Tunnel Lighting Design

With a well-designed tunnel lighting, at the tunnel entrance, the driver could very easily adapt without discomfort to the light inside the tunnel. It should be designed in a way to create a safe driving pathway by providing proper lighting throughout the tunnel.

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Tunnel lighting is more critical during the daytime. Lighting at the entrance of the tunnel should be good enough and well-lit so that the driver entering the tunnel from outside daylight should not feel blind.

Daytime tunnel lighting has the following five zones: access zone, threshold zone, transition zone, inner zone, and exit zone.

  1. Access Zone – This zone is having a length equal to the safe breaking distance of the roadway and is the area just before the entrance of the tunnel ending at the tunnel entrance. This zone should be properly lit so that driver’s eye could adapt and should not experience a black hole effect just before and immediately after entering the tunnel.
  2. Threshold Zone– Length is the same as safe breaking distance. Starts from the entrance of the tunnel to a safe breaking distance ahead. The light level in the threshold zone should match with the lighting level outside. At the end of the threshold zone, the light level should be reduced to 40 %.
  3. Transition Zone – Area in between threshold and interior zones. The light level is gradually reduced so that the light level at the end is equal to that of the interior zone.
  4. Interior Zone– The area between transition and exit zone. The light level depends on the speed of traffic and traffic intensity.
  5. Exit Zone– is between the interior and the exit portal of the tunnel. The light level should be well enough so that the driver’s eye could adapt exiting to the bright light outside.

During night time the tunnel light level is maintained as same throughout the length of the tunnel which is maintained the same as the adjacent roadway light level.

The guideline used for tunnel lighting in Europe and other countries is “CIE 88 – Guide for the Lighting of Road Tunnels and Underpasses”

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As per CIE88: 2004: the tunnel luminance level throughout the tunnel should follow or should be in compliance with CIE curve which is as follows:

Luminance evolution along the tunnel

Ref CIE 88:2004: Fig. 6.6. Luminance evolution along the tunnel.

Information Required for Tunnel Lighting Design

In order to do a tunnel lighting design and prior to starting the design work following details are required:

  1. Tunnel Construction details.
  2. Design Speed of Tunnel
  3. Safe Stopping Distance in meters
  4. Traffic Flow Rate and one way or two-way traffic
  5. The direction of Tunnel –North / South or East /West

Based on the above Tunnel design / Site parameters and using CIE88 guidelines we need to calculate the following:

  1. Access Zone Luminance L20 (cd/m2)
  2. K factor value based on Tunnel Speed Limit
  3. Threshold Zone Luminance based on L20 x K (cd/m2)
  4. Length of Threshold Zone same as Safe Stopping distance
  5. Interior Zone Luminance based on Speed Limit, Safe stopping distance & Traffic rate (cd/m2)
  6. Exit Zone Luminance = 5 x Interior Luminance (cd/m2)
  7. Exit Zone Length (m)
  8. Road Surface material Classification (R1, C2, etc)
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Tunnel Lighting Design Compliance

After finishing the tunnel lighting design, the following parameters should be checked for compliance:

  1. Compliance with tunnel luminance curve (CIE 88:2004) shown above
  2. Overall Uniformity (UO) of the Roadway
  3. Longitudinal Uniformity (UL) of the Roadway
  4. The average luminance of the tunnel walls, up to at least a height of 2 m, must be at least 60% of the average road surface luminance at the relevant location as per (Ref: CIE 88:2004)
  5. TI < 15%
  6. For travel time inside a tunnel of more than 20 s, the frequency range between 4 Hz and 11 Hz be avoided, particularly when small light sources with a sharp run-back are used. ((Ref: CIE 88:2004)

So, in short, a good tunnel lighting should ensure the safety and security of passers-by. Once you design strictly in compliance to the guidelines as mentioned above these two factors will be met automatically.

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