Through participation in the IESNA Roadway Lighting Committee, Marshall Design has stayed current with the developments in the American National Standard for Roadway Lighting, ANSI/IESNA RP-8-00, which was revised in 2000. The standard provides for three separate methods for demonstrating compliance, known as the Illuminance, Luminance and Small Target Visibility (STV) methods. Marshall Design developed the RoadSoft calculation program to evaluate simple roadway models according to the 2000 standard.
From the RoadSoft software, Marshall Design also developed specialized optimization software, used to investigate the design potentials in the new standard. The metric of Unit Power Density was used to evaluatue roadway lighting systems. The significance of the UPD metric is based on the clear correspondence between UPD and energy use, initial equipment and installation costs, and operation and maintenance costs. Changes in UPD are directly proportional to changes in these aspects of roadway lighting systems. Lower UPD values correspond to more efficient lighting systems. The first results from this research were presented to the Roadway Committee and at the IESNA Conference in 1999, and published in the Journal of the IES in the Summer 2000 (v29n2).
This work demonstrated the effectiveness of the optimization algorithm, using a version of the software which adjusted the mounting height to determine the longest spacing. Luminaires were all located at the edge of the roadway. There were 73 luminaires included, all high pressure sodium, with 150W, 250W and 400W lamps and full cutoff, cutofff and semi-cutoff distributions. Sixteen different combinations of roadway classifications, roadway widths and lamp wattages were established. In total 6,887 runs were made, each meeting the full set of criteria for both the illuminance and luminance methods for the specified roadway classification.
For each of the optimum roadway designs, the Unit Power Density (UPD, watts/unit area of roadway) and Unit Uplight Density (UUD, uplight/unit area of roadway) were calculated. In addition, some designs were re-optimized to meet just the luminance or STV methods.
The conclusions from this study include:
Further research has expanded significantly on the initial work. The photometric set now includes 234 files with high pressure sodium (HPS) lamps and 153 with metal halide (MH) lamps, and with distributions ranging from non-cutoff to full cutoff. The software has been improved to adjust the overhang or setback as well as mounting height - within specified limits - to find the greatest spacing. The procedure for determining the optimum spacing for luminaires is effective for either the illuminance or luminance methods of the standard.
This further research was presented at the 2001 IESNA Annual Conference in a paper which will be published in the upcoming issue of the Journal of the IES. The conclusions from this work include:
The Street and Area Lighting Committee of the IESNA invited David Keith to make a presentation about roadway ligthing at their 2001 Conference (download a copy here). In addition to summarizing the previous work, he also showed recent work which included changes in the rated lumens for metal halide lamps, investigation of the sensitivity of UPD to system Light Loss Factors (LLF), and changes in UPD as overhang was restricted at or off the traveled way. Examples of Unit Uplight Density results for specific luminaires and lighting systems were also detailed. A report showing these UUD example calculations is available here, and the photometry used in these examples is available here. In addition there is a thorough investigation of the increase in UPD for roadway lighting systems when full cutoff distributions are required.
The increase in prescriptive lighting ordinances requiring full cutoff distributions has been very rapid, but not well informed. While some professionals and agencies have publically testified that there are no costs associated with requiring full cutoff distributions, many lighting design professionals have questioned that opinion. From extensive investigations looking at thousands of optimized roadway designs, the only possible conclusion is that requiring full cutoff distributions increases Unit Power Density, and therefore the initial costs, energy use and costs, operating costs and environmental and light pollution. The relative increase in UPD compared to using the most efficient available equipment is shown in a table and a separate chart in the SALC presentation, indicating increases of 12% to over 25% in UPD for most local and collector roadway conditions. As lighting designs are refined to reduce UPD, the penalty for requiring full cutoff distributions increases.
The table and chart from the presentation to the 2001 SALC Conference are available here.
The overall conclusion from this work is that roadway lighting systems can now be designed to be more efficient. Such improvement will reduce costs and pollution (both energy and light) and still provide lighting which meets the current National Standard. To reach this goal, the designer should thoroughly investigate which wattage, lamp type, luminaire distribution and pole combinations provide the most appropriate system.
The use of full cutoff and cutoff distributions and the use of metal halide lamps leads to less efficient systems, with higher initial costs (for both installation and equipment) and higher operating costs (for both energy and maintenance). In addition, the use of such equipment increases the uplight from the lighting system as well as the energy consumption.
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