# Reliability Enhancement Driven by ANN for Lighting Control System in Highway Tunnels
## 2 Research Status of Tunnel Lighting System
### Progress of Tunnel Lighting Strategies
#### 2.3.1 Characteristics of Visual Adaption in Highway Tunnels
Restriction of Glare and Flicker Effect  
When the luminance level of a light source or reflection surface in the field of view is too high, or the brightness contrast between a light source and its background is extremely high, this will cause visual discomfort for the driver and reduced visibility of the visualtarget, this is called glare. American physicist [PERSON] put forward the concept of glare in 1926 and thought that the effect of glare on visual function was mainly caused by equivalent veiling luminance in people's eyes. Glare is one important factor affecting lighting quality and the comfort of the light environment. When glare exists, a driver's pupils shrink significantly and a light spot is formed in eyes, causing visual discomfort or vision loss, and greatly impairing the visual system's adaptation to the surrounding physical space, thus, it is difficult for drivers to visually recognize obstacles or sudden information ahead, which is likely to trigger traffic accidents. There are many possible causes of glare, as listed below:  
1. Unreasonable design of the tunnel lighting, for example, improper location and angle of the lighting fixtures. 2. Reflected light from a mirror or reflective material on the side wall. 3. Strong intensity of the active luminous sign profile, especially in the tunnel curve section. 4. Direct sunlight from the tunnel exit. 5. Dense vehicle exhaust fumes illuminated by high-power lights. 6. Sudden increase in the luminance level when approaching the tunnel exit. [PERSON] et al. conducted a comparative evaluation test with LED and other lighting sources on the impact of target response time, recognition accuracy and impact of visual effects from subjective discomfort caused by glare, and proved that LED lighting was conducive to improving the visual recognition effect under mesopic vision [17]. Additionally, many measures have been tried to restrict this undesirable glare effect, including:  
1. Reasonable tunnel orientation to avoid direct sunlight from the tunnel exit. 2. Similar color temperature in interior zone and outside tunnel. 3. All enhanced lighting fixtures switched off at nighttime. 4. Use of tunnel portals with lower reflectivity. 5. Dark material used for pavement inside tunnel. 6. Dark material paving at least one stopping distance in the access zone and parting zone. 7. Vegetation or shrubs planted around the tunnel portals. 8. Buildup of anti-glare structures, such as shading shed or shading panel, etc. In addition to glare, the flicker effect should also be restricted, as suggested by CIE. The flicker effect of tunnel lighting is similar to that of light sources. While driving in the tunnel, if the spacing of light fixtures leads to a zebra crossing pattern with alternating light and dark, and if the alteration frequency of light and dark falls within 2.5-15 Hz, this flicker effect will cause visual discomfort and psychological interference to drivers. How severe flicker effect caused visual discomfort depends on the following factors:  
1. Number of luminance changes per second (flicker frequency). 2. Duration of this flicker effect. 3. Ratio of peak luminance to trough luminance. All these factors rely on vehicle speed and light fixture spacing. Flicker effects lower than 2.5 Hz and above 15 Hz are negligible. When this frequency is within 4-11 Hz and the duration exceeds 20 s, if no measures are taken, drivers will feel uncomfortable. In this case, it is recommended to avoid the frequency range of 4-11 Hz [11].