The primary objectives of tunnel lighting is to ensure traffic safety by illuminating all important objects during the journey of vehicles, and to improve visibility and comfort. Proper lighting in road tunnels will ensure that traffic can enter, pass through and leave the tunnel with a great level of safety. These aims are achieved by the adequate illumination of the tunnel interior, which allows drivers to quickly adjust to the light within, identify possible obstacles, and manage to leave the tunnel without reducing speed.
For that purpose, it is compulsory that the carriageway and the tunnel walls give a clear picture of the road. The road markings and stripes should also be visible, and tunnel lighting should ensure that objects on the road are visible at a sufficient distance.
Lighting must then ensure an adequate luminance level on the road, throughout the tunnel, and that luminance ha to be distributed uniformly on the road, in wet or in dry conditions. The lighting should not give rise to glare.
Tunnel lighting is critical during the day as the driver ‘s eye is accustomed to daylight, and cannot adapt immediately to the darkness of the tunnel. For that reason, it is necessary to adapt the light intensity to the lower level needed in the tunnel, compared with the daylight.
Different zones and luminance levels will be defined in the tunnel. Each of the tunnel section will have a specific lighting requirement. The threshold zone is the section that is visible before one arrives to the tunnel The transition zone follows the threshold zone, and is a section in which the luminance declines to the level in the inner zone of the tunnel. The inner zone luminance is the carriageway luminance necessary for an acceptable level of traffic safety in the tunnel by day. The exit zone is the last part of the tunnel, in which the significant, but also during the night when it is less, but reversed.
Although it is difficult to correlate lighting and accidents in tunnels, the role of lighting is crucial to enhance the safety and comfort of drivers, and it is essential to define very precisely the needs of security and respect for the environment, to choose the most suitable lighting solutions and more effective for energy cost savings.
There are 4 categories of lighting fixtures for tunnels:
– Security lighting : these light fixtures must continue to operate even in case of power failure.
– Evacuation lighting : it runs continuously and is used to indicate low power for a safe the pedestrian route in an emergency .
– The basic lighting : present along the entire length of the tunnel, it works day and night in different levels: day, night and reduced night. It is not useful for tunnels with a short length.
– The main lighting : the light fixtures used to overcome the risk of glare and difficulties of accommodation to lower brightness. It requires a high level of luminance during the day and throughout the year essentially at the tunnel entrance and exit These requirements vary depending on weather conditions , the number, and speed of vehicles.
Increasingly , lighting levels can adapt not only to weather but also to traffic and the drivers speed, so as to optimize the power required for lighting and therefore the energy consumption.