Tunnel Lighting
Designing tunnel lighting is a complex process that requires consideration of several factors, including the tunnel’s location, configuration, usage, environment, and local standards. The CIE 88:2004 Guide to Illuminating Road Tunnels and Underpasses provides the industry framework for lighting design. It highlights that the lighting requirements inside a tunnel vary significantly between daytime and nighttime conditions.
At night, the challenge is relatively straightforward: maintaining brightness levels consistent with the illuminated roads outside the tunnel. However, during the day, the design becomes more critical due to the human visual system’s adaptation process. When drivers transition from a brightly lit exterior to a relatively darker tunnel interior, they may experience temporary blindness, as their eyes are unable to perceive details on the road immediately. To mitigate this, tunnel lighting must be designed to accommodate this visual adaptation process.
CIE 88 divides tunnel lighting into five zones, each with specific lighting needs:
Access Zone
Begins at a distance from the tunnel entrance and ends at the entrance portal. The adaptation of the driver’s eye as they approach the tunnel is managed here. The lighting design uses methods like the L method or the Perceived Contrast Method (L20) to calculate visual adaptation.
Transition Zone
Extends from the end of the threshold zone to the start of the interior zone. Here, luminance gradually decreases, starting from 40% of the threshold luminance, until it reaches the interior zone's stable level.
Threshold Zone
This is the first section of the tunnel starting from the entrance portal. The luminance here is determined by the outside lighting conditions (L20 or Lseq) and must be equal to or greater than the stopping distance.
Interior Zone
The core part of the tunnel where the minimum necessary luminance is maintained consistently for visibility.
Exit Zone
Begins about 20 meters before the tunnel exit, allowing the driver’s eyes to adjust from the dark tunnel to the bright exterior environment. The luminance level here gradually increases, reaching up to five times the interior zone level.
Daytime lighting requirements are based on specific conditions, as outlined in the CIE 88:2004 guide, and consider factors like design speed, stopping distance, traffic flow, and veiling luminance.
Definitions
Design speed
The design speed to be taken into consideration for the design of a lighting system of a tunnel must be specified by the prime contractor.
Stopping distance
Stopping distance includes the distance traveled while the driver notices a hazard and applies the brakes, and while the vehicle comes to a full stop from its initial speed.
Traffic flow
The number of vehicles passing a specific point in a stated time in a stated direction(s). In tunnel design, peak hour traffic, vehicles per hour per lane, will be used.
Veiling luminance
The overall luminance veil consists of the contribution of the transient adaptation, the stray light in the optical media, in the atmosphere and in the vehicle windscreen.
Access zone luminance Land L 20 seq
Average luminance contained in a conical Field of view.
Overall uniformity (U) O
The ratio of the minimum to the average.
Threshold zone luminance (L) th
Average road surface luminance of a transverse strip at a given location in the threshold zone of a tunnel.
Transition zone luminance (L) tr
Average road surface luminance of a transverse strip at a given location in the transition zone of a tunnel.
Interior zone luminance (L) in
The average road surface luminance at that location. Visual guidance
The means that ensure that motorists are given adequate information on the course of the road in the tunnel.
Contrast revealing coefficient (q) c
The ratio between the luminance of the road surface and the vertical illuminance E at a specific location in v the tunnel qL/E. The method of tunnel lighting may c v be defined in terms of the contrast ratio in three ways: Symmetric lighting, Counter-beam lighting and Probeam lighting.
Vertical Illuminance (E) v
The vertical illuminance at a particular location at a height of 0,1 m above the road surface, in a plane facing and at right angles to the direction of oncoming traffic. The height of 0,1 m above the road surface is meant to represent an object of 0,2 x 0,2 m2.
Equivalent veiling luminance (L) seq
The light veil because of the ocular scatter, Lis seq quantified as a luminance
Longitudinal uniformity (U) I
The ratio of the minimum to the maximum road surface luminance.
Light distribution options
Symmetric lighting
The lighting is where the light equally falls on objects in directions with and against the traffic. Symmetric lighting is characterized by using luminaires that show a luminous intensity distribution that is symmetric in relation to the plane normal to the direction of the traffic.
Counter-beam lighting (CBL)
The lighting is where the light falls on objects from an opposite direction to the traffic. Counter-beam lighting is characterized by using luminaires that show a luminous intensity distribution that is asymmetric in relation to the plane normal to the direction of the traffic, where the maximum luminous intensity is aimed against the direction of the traffic. The term refers only to the direction of normal travel.
Pro-beam lighting
The lighting is where the light falls on objects in the same direction as the traffic. Pro-beam lighting is characterized by using luminaires that show a luminous intensity distribution that is asymmetric in relation to the 90/270 Cplane (the plane normal to the direction of the traffic), where the maximum luminous intensity is aimed in the same direction as the direction of the traffic.
Initial design requirements
Before beginning a lighting design or creating a lighting design specification for a road tunnel, the following criteria must be determined:
- Structural details of the tunnel.
- Design/speed limit of approach road(s).
- Stopping Distance (m)
- Tunnel Class based on:
- Traffic Flow Rate
- Traffic Mix
- Access Zone Luminance L20 or Lseq (cd/m2)
- Threshold Zone Luminance (cd/m2)
- Total Threshold Zone Length = Stopping distance (m)
- Interior Zone Luminance based on Tunnel Class & Speed Limit (cd/m2)
- Exit Zone Luminance = 5x Interior Luminance (cd/m2)
- Road Surface Category (R1, C2 etc).
