# Reliability Enhancement Driven by ANN for Lighting Control System in Highway Tunnels
## 2 Research Status of Tunnel Lighting System
### Progress of Tunnel Lighting Strategies
#### 2.3.2 Existing Lighting Strategies on Luminance Level  
CIE Visual Adaptation Curve and Reduction in Luminance Level  
As defined in CIE88:2004, tunnel lighting should meet drivers' visual requirements and ensure that, in principle, obstacles can be recognized ahead of the stopping distance (SD). Tunnel area is distinguished into different zones to determine the longitudinal level at daytime lighting: access zone, threshold zone, transition zone, interior zone and exit zone. The parting zone is the first part of open road directly after the exit zone and closely related to tunnel lighting. The CIE guide specifies the different lighting requirements for these zones based on several properties of the tunnel in consideration, such as geometric length,linear shape, wall reflectance and traffic volume, etc. The length and luminance level of these zones are suggested here and not repeated for each zone. Typically, luminance levels of threshold and transition zones follow the stepped visual adaption curve, as in Figure 1.  
Here, \(L_{th}\) is luminance level at the beginning of the threshold zone. For the first half of the threshold length (\(\geq\)SD), the luminance level must be equal to \(L_{th}\), and it is recommended that from the half SD onwards, the lighting level may gradually and linearly decrease to about 0.4 \(L_{th}\) at the end of the threshold zone. Additionally, \(L_{tr}\) is the luminance level of the transition zone as the function of driving time and speed, as in Equation (1). \[L_{tr}=L_{th}(1.9+t)^{-1.4},\hskip 14.226378 ptL_{th}=100\%,\hskip 14.226378 ptt\geq 0 \tag{1}\]  
Chinese Guidelines for Tunnel Lighting and Segments of Lighting Area  
As defined in the Chinese tunnel lighting guidelines [12], the tunnel lighting area is subdivided into access zone, threshold zone (TH\({}_{1}\) and TH\({}_{2}\)), transition zone (TR\({}_{1}\), TR\({}_{2}\) and TR\({}_{3}\)), interior zone and exit zone (EX\({}_{1}\) and EX\({}_{2}\)) as in Figure 2. This is similar to the CIE guide, but many differences exist too. In practice, the segmentation of these zones is flexible in scale in accordance with different tunnel properties (especially tunnel length) and design requirements. Normally, the following rules about zone segmentation and luminance reduction will be followed, and specific requirements definitely need to be considered for scenarios not covered in these guidelines. Before this, one vital parameter, \(L_{20}(S)\), should be given a clear explanation. \(L_{20}(S)\) is the reference parameter to be used by subsequent lighting zones. It is measured with an outside luminance detector (Konica Minolta LS-100) at the reference point that is in principle located in the center of the approaching lanes with 1.5 m height and a distance equal to the stopping distance ahead of the tunnel entrance, as in Figure 3. This detector should have a luminance range as wide as 1-7000 cd/m\({}^{2}\) with 0.1 cd/m\({}^{2}\) resolution and IP65 protection level, it senses the outside light conditions and reports the luminance level at the stopping distance with a 20\({}^{\circ}\) field of view opposite the tunnel entrance. Figure 1: CIE luminance evolution along the tunnel [11]. (1). Daytime, threshold zone: luminance level and length. \[L_{th1}=kL_{20}(S) \tag{2}\]  
\[L_{th2}=0.5 kL_{20}(S) \tag{3}\]  
\[D_{th1}=D_{th2}=\frac{1}{2}\bigg{(}1.154D_{\rm s}-\frac{h-1.5}{tan10^{o}}\bigg{)}. \tag{4}\]  
The key to this equation is as follows:  
\(k\), luminance reduction coefficient of threshold zone. \(L_{th1}\) and \(L_{th2}\), luminance level of \(\rm TH_{1}\) and \(\rm TH_{2}\). \(D_{s}\), stopping distance. \(D_{th}\) and \(D_{th2}\), length of \(\rm TH_{1}\) and \(\rm TH_{2}\). \(h\), headroom of tunnel entrance. (2). Daytime, transition zone: luminance level and length are defined in Table 3.