner over the traveled portion of the roadway. Partial lighting is the illumination of key decision areas, potential conflict points, and 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. Considerations 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 23). See Chapter 11 for further details on light trespass limits and the current AASHTO guide for discussion of the 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. The only effective way to address issues of perceived glare 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, so as to limit the amount of side or backlight. LED high-mast luminaires, however, often use the LED source spread across the entire emitting surface of the luminaire, which makes it difficult to fully obscure all of the 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 that distribute shields across the emitting surface of the luminaire also could be effective. Other considerations when SSL is being used for high-mast installations are potential savings in pole and foundation requirements because of fewer luminaires, less weight, and lower luminaire effective projected area. Also worth considering are the requirements for lowering devices to balance the expected need to access luminaires against the cost and weight of the lowering system. Figure 23.   High-mast lighting light trespass. 

28   Solid-State Roadway Lighting Design Key Issues During Lighting Design • Consider an adaptive lighting system (or at least provide a controls-ready system) and establish light levels for dimming. • Select the CCT for the installation on the basis of visibility differences for certain spectral content sources as well as preferences determined by the DOT or authority. A CCT of 4000K generally provides consistent results, but consider other factors as discussed in other sections of this guide. • Include the shoulders adjacent to the roadway in the lighting design. When pedestrians 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 sound ratio levels discussed in this chapter. • Consider warrants and other factors as part of the decision process for roadway safety improvements. • Consider high-mast lighting only for large area lighting or very wide roadway cross sections, as it can create more light trespass and perceived brightness issues for abutters. • Identify sensitive receptors and housing areas close to the roadway right-of-way early in the design process. • Consider shielding in lighting system design. • Select shielding types carefully, because individual LED shields perform better than luminaire shields for high-mast lighting. • Note that LED luminaires may help reduce structural requirements for poles (fewer fixtures/ less weight/lower effective projected area). • Consider longer periods between needed access to luminaires.