Zoning and Installation for Safe Tunnel Lighting

Utilization of tunnels relieves the pressure of crowded road traffic and shorten the distance from one place to another, providing a faster and more convenient transportation way. Within tunnels, where is darker and narrower than open road, reliable and high performance of lighting is vital for safe traffic. Enclosed environment inside the tunnel may cause discomfort, anxiety and fear. Drivers are not likely to slow down as they are pressing to pass through the tunnel as fast as possible. Moreover, the brightness contrast between the outside and inside of tunnel impedes the driver vision. Combination of desperation to escape the tunnel and the blurry vision increases the risk of accident. To ensure visual adaption and safety, tunnel lighting is divided into 5 zones with different lighting levels. The zoned tunnel lighting reduces the brightness contrast and increase the visibility of drivers. However, structure and size of tunnels vary from each other. Improper installation may result severe glare and uneven light distribution. Thus, installations of tunnel lights should match the tunnel. This post would like to the zoning and installation for safe tunneling lighting.

Aims of tunnel lighting

There are two aims of tunnel lighting. One is to keep the safety of traffic entering, transiting, and exiting the tunnel. The other is to ensure the smooth traffic flow in the enclosed section of tunnel. To achieve the aims, tunnel lighting should be adequate, comfortable, and high quality. Adequate lighting increases the visibility of drivers, offering excellent visual guidance. It also allows drivers to identify road signs and possible obstacles quickly, reducing the accident. Comfortable tunnel lighting is supposed to be flicker-free and low glare. This requires appropriate lighting levels that are adapted to different zones of tunnel and proper installations. Good color rendering is the key point of high quality tunnel lighting. Lighting with higher CRI enhances the actual appearance and details of the interior tunnel environment. Qualified tunnel lighting makes traffic safe and provides a comfortable visual experience like driving on the open road.

Why zoning the tunnel lighting

It goes without saying that safety is critical to tunnel lighting, but challenges remains for safe tunnel lighting. Black hole effect is the major threat for safe tunnel lighting. To figure out the reason for black hole effect, let’s take a look at the environment changes along the tunnel. Image that you are a driver driving on the open road (during daytime), and there is a tunnel ahead. Surrounding environment is bright due to the reflection of sunlight. And this makes you feel safe as you can recognize what’s going on around you. But the entrance of tunnel appears like a black hole as the entrance of tunnel is much darker than open road. The significant brightness contrast impedes your vision and endangers the fear and insecurity feelings. Thanks to visual adaption, you can adapt to the dark tunnel. But visual adjustment is not instantaneous. It is dangerous for driving during this period. Visual adaption must be occurred at least before you enter the tunnel. Otherwise, your vision is blurry. After a while, you adapt the lower luminance inside the tunnel. Later, when you are ready to arrive the exit of tunnel, lighting levels outside the tunnel is higher than the interior of tunnel. Again, the brightness contrast may impede your vision. Therefore, tunnel lighting is zoned into 5 areas with different lighting level requirements to ensure the visual adaption. Lighting levels at the entrance should be brighter and then reduced slowly inside the tunnel. At the exit of tunnel, lighting level is increased.

5 zones of tunnel lighting  

CIE claimed that lighting requirement at a tunnel should be applied to the outside lighting level and the point in the tunnel where visual adaption must be occurred. There are 5 zones of tunnel lighting. When planning the tunnel lighting, they are the basements for safe and successful tunnel lighting system.

1. Access zone

Access zone is the stretch area approaching to the tunnel entrance. In this zone, drivers must be able to identify possible obstacles. Additionally, visual adaption must be occurred in this zone. L20 method is widely used to calculate the visual adaption, which calculates the average luminance within a 20° viewing cone of the driver’s sight, starting from the access zone and centered on the entrance. Lighting level in the next zone of tunnel is affected by visual adaptation of driver.

2. Threshold zone

Threshold zone is the entrance of the tunnel. It is the most important zone for safety as black hole often occurs in this zone. Thus, luminance must be constant and uniform in the first part of threshold zone and then reduced slowly. Lighting level at the end of threshold zone can be reduced to about 40% of the initial value.   

3. Transition zone

Transition zone is the second part inside the tunnel. Lighting level is decreased slowly in this zone to allow visual adaption in next zone. The reduction of lighting level must not exceed a 1:3 ratio, which means lighting level at the beginning of transition zone must not higher than 3 times that at the end of this zone.

4. Interior zone

Interior zone is often the longest area of the tunnel. Lighting level in interior zone depends on the speed and traffic flow. Here is a table about the recommended lighting levels for tunnels with different traffic and speed.

5. Exit zone

Exit zone is the last part of the tunnel. Lighting level outside the tunnel is higher than exit zone. Although human eye can adapt instantly from low to high lighting levels, the exit may be invisible due to the brightness contrast. Thus, lighting level at exit zone must be increased to allow drivers to adapt to the bright outside. Lighting level of the exit zone should be 5 times the level of interior zone.

How to install tunnel lights

Ceiling mounting and wall mounting are the common options for installing tunnel lights. Except for mounting ways, arrangement and the mounted height also affect the lighting performance. Improper installation may result uneven lighting distribution and glare. We should keep in mind that each tunnel is unique. Therefore, the answer of how to install tunnel lights is varied. Knowing the effects of installation on lighting performance helps us select the right installation more effective.

Generally speaking, ceiling mounting is suitable for the tunnels that are higher than legal and minimum protection height. While the wall mounting is the opposite. In certain range of mounted heights, the lower mounted height and smaller installation angle, the greater road illumination and more uniform lighting.

Here is a table about the mounting options of tunnel lights and their respective effects.