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31 Chapter 4 â Tunnels and Underpasses Current Guide The current AASHTO guide discusses the differences between underpasses and tunnels and supplemental daytime lighting systems necessary to maintain visibility in longer tunnels where drivers entering the tunnel cannot see hazards or stopped vehicles because of inadequate daylight within the tunnel structure (Figure 25). For detailed design guidance, refer to ANSI/IES RP-8-18 and CIE 88 as well as National Fire Protection Association (NFPA) 502 for emergency and egress lighting requirements. Additional Considerations for Solid State Lighting Methodology for determining the lighting levels in a tunnel does not change with LED lighting systems. The primary difference with SSL technology for tunnel lighting design is the ability to control the light levels within the tunnel in a more precise manner. SSL technology allows better consideration of ambient lighting conditions outside of the tunnel because it can vary based on time of year, time of day, and atmospheric conditions. Figure 18. LED Lighting System with Adaptive Controls As defined in ANSI/IESNA-RP-8-18, a tunnel is broken into several transition zones for lighting. Lighting design must allow for decreasing at a similar rate as the human eye adapts to the darker tunnel
32 interior. The required values for pavement luminance (Lth) in the threshold zones (entry portals) depend on posted speed, orientation of the tunnel, ambient daylight conditions, and most importantly, the visual environment immediately around the tunnel portal. The equivalent veiling luminance (Lseq) method is used to determine the required luminance values. The Lseq evaluation uses the measured surface luminance(s) measured from objects that fall within the visual field of the drivers as they approach the tunnel portal. These data are then analyzed to establish a relationship between the average approach luminance and the required threshold luminance. An adaptive control system installed in the tunnel adjusts the lighting level within the tunnel to accommodate for the ambient exterior daytime luminance and weather condition outside of the tunnel, which is controlled by a luminance meter outside the tunnel portal. Using a luminance meter that continuously measures the brightness of the tunnel portal, connected to an adaptive lighting system that has a large number of dimming levels, allows for more precise control of required lighting. In contrast, traditional HID tunnel lighting systems switch luminaires on or off via contactors throughout the day to achieve 3 to 5 step levels to adjust to ambient conditions. In addition, HID lamps need time to cool before restarting, requiring longer hold-on periods to limit frequent cycling of the luminaires. Using fewer step levels and holding the levels for longer periods causes the system to use higher light levels than required. An LED system with adaptive controls allows for continuous dimming of the system throughout the day to achieve required lighting levels. In Figure 26 taken from data at a recently converted tunnel, the red line shows typical dimming for an LED system with adaptive controls throughout one 24-hour period. The dark blue line shows the contrasting HID system with step switching. The area hatched in dark blue shows the energy savings in one 24 hour period for one tunnel portal. SSL can provide improved tunnel lighting systems with more discrete control of lighting levels in the tunnel to compensate for the lighting conditions outside the tunnel
33 Another benefit of applying LED tunnel lighting systems with adaptive controls is reduced frequency of maintenance and costs as a result of longer operating life from LED luminaires. Operating at lower than full output considerably extends the useful life of the luminaire components. Another consideration when using LED sources in a tunnel for supplemental daytime lighting is the CCT of the source. Typical daytime CCT can be in the range of 5,000K to 20,000K depending on sky conditions and where you are looking. For this reason, consider higher CCT luminaires for daytime lighting for higher efficiencies. The luminaire used for nighttime lighting within the tunnel should match the CCT of the approach roads, and some tunnels use a mix of CCT to obtain the highest efficiencies (Figure 27). Consider temperature when using SSL in tunnels. Often temperatures at a tunnel ceiling can be higher than ambient temperatures so the design of the SSL luminaire must account for those elevated temperatures. Figure 26. Differences in Step Switching vs an Adaptive System
34 Figure 19. LED Lighting System with Mixed CCT Sources Key Issues for LED in Tunnels â¢ Use an adaptive control system with luminance sensors outside the tunnel for better control of required threshold and transition zone lighting. â¢ Evaluate source CCT and potential benefits. â¢ Evaluate tunnel luminaires and operating characteristics relating to temperature and expected environment of the installed luminaires.