The much-anticipated Port of Miami Tunnel recently opened for business after four years of construction. Costing in excess of USD 1bn, the tunnel was funded through a public-private partnership