The trick to designing tunnel lighting that lets drivers’ eyes adjust

Good lighting should make driving through a tunnel like driving on the open road: it must allow drivers to enter, transit and exit the structure in safety and comfort.

The standards for tunnel lighting vary from country to country, but they commonly state that the amount of light needed inside a tunnel depends on the level of light on the approach – which affects how easily drivers’ eyes can adapt to the change.

Generally, the lighting of a tunnel is divided into zones.

Access zone
First, the access zone, which is formed by the approach road itself, taking outside lighting into consideration. Drivers should be able to see clearly into the tunnel to detect any obstacles and react safely, instead of being confronted by a black hole.

Threshold zone
The first zone in the tunnel itself is the threshold zone, which extends for the same length as the stopping distance for the design speed of the road. The target luminance level for this zone (when using the L20 method) is derived from the portal luminance (L20) value factored for the class of tunnel. This level is maintained at 100 per cent for the first half of the threshold zone, and can be reduced to 40 per cent by the end of the zone.

Transition zone
Following the threshold zone, the transition zone starts, which continues until the specified daytime interior zone level is reached. Throughout the transition zone the luminance levels are gradually reduced at a rate not exceeding a ratio of 3:1, to enable the human eye to accommodate the lowering lighting levels. The length of the transition zone depends on the design speed of the road.

Interior zone
During the day, the interior zone stretches from the end of the transition zone to the beginning of the exit zone. The lighting levels required in the interior zone will be scheduled in the standard being used, considering the characteristics of the tunnel use. At night-time, the interior zone stretches the whole length of the tunnel.

Exit zone
The final zone is the exit zone, which normally stretches for a distance in metres equal to the speed of traffic in kilometres per hour (e.g. 50kph, 50 metres), with a luminance level of five times that of the interior zone. The eye adapts much more quickly to increasing lighting levels than to decreasing levels, with exit lighting assisting rear vision as vehicles leave the tunnel, as well as preventing smaller vehicles being hidden behind trucks in the tunnel against the bright exit portal.

The lighting in the threshold, transition and exit zones will also be subject to multiple-stage switching or continuous dimming to maintain the correlation between the outside approach luminance of the entrance portal and the luminance levels inside. The interior zone will generally have two active stages, one for the daytime and one for the night.

Of course, the above description assumes a perfect scenario for a long tunnel where all the unique aspects of the structure fit the standard being used. If the tunnel is shorter, or doesn’t fit the standard, then professional engineering judgement will be required to make sure the lighting scheme meets the objective of tunnel lighting, enabling drivers to conduct the visual task safely, while also providing an economic solution in terms of energy usage and carbon footprint.

 

John Rands is a tunnel lighting expert with UK-based Designs for Lighting, and sits on the BSI, CEN and CIE tunnel lighting standards groups. He is among the experts speaking at our Road Tunnel Lighting conference in Barcelona on 8-9 October. Rands will assess the current standards landscape and evaluate the state of both current and new national standards. The event is free for specifiers, tunnel operators and consultants working in the field. To register, visit www.tunnellightingconference.com

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