# Comparative Study of Energy Savings for Various Control Strategies in the Tunnel Lighting System
###### Abstract  
Tunnel lighting is the most significant component in total energy consumption in the whole infrastructure. Hence, various lighting control strategies based on light-emitting diode (LED) technology have been investigated to conserve energy by decreasing luminaines' operating time. In this study, four kinds of tunnel lighting control strategies and the development of their associated technologies are evaluated: no-control low-consumption lamps (LCL), time-scheduling control strategy (TSCS), daylight adaptation control strategy (DACS), and intelligent control strategy (ICS). This work investigates the relationship between initial investment and electrical costs as a function of tunnel length (\(L\)) and daily traffic volume (\(N\)) for the four control strategies. The analysis was performed using 100-day data collected in eleven Chinese tunnels. The tunnel length (\(L\)) ranged from 600 m to 3300 m and the daily traffic volume (\(N\)) ranged from 700 to 2500. The results showed that initial investment costs increase with L for all control strategies. Also, the electricity costs for the LCL, TSCS, and DACS strategies increased linearly with \(L\), whereas the electricity cost for the ICS strategy has an exponential growth with \(L\) and \(N\). The results showed that for a lifetime equal to or shorter than 218 days, the LCL strategy offered the best economical solution; whereas for a lifetime longer than 955 days, the ICS strategy offered the best economical solution. For a lifetime between 218 and 955 days, the most suitable strategy varies with tunnel length and traffic volume. This study's results can guide the decision-making process during the tunnel lighting system's design stage. l 20  
improving drivers' visual adaptation, thus ensuring traffic safety. All strategies used require the use of green energy for the tunnel lighting. Green energy comes from natural sources such as sunlight, wind, tides, and geothermal heat. The solutions used up to date can be grouped into three:  
1. Shifting the threshold zone (most consuming area) out of the tunnel by utilizing structures allowing certain solar flux to pass. These structures are placed right before the portal gate of road tunnels and their effect is to reduce the amount of electrical luminous flux requirement, that is, complementing it with natural light. [PERSON] and [PERSON] [7; 8] applied this strategy with concrete pergolas and without diffusers to optimize the uniformity. [PERSON] et al. [9] used a punctured structure (filter) before the tunnel entrance to mitigate the elevated difference of luminance in the threshold zone, enhancing the incoming drivers' visual adaptation. [PERSON] et al. [10; 11] proposed new alternative ways to use these shifting pre-tunnel structures. [PERSON] et al. [12] proposed setting the dark shading sheds near the entrance and the light color shading sheds at the other end far away from the tunnel entrance to achieve a smooth luminance transition. [PERSON] et al. [13] proposed that a tunnel entrance sunshade should be set up to mitigate the sudden change of the luminance intensity at the tunnel entrance and proposed that the sunshade's length should be less than the SSD (safe stopping distance). However, utilizing the shading structures could result in increased maintenance workload and poor pavement illuminance uniformity. 2. The second option is using the light of the sun to power lighting devices or simulate projectors themselves. [PERSON] et al. [14] used a solar LED (light-emitting diode) tunnel lighting system to reduce the high operating cost and high energy consumption. [PERSON] and [PERSON] [15] showed that installing black solar panels around the tunnel portals decreases the energy requirement in the entire tunnel, which leads to significant savings due to the darkening in the tunnel surroundings with lower lighting requirements and also the energy produced by the panels. [PERSON] et al.