Daytime Lighting in Short Tunnels
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2023-12-01
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Edition:Final Report (December 2022–December2023)
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Abstract:Daytime lighting is warranted when natural sunlight fails to provide sufficient visibility for tunnel users. According to the American Association of State Highway Transportation Officials (AASHTO) Roadway Lighting Design Guide, no supplemental lighting is required if the tunnel is less than 80 ft in length, whereas supplemental lighting is needed if the tunnel length exceeds 410 ft. However, inadequate guidance is provided for the interim tunnel lengths (i.e., 80 ft ≤ tunnel length ≤ 410 ft). As such, for the short tunnels in this particular range, specific guidelines are necessary for daytime lighting warrant analysis and determination of the low-visibility area for lighting installation. In addition to the tunnel length, visibility in short tunnels is dependent on a variety of factors, including tunnel type, light penetration, median wall presence and type, orientation, whether the tunnel is straight or curved, reflectivity of pavement and interior surfaces, etc. Short tunnels, particularly on clear and sunny days, may experience visibility challenges due to the significant contrast in luminance between the tunnel interior and the exterior environment. This stark difference can create a “black hole” or “black frame” effect, temporarily blinding drivers and impeding their ability to detect potential hazards or small objects within the tunnel. To better understand this problem and address the associated safety concerns, field measurements of luminance and illuminance have been conducted for a number of short tunnels in Georgia. This research study leveraged this invaluable dataset to develop an analytical process consisting of decision trees and regression models aimed at achieving two primary objectives: (1) correlate adequate visibility and daytime lighting in short tunnels by evaluating the key variables, and (2) identify the specific areas within a tunnel that need artificial illumination. However, it is important to note that the conclusions and suggestions outlined in this study are tailored to tunnels falling within the restricted length range of 80 to 250 feet. This limitation stems from the dataset's constraint, encompassing tunnels strictly below the 250-foot threshold. A more inclusive and comprehensive investigation is needed to incorporate an expanded dataset covering the entire spectrum of short tunnels, including those up to 410 feet in length.
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