A tunnel is defined as a structure over a roadway that restricts the normal daytime illumination of a roadway section so that the driver’s visibility is substantially diminished. Design of tunnel lighting requires adaptation for driver needs in the approach, and the
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FIGURE 7.59 Lighting adaptation zones for tunnel lighting. Approach: The external roadway area leading to the tunnel. Portal: The plane of entrance into the tunnel. Threshold zone: The area inside the tunnel where a transition is made from the high natural lighting level to the beginning of the transition zone. Transition zone(s): Areas that allow the motorist to achieve appropriate eye adaptation by incrementally reducing the level of luminance required in the threshold zone to the luminance of the interior zone. Interior zone: Area within the tunnel after eye adaptation has been completed. |
threshold, transition, and interior zones, as presented in Fig. 7.59. Tunnels are classified by structure length and geometric alignment (visibility through the structure). A straight tunnel having an overall length from portal to portal equal to or less than the safe stopping sight distance (SSSD; Table 7.18) is considered to be a short tunnel. A tunnel with an overall length greater than one SSSD, or having an alignment or curvature that prevents motorists from seeing through the structure to the exit end, is considered to be a long tunnel. Overpasses and underpasses are those structures in which the length does not exceed one width of the roadway over (or under) which they are constructed.
Underpasses with a length-to-height ratio of approximately 10:1 or less will not normally require daytime lighting. When the length-to-height ratio exceeds 10:1, it is necessary to analyze the specific conditions, including vehicular and pedestrian activity, to determine the need for daytime lighting. Roadways that are not continuously lighted warrant underpass lighting in areas having frequent nighttime pedestrian traffic or where
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TABLE 7.18 Safe Stopping Sight Distances
*Assumes average prevailing speeds in a straight and level tunnel approach roadway are at, or near, the posted speed limit of the facility. For other geometric conditions, refer to the AASHTO documents. Source: A Policy on Geometric Design of Highways and Streets, American Association of State Highway and Transportation Officials, Washington, D. C., 2004, with permission. |
unusual or critical roadway geometry occurs under or adjacent to the underpass area. On roadways with continuous lighting, favorable positioning of luminaires adjacent to the underpass can often provide adequate lighting without supplemental luminaires. Lighting levels and uniformities should match the values on the adjacent roadway when practical. Because of limited mounting height, when lights are placed within a tunnel, special consideration should be given to glare and uniformity. Raised lighting levels may be achieved by using closely spaced low-wattage luminaires. Such increased levels should not exceed twice that of the adjacent roadway.
