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32 Work Zone Lighting and Temporary Roadway Lighting Current Guide The current AASHTO Roadway Lighting Design Guide (AASHTO 2018) provides direction for work zone lighting and references more detailed information included in NCHRP Report 498: Illumination Guidelines for Nighttime Highway Work (Ellis et al. 2003) and the Manual on Uniform Traffic Control Devices (FHWA 2009). Additional Considerations for Using LED Sources The use of 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 luminaires that can be better aimed than older HID work zone lighting. LEDs also can cause a higher level of discomfort glare than HID sources, however, which makes optical control and considerations of traffic moving through the work zone an important aspect of such system design. When illuminance levels for work zones are being selected, the ambient illuminance levels of the area, the presence of existing roadway lighting, and the 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 surround- ings, 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 resulting from possible glare sources should be part of establishing illuminance levels. Work area lighting should be designed to limit glareâparticularly disability glareâto both the workers and the passing motorists. Shielding, including internal louvers, shields, or visors, is suggested to limit glare and uplight. Both the disability and nuisance glare of 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 CCT (e.g., lower than 4000K). The orientation of portable light towersâspecifically, the angle between the beam axis and the driver line of sightâplays a crucial role in visibility and driver perception of glare. This angle should always be greater than 90Â°. Figure 27 shows recommended orientations. The angle between the axis of the light beam and vertical should be less C H A P T E R 5 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.
Work Zone Lighting and Temporary Roadway Lighting 33 than or equal to 30Â° (for portable light towers that can be aimed). In two-way traffic conditions, Configurations 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. To limit the light into an opposing lane, base the mounting height of the portable light tower on the location of the light tower relative 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 the arithmetic mean value at the windshield of a driver through the work zone of less than 1.6 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 driverâs eye level [height of 4.76 ft from the ground (a slightly higher mounting height is acceptable if the vehicleâe.g., a pickup truckâdoes not allow for a 4.76-ft mounting height)], as illustrated in Figure 28 and Figure 29. 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 30). The arithmetic mean of the vertical illuminance at these four distances to the portable light tower should not exceed 1.6 fc. Figure 27. Recommended orientations for portable light towers. 1.45 m (4.76 ft.) Figure 28. Height for light meter inside the vehicle.
34 Solid-State Roadway Lighting Design 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. 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. Lighting levels for the roadway traveling through the work zone should meet the required lighting levels included in the AASHTO Roadway Lighting Design Guide, Chapter 3, for the type of road on which the work zone is located. Figure 29. Mounting of light meter inside vehicle. SSL can sometimes be brighter than older HID technologies, so limiting glare toward drivers is an important issue. This section describes proper aiming and limits to the amount of light reaching the driver. Figure 30. Distances for vertical illuminance readings.
Work Zone Lighting and Temporary Roadway Lighting 35 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 according to AASHTO lighting requirements. â¢ Limit glare in work zones by proper selection and use of equipment. Use the glare limits included in this document to assess when work zone glare would become an issue.