Well-designed, high-quality lighting plays a key role in the optimal operation of vehicular traffic in a tunnel, ensuring a safe environment for travel in both daytime and nighttime conditions. High performance tunnel lighting ensures that the visual perceptions of drivers will be maintained during daytime and nighttime driving conditions, avoiding sudden variations in lighting levels when entering and exiting the tunnel. At night, the luminance level in a tunnel should be constant and equivalent to the level on the road leading into the tunnel. However, since there is a high level of external light during the day, it is necessary to increase the levels of luminance at the tunnel entrance to avoid a ‘black hole effect’ and a reduction of visual perception. At the tunnel exit, the luminance levels should also be increased to help drivers avoid glare effects by the ambient light visible outside the exit portal.
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Visual Adaptation
When drivers enter a tunnel during the day, they are confronted with the dual problem of visual adaptation.
There are two elements to visual adaptation: spatial adaptation and temporal visual adaptation.
Spatial adaptation occurs when the driver’s field of vision outside the tunnel is very wide and corresponds to the field of visibility offered by the vehicle’s windshield. As the driver approaches the tunnel, the field of vision narrows and is limited to an angle corresponding to the opening of the tunnel entrance.
Temporal visual adaptation occurs when entering a tunnel; drivers suddenly move from a high level of ambient luminance to a very low level of luminance inside the tunnel. Consequently, the eye needs time to adapt. During this time, the vehicle travels a distance that is greater relative to the travel speed. If this temporal adaptation does not properly occur, drivers lose visibility of possible obstacles on the road and traffic safety can be compromised. At the same time, when approaching the entrance to the tunnel, the average luminance in the driver’s field of vision decreases; yet within this field of vision, the percentage of space occupied by the tunnel entrance increases as the driver approaches it. In order to maximise the driver’s visual adaptation, the first part of the tunnel, the ‘threshold zone’, should be strongly illuminated over a distance equal to one ‘safe stopping distance’ based on the posted speed limit.
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By GlobalDataThe higher the speed limit, the longer the safe stopping distance will be. Proper lighting in the threshold zone helps a driver to better see obstacles, inside the tunnel, from the outside. The ‘transition zone’, an area where the level of luminance is gradually reduced over a distance determined by the authorised speed limit, is the next tunnel zone immediately following the threshold zone. The transition zone serves to support the curve of acceptability for the reduction in luminance perceived by the eye and to control temporal adaptation. Spatial adaptation disappears once the threshold zone has been crossed.
At the end of the transition zone, luminance is reduced to the lighting level selected for the ‘interior zone’ of the tunnel. The interior zone represents an area of the tunnel whereby the eye has adapted to the extent that minimal luminance levels can be used. The ‘exit zone’, less critical in terms of visual perception, is illuminated slightly higher to prepare drivers for the return to external ambient luminance (as well as the perception of obstacles within the exit zone). The need to reinforce luminance at the exit of a long tunnel depends upon several factors including: physical orientation of the tunnel to the sun; degree of driving complexity; and levels of danger within the exit zone.
It is important to note that tunnel lighting is designed based on luminance (light reflected off the tunnel interior surfaces – pavement, walls and ceiling – into the driver’s eyes at a fixed angle and distance), not illuminance (the amount of luminous flux per unit area) as most roadway lighting is designed to. It is a more scientific approach than conventional roadway lighting design, and is preferred due to the visual adaptation required as a driver approaches, enters, moves through, and exits a tunnel.
Flicker Effect
Traveling through a tunnel, the driver must not be distracted by ‘flicker’. Depending on the speed limit and the spacing between luminaires, flicker can occur when the frequency of perception of the ‘flashes’ emanating from the light sources range from 4Hz to 11Hz. These frequencies can be disorienting to a driver, and must be avoided (with respect to luminaire spacing) to ensure maximum safety while performing the driving task. Luminaires must be spaced at a certain range based on speed limit, to avoid this phenomenon of flicker. See IESNA Publication RP-22-2011 for more on ‘flicker effect’, as well as on recommended practices for tunnel lighting design.
Contrast
Drivers must be able to clearly and safely detect obstacles that arise in the various zones throughout the tunnel. For this purpose, a contrast must be created between the obstacle and the background in which it is viewed against (e.g., road or wall). The obstacle should stand out by being either lighter than its background (through ‘positive contrast’), or darker than its background (through ‘negative contrast’).
Several tunnel lighting systems may employ an increase in contrast, whether positive or negative.
