# Comparative Study of Energy Savings for Various Control Strategies in the Tunnel Lighting System
## 2 Tunnel Lighting Controls: Strategies and Technologies
### Comparison of Hardware Cost and Energy-Saving for Four Control Strategies  
In this section, Madang tunnel is used as the case study to evaluate four tunnel lighting control strategies in terms of hardware cost and energy savings. The evaluated control strategies are no-control low-consumption lamps (LCL), time-scheduling control strategy (TSCS), daylight-adaptation control strategy (DACS), and intelligent control strategy (ICS). The LCL control strategy maintains a maximum 24-h luminance level, regardless of the weather and vehicle conditions. This method leads to large energy consumption; however, it does not require any equipment for its operation. A programmable scheduling routine regulates the LED light fixtures for the TSCS. The scheduling is based on the time of day. For tunnel lighting, as shown in Table 1, the 24-h period is often divided into six sub-periods, each with a luminance level. Figure 2(a) presents the hourly lighting for the LCL and TSCS strategies. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sun Location** & **Time of Day** & **Luminance Level** \\ \hline Early morning & 02:00–06:00 & 60\% \\ Morning & 06:00–10:00 & 80\% \\ Noon & 10:00–14:00 & 100\% \\ Afternoon & 14:00–18:00 & 80\% \\ Evening & 18:00–22:00 & 60\% \\ Midnight & 22:00–02:00 & 40\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Scheduling routine of TSCS. Figure 3: Hourly lighting luminance level for various control strategies. For DACS, the light levels of light fixtures are controlled in response to varying ambient luminance, regardless of vehicle presence and vehicle speed. Ambient luminance is read from the luminance meter placed 1.5 m above ground at the SSD from the tunnel entrance port. The data collection interval is 10 s. Figure 2(b) shows the hourly lighting for the DACS strategy. For the ICS, this study considers the \"vehicle in, light on; vehicle out, light off\" approach [23]. The light levels of light fixtures depend on vehicles' presence, vehicle speeds, and ambient luminance. LEDs are dimmed to their minimum luminance level when there are no vehicles in the tunnel; otherwise, the luminance is varied with the ambient luminance and vehicle speed. Vehicle detectors, luminance meters, and surveillance cameras are required in the system. The hourly lighting for this strategy for one day is shown in Figure 2(c). Table 2 presents the itemized initial installation costs of various control strategies in the Madang tunnel in China. The total cost in Table 2 does not include maintenance and operation costs. According to Table 2, the initial installation cost for the LCL, TSCS, DACS, and ICS strategies are $38,432, $51,756, $53,613, and $60,795, respectively. This shows that the LCL strategy has a lower initial installation cost compared to the TSCS, DACS, and ICS strategies. The data in Figure 3 and Table 2 are combined in Figure 4. As can be observed from Figure 4, the ICS strategy could consume about 10 percent less electricity than the DACS strategy, 27 percent less than the TSCS strategy, and 57 percent less than the LCL strategy. Likewise, the system cost for the ICS strategy is 19 percent higher than the DACS strategy, 24 percent higher than the TSCS strategy, and 58 percent higher than the LCL strategy. Because traffic volume and tunnel length could impact the energy saving of the ICS strategy, data collected in eleven tunnels with different lengths and traffic conditions were used to determine a relationship between these variables. Table 3 presents the advantages and disadvantages of several control strategies.