Methods to consider when designing roadway lighting
Roadways are not parking lots-higher vehicle speeds on roadways call for lighting designed to help drivers quickly identify hazards and react to them. The American National Standard Institute's Practice for Roadway Lighting (ANSI/IESNA RP-8-00) provides three design methods for roadway lighting: illuminance, luminance, and small target visibility (STV). Illuminance is the amount of light reaching the roadway surface, measured in footcandles (lumens/ft2) or lux (lumens/m2). Luminance is the measure of light reflected off the roadway surface, measured in candelas per square meter (cd/m2). STV predicts visibility of a standard object on the roadway, and accounts for the contrast between a standard target and its background, taking into account factors including driver age, viewing time, pavement reflectance, and glare from the luminaire. The larger the STV number, the more visible an object will be.
Which method should you use? Illuminance is the most common, since it is also the oldest, having been around since the early days of lighting design; however, illuminance levels do not correlate with visibility or driver performance. Standard RP-8-00 addressed one shortcoming of the illuminance method by adding a maximum veiling luminance ratio intended to limit glare from the luminaire. A luminance calculation is necessary to calculate this veiling luminance ratio. Luminance design, including a maximum veiling luminance ratio, has been in use since 1983. Luminance describes the reflected light from the pavement seen when driving, so evaluating the quality of a lighting project by how it looks at night is actually the same as evaluating its luminance.
STV, which was added as a design method in 2000, predicts how easily a pedestrian or an object will be seen in the roadway. Making it easier to see people and objects is the goal of roadway lighting design, so it is logical to use a method that predicts visibility. The one drawback to STV is limited designer experience, given the method's relative newness.
Most U.S. designers still use illuminance as the primary roadway lighting design criteria. Worldwide, however, luminance is the dominant method, and practitioners in the United States would be well served to make the change, as it will help to better predict the performance of their roadway lighting designs. Designers should run luminance and STV calculations and select a design that meets the luminance criteria while maximizing the STV value.
PRODUCING A ROADWAY LIGHTING SCHEME
With multiple design methods and many criteria to consider, it is hard to know where to start when designing a roadway lighting scheme. Software that is capable of STV calculations is a must. (A list of programs is available at www.darksky.org.) Because the design process is iterative, producing multiple answers that meet the criteria, there are several decisions to make before entering numbers into a software program. The luminaire and lamp type, along with mounting height, spacing, configuration, and orientation all influence the results.
One design approach is to begin by selecting a mounting height that is in scale with the roadway cross section. A mounting height equal to the roadway width times 0.62 produces a visual scene that approximates the Golden Rectangle, the proportional measure determined by ancient Greek scholars. Using this mounting height, adjusted to match the nearest commercially available option, will produce a design that appears in scale with the roadway.
Specifying a luminaire mounting height of 12 to 15 feet is a common mistake designers make. This height correlates to a pedestrian-scale walkway; it does not work well for lighting the width of a roadway. However, the designer can relate a roadway-scale solution to the pedestrian by specifying an architectural base that provides visual interest at the pedestrian's eye level. A roadway-scale lighting system can also be supplemented with separate luminaires and light standards at the pedestrian scale.
After choosing a mounting height, determining luminaire configuration is the next step. Possible layouts that will usually offer better visibility include luminaires on one side of the road, luminaires on both sides of the road, or luminaires in a center median. Two-sided staggered, a popular configuration intended to provide a more uniform lighting level, will have poorer visibility results, because the increased uniformity actually decreases visibility on the roadway. One-side and median configurations often have the additional advantage of requiring less wire and conduit, resulting in lower construction costs.
LUMINAIRE AND LAMP TYPE
Next, the designer needs to select a luminaire and lamp. Type II or Type III (IESNA terms that refer to width of beam spread) luminaire distributions are typical for roadway lighting. Cutoff or full-cutoff luminaires should be specified to limit light pollution. Semi-cutoff luminaires are a good choice for applications such as a downtown area, where the luminaire might also have to provide vertical illumination for building façades. Non-cutoff luminaires produce significant glare and sky glow, and should not be specified for roadway lighting.
High-pressure sodium lamps are most common for roadway lighting. Metal halide is used where color rendition is a concern or 'white light' is desired. Lamps typically range from 100 to 400 watts, depending on the mounting height and roadway cross-section. A light-loss factor (LLF) is selected to account for the depreciation in lamp output and luminaire performance over time. Typical light-loss factors are 0.60 to 0.70 for high-pressure sodium lamps and 0.45 to 0.55 for metal halide lamps. The LLF will also vary depending on environmental conditions and maintenance procedures.
After selecting the inputs, luminaire spacing can be calculated. Most software programs have optimizers that run multiple iterations, varying the distance between luminaires in order to determine the allowable separation. Even if the first trial works, it is helpful to change the inputs to identify other possible designs. The software can provide a number of 'right' answers, but it is only a tool; the designer must still compare construction and operating costs and visibility performance in order to select the best lighting system for the roadway. Adopting this design approach will result in an efficient and effective lighting system that improves safety.
j. delvin armstrong & benjamin j. jordan
J. Delvin Armstrong, president of Armstrong Engineers, specializes in outdoor illumination projects in the Pacific Northwest. He is a member of the IESNA Roadway Lighting Committee. He is also an instructor in the University of Wisconsin-Madison, Department of Engineering Professional Development continuing education course, 'Effective Roadway Lighting.'
Benjamin J. Jordan is a program director for the University of Wisconsin-Madison, Department of Engineering Professional Development. He is also an instructor in the 'Effective Roadway Lighting' course, information about which is available online at www.epdweb.engr.wisc.edu/webH123.