Tunnel lighting differs from standard roadway lighting because it must remain operational 24 hours a day; notably, the required illumination intensity is higher during the day than at night, making electricity costs a significant component of operational expenses. Furthermore, tunnel lighting possesses unique characteristics distinct from general roadway lighting, such as accounting for human visual adaptation to light and dark, as well as the lighting requirements within transitional spaces. These factors are integral to tunnel lighting design and are inextricably linked to user safety. Tunnel lighting is typically categorized into entrance, interior, and exit lighting. Entrance lighting is subject to particularly strict requirements, necessitating a gradual reduction in brightness from levels comparable to the external environment. Specifically, daytime entrance lighting levels are determined by factors such as external brightness, vehicle speed, the field of view at the entrance, and tunnel length. The International Commission on Illumination (CIE) divides tunnel entrance lighting into two zones (starting from the portal): the threshold zone and the transition zone. The threshold zone aims to eliminate the "black hole" effect, enabling drivers to clearly identify obstacles at the entrance, while the transition zone is designed to mitigate the stark contrast between the threshold zone and the basic interior lighting by further gradually reducing illumination levels. Japanese standards further subdivide entrance lighting into the access zone, adaptation zone, and transition zone.
The interior zone is the area within the tunnel unaffected by external natural light, where drivers' vision relies solely on the tunnel's artificial lighting. This zone is characterized by uniform illumination levels throughout. Since the lighting level remains constant, the design focuses on providing an appropriate brightness value, determined by traffic volume and vehicle speed. The layout and brightness levels of the entrance, transition, interior, and exit zones within the tunnel.