Tunnels are divided into 5 transition zones of light:
- 1- The access zone: Part of the road leading to the tunnel entrance.
- 2- The threshold zone: Known as the “braking distance.”
- 3- The transition zone: Brightness is slowly reduced towards as users enter the tunnel.
- 4- The inside zone: Often the longest zone. The brightness level depends on traffic and speed.
- 5- The exit zone: The maximum length is 50 meters and the light is 5 times brighter than in the inside zone.
In daylight, road safety rules require that drivers’ eyes adapt from lighting conditions outside of tunnel entrances to levels applied inside the tunnel.
The initial adjustment occurs within the threshold zone. When drivers’ eyesight adjust as they move inside the tunnel, brightness levels can be slowly diminished in order to decrease energy consumption and ensure that eyesight adaptation is not affected.
The transition zone then facilitates the adaptation to an acceptable level of brightness before entering the inside zone.
The brightness levels in various zones are determined by a variety of elements that include: speed and traffic volumes, tunnel lengths, geographical position, and approach characteristics.
At night, drivers’ eyesight is already adapted to the tunnel’s low level of brightness; usually no adjustment to the inside zone is necessary.