A conventional symmetrical lighting system uses luminaires that provide a light distribution with a vertical symmetry plane perpendicular to the axis of the tunnel. In other words, the luminous flux is mainly sent across the tunnel on the road and on the walls. Light is directed symmetrically in the parallel plane to the direction in which the traffic is travelling. Symmetrical lighting systems provide a positive contrast of obstacles, relatively low glare, and good adaption to high traffic densities, enabling appropriate luminaire locations in a tunnel section and providing optimal lighting on the walls.
Asymmetric counter-beam lighting systems provide light that is distributed asymmetrically in the parallel plane to the direction in which the traffic is travelling, with the maximum luminous intensity directed toward oncoming traffic. This system enhances negative contrasts and reinforces the road’s level of luminance as observed by drivers.
Asymmetric pro-beam lighting systems provide light that is distributed asymmetrically in the parallel plane to the direction in which the traffic is traveling and the maximum luminous intensity is directed in the direction in which the traffic is travelling. This system enhances positive contrasts and reinforces an obstacle’s level of luminance as observed by drivers.
Light distribution and Sources
The geometry of tunnels is typically different in every case. To obtain the optimum light distribution in a given tunnel, the most suitable photometry should be examined and implemented (for each individual project as well as the specific elements of each type of application). For this reason, a very wide range of reflectors can be integrated into various tunnel luminaires depending upon project needs.
Photometry should ideally be independently tested and current/ updated within the last few years.
Most tunnels are currently illuminated with luminaires using HID lamps or fluorescent lamps. New energy efficient LEDs are growing in both interest and application for tunnels, though LEDs currently (typically) still do not quite deliver the high lumen package required to cost-effectively light the threshold and transition zones.
Still, it is expected that everimproving LED efficacies will, in the near future, allow LEDs to become a viable light source for illuminating tunnels.
Emergency lighting
Public safety inside a tunnel largely depends on the main source of vehicular traffic lighting; but also, in case of a major incident, on the emergency lighting within the tunnel. The function of emergency lighting is to guide and assist drivers, passengers and first-responders in case of fire, which in a tunnel is often accompanied by very dense smoke. It is important to provide reinforced lighting for emergency egress areas, fire doors and pedestrian evacuation tunnels. Appropriate ‘marker lights’ are designed to guide emergency responders toward emergency egress areas.
Tunnel luminaire considerations – Environment
Tunnels are generally characterised as ‘aggressive’ environments that demand luminaires with a rigorous mechanical design. In road tunnels, traffic exhaust fumes generate a high level of pollutants and a highly corrosive atmosphere when combined with humidity and radical swings in temperature over time. High levels of alkaline or acid pH, and galvanic coupling may result, and tunnel luminaires are therefore confronted with all types of corrosion including chemical, bacteriological and electrolytic corrosion. It is essential that tunnel luminaires be made of carefully selected materials that will resist these extreme conditions. Extruded aluminum, die cast aluminum with a very specific grade of aluminum and coating, stainless steel and synthetic materials will be the materials of choice in most tunnels.
Corrosion tests can be performed in laboratories and on site, to provide technical data/courses of action for such issues. The specified level of corrosion protection must ensure an optimal level of tightness (ingress protection, whereby IP Ratings are derived) to avoid the infiltration of air pollution, dust particulates and moisture into the luminaire over time. Hosedown tests can also be performed on tunnel luminaires, to assist in properly designing and specifying luminaires to protect against water infiltration during maintenance and cleaning with high-pressure water jets.
Tunnel luminaire considerations – Physical
As trucks and other vehicles pass through a tunnel, tunnel luminaires may be subjected to intense vibrations due to wind tunneling effects. Tunnel luminaires should be systematically subjected to the appropriate ANSI (American National Standards Institute) vibrations tests, and they should accordingly be designed to withstand the expected vibrations within the tunnel. Tunnel luminaires should also be subject to shock resistance tests. Stones and other objects propelled by vehicles passing through the tunnel, as well as random acts of vandalism, must be taken into account when designing tunnel luminaires. Accordingly, lenses should be designed and fabricated with the proper physical properties and the IK Ratings in mind.
The durability of tunnel luminaires during fires is of the utmost importance. In the event of fire in a tunnel, luminaires must continue to function for enough time to allow emergency workers to intervene and reach the emergency egress shelters. Non-flammable materials must be used in the production of tunnel luminaires to avoid the emission of toxic fumes
