Tunnel Lighting

The safety of a transportation tunnel depends highly on the ability of the motorists to see the interior and objects in the tunnel. The geographic location, orientation, and portal design can all influence the time it takes for human eye to adapt to new settings. This is usually more significant during the day as motorists enter from the bright daylight into the darker settings of tunnel interior.

In this article, I will discuss units of measure, common terminology, and lighting best practices.

Units of Measure

There are three common units of measure that describe lighting: lumen, candela, and lux.

A lumen [lm/lumen] describes the total quantity of visible light emitted by a source. It is the amount of light that is radiated.

Candela [cd] measures the amount of light emitted in the range of a three-dimensional angular span. Candela indicates the strength of the light emission. For instance, a full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 (cd) × 4π (sr) = 4π cd?sr ≈ 12.57 lumens.

Lux [lx] indicates the illumination intensity of a surface area. One lux is one lumen per square meter (lumen/m2).

Common Terminology

There are a variety of terms specific to tunnels and lighting that are important to understand. First is adaptation. This is the process by which the retina of the eye adjusts to increases or decreases in light. The design of the tunnel itself and the associated lighting should make it as easy as possible for motorists to adapt to these increases or decreases.

The lighting of the tunnel is typically divided into zones. The portion of the open approach of the highway immediately preceding the tunnel facade or portal is called the access zone. From this area, motorists should be able to see into the tunnel to detect possible obstacles and should not have to reduce speed.

Once motorists pass the access zone, they enter the threshold zone. This is the section at the entrance of tunnel where the first decrease in daylighting takes place. In the first part of this zone, the required luminance must remain constant and is linked to the outside luminance and traffic conditions.

The transition zone is a distance following the threshold zone where luminance is reduced gradually to reach the level required in the interior of the tunnel. It must be long enough to provide for adequate eye adaptation time from open road to interior tunnel brightness.

The most important part of tunnel lighting is the entrance, which comprises the threshold and transition zones. Requirements for illumination and luminance levels in this area are typically different depending on the region or country.

Once motorists have passed the transition, they are in the interior zone, which continues until the exit of the tunnel. Here, eye adaptation is no longer a concern as there is a consistent amount of light. This zone is usually the longest stretch of a tunnel, and luminance levels must guarantee safe driving conditions.

Finally, the exit zone is the area at the end of the tunnel where, during daytime, it appears bright for the drivers who have driven for some time in the interior. It’s important to understand that human eyes need more time to adapt from brightness to darkness than the other way around. This is called temporal adaptation. Therefore, the lighting processes for entering the tunnel is not simply reversed for the exit zone. In some cases, adaptive lighting will be required.

The other reason that more time and distance is necessary to enter the tunnel is the “black hole” phenomenon, which is triggered by the larger difference in luminance when approaching the tunnel. This effect is also partly psychological for drivers, causing a feeling of insecurity that is not equally experienced when exiting the tunnel.

Once motorists exit the tunnel, they are in the parting zone. The length of the parting zone is typically two to three times the required stopping distance. Stopping distance is the distance needed by a car at any given speed to stop when an unanticipated danger occurs on the road. This distance might depend on driver’s visual sensitivity to surroundings, his/her response time, and road conditions.

Tunnel Lighting

There are two common systems used to light tunnels – transverse and longitudinal. Both are dependent upon the utilization factor.

Utilization factor is the ratio of the lumens emitted on the surface to the total lumen output of the source. It depends on photometric characteristics, mounting heights, tunnel dimensions, and surface reflectance (the ratio of the flux reflected by a surface or medium to the incident flux).

In a transverse system, the light is emitted almost perpendicular to the axis of the tunnel. The familiar example is the continuous line of tubular fluorescents. The advantages of this system are the good visual guidance, minimal glare, light penetration between vehicles, and simple switching.

In a longitudinal system, the light is emitted more parallel to the tunnel axis. High-pressure sodium or mercury lamps can be used in this system. The advantages include the high luminous efficacy and bigger luminaire spacing that results in a power consumption savings.

Finally, in the event of a power failure, it is crucial to provide emergency lighting to maintain a minimum visibility in the tunnel. This is required both to avoid collisions and also to aid emergency crews should an accident occur.

Sound tunnel lighting design not only lowers the number of accidents from happening, but also can improve security, aesthetics and create an enjoyable driving (or walking) experience for passengers. Next time you drive through a tunnel, see if you can notice the concepts we talked about.

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