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35 Chapter 5 â Work Zone Lighting and Temporary Roadway Lighting Current Guide The current AASHTO guide provides direction for work zone lighting and references more detailed information included in NCHRP Report 498 âIllumination Guidelines for Nighttime Highway Workâ and FHWAâs âManual on Uniform Traffic Control Devices.â Additional Considerations for Using LED Sources Using LED sources does not change any of the recommendations in the current AASHTO guide or references. However, LED sources provide benefits and adaptability that includes better control and lights that can be better aimed than older HID work zone lighting. LEDs also can cause a higher level discomfort glare than HID sources, however, making optical control and considerations of traffic moving through the work zone an important aspect of such system design. When selecting illuminance levels for work zones, the ambient illuminance levels of the area, presence of existing roadway lighting, and type of lighting must be considered. For example, a rural area work zone with minimal lighting in the approach and surroundings would deserve a minimal illuminance level in the work zone, which would be particularly appropriate if traffic were one-way and there were no oncoming headlights or other fixed glare sources. However, for work zones in urban areas, with complex and highly illuminated surroundings, increased illumination is appropriate, especially when glare sources are in view. Although glare sources may be mobile (such as headlights or fixed lights), potential degradation of visibility due to possible glare sources should be part establishing illuminance levels. Work area lighting should be designed to limit glare to both the workers and the passing motorist, particularly disability glare. Consider shielding, including internal louvers, shields, or visors, to limit glare and up- light. Both the disability and nuisance glare of the work area lights can be reduced by using diffuse light sources (e.g., balloon lights) that limit luminance. The LED light source can also be made less objectionable by using light sources with a lower correlated color temperature (e.g., lower than 4000K). Orientation of the portable light towers, specifically the angle between the beam axis and the driver line of sight, plays a crucial role in driver perception of glare and visibility. This angle should always be greater than 90 degrees. Figure 28 shows recommended orientations. The angle between the axis of the light beam and vertical should be less than or equal to 30 degrees (for portable light towers that could be aimed). In two-way traffic conditions, configuration b and c should only be used if the median separating the driving lanes is sufficient to block or minimize the impact of the work zone lighting. A recommendation to limit the light into an opposing lane is provided below. Lighting in work zones has two specific requirements: lighting in the work area and lighting for the roadway adjacent to the work area to compensate for glare and allow safe passage through the work zone.
36 Figure 28. Recommended Orientations for Portable Light Towers Base the mounting height of the portable light towers on the location of the light towers to the roadway, and make sure that it is at least 20 ft (6 m) when located closest to the roadway (e.g., in the lane being worked on). If the work zone is using balloon-type portable light towers (lumen output greater than 440,000 lumens), then locate the light tower in the shoulder and mount it at a height of at least 25 ft (8 m). For lights and light towers mounted on vehicles, aiming depends on type of light source. Aimed light sources follow the same orientation guidelines as those for portable light towers. Mount balloon light sources that produce diffused light in all directions at a height of at least 20 feet (6 meters) to reduce glare for oncoming traffic. In some instances, lower the mounting height to allow for travel under overpasses and overhead wiring. To quantify glare and limits for glare in a work zone, use an average vertical illuminance or arithmetic mean value at the windshield of a driver through the work zone less than 1.6 footcandle (fc). Vertical illuminance is defined as the amount of light incident on a vertical plane inside the windshield of a vehicle within the work zone, measured at the driver eye level (height of 4.76 feet from the ground (a slightly higher mounting height is acceptable if the vehicle (such as a pickup truck) does not allow for a 4.76-foot mounting height), as illustrated in Figure 29 and Figure 30. Calculate this vertical illuminance at a distance of 260, 200, 130, and 65 ft (80, 60, 40 and 20 m) from the portable light tower (see Figure 31). The arithmetic mean of the vertical illuminance at these four distances to the portable light tower should not exceed 1.6 fc. Measure the vertical illuminance level with the help of a cosine-corrected illuminance meter; if a cosine-corrected meter is not available, a cell phone with a light sensor application can be used but may not be as accurate. If the average vertical illuminance level at the measured distances is greater than 1.6 fc, reorient the portable light tower and remeasure the vertical illuminance levels. SSL can sometimes be brighter than older HID technologies so limiting glare towards drivers is an important issue. This section describes proper aiming and limits to the amount of light reaching the driver.
37 Continue this process until the mean vertical illuminance level is below the recommended value of 1.6 fc in the critical range to the portable light tower. Repeat the process of measuring the vertical illuminance if the orientation of the light tower is altered. If the width of the median between the two directions of traffic flow is less than 36 ft (11 m), then the mean vertical illuminance levels in the opposing traffic lane should not exceed 1.6 fc. In addition to limiting the mean vertical illuminance to 1.6 fc, the vertical illuminance at any location throughout the work zone should not exceed a maximum value of 5 fc at the windshield of the driver. Figure 29. Height for Light Meter Inside the Vehicle Figure 20. Mounting of Light Meter Inside Vehicle Figure 21. Distances for Vertical Illuminance Readings
38 Lighting levels for the roadway travelling through the work zone should meet the required lighting levels included in AASHTO Lighting Guide, Chapter 3, for the type of road the work zone is located. Key Issues for Work Zone Lighting with LED Light Fixtures â¢ Base work zone lighting on the ambient environment in which it is used and illuminate the roadway in the work zone to AASHTO lighting requirements. â¢ Limit glare in work zones by proper selection of equipment and its use. Use the glare limits included in this document to assess when work zone glare would become an issue.