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29 Tunnels and Underpasses Current Guide The current AASHTO Roadway Lighting Design Guide (AASHTO 2018) discusses the differ- ences 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 24). For detailed design guidance, refer to ANSI/IES RP-8-18 (IES 2018) and CIE 88 (CIE 2004) as well as National Fire Protection Association (NFPA) 502 (NFPA 2020) for emergency and egress lighting requirements. Additional Considerations for Solid-State Lighting The 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 on the basis of time of year, time of day, and atmospheric conditions. As defined in ANSI/IES RP-8-18, a tunnel is broken into several transition zones for lighting (IES 2018). Lighting design must allow for the light to decrease at a rate similar to that of the human eye as it adapts to the darker tunnel interior. The required values for pavement luminance in the threshold zones (entry portals) (Lth) depend on posted speed, the 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 surface luminance values measured from objects that fall within the visual field of 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. The system is controlled by a luminance meter outside the tunnel portal. Using a luminance meter that continuously measures the brightness of the tunnel portal and that is 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 lumi- naires on or off via con tactors throughout the day to achieve three to five step levels to adjust to ambient conditions. In addition, HID lamps C H A P T E R 4 SSL can provide improved tunnel lighting systems with more discrete control of lighting levels in the tunnel to compen- sate for the lighting conditions outside the tunnel.
30 Solid-State Roadway Lighting Design need time to cool before restarting, which requires 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 25, taken from data at a recently converted tunnel, the dotted red line shows typical dimming for an LED system with adaptive controls throughout one 24-hour period. The solid 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. Figure 24. LED lighting system with adaptive controls. Figure 25. Differences in step switching versus an adaptive system.
Tunnels and Underpasses 31 Another benefit of applying LED tunnel lighting systems with adaptive controls is reduced frequency of maintenance and costs as a result of the longer operating life of 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 5000K to 20000K, depending on sky conditions and where you are looking. For this reason, higher CCT luminaires are suggested for daytime lighting for higher efficiency. The luminaire used for nighttime lighting within the tunnel should match the CCT of the approach roads; some tunnels use a mix of CCTs to obtain the highest efficiency (Figure 26). Temperature is a consideration when SSL is used 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. 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 the source CCT and potential benefits. â¢ Evaluate tunnel luminaires and operating characteristics related to temperature and the expected environment of the installed luminaires. Figure 26. LED lighting system with mixed CCT sources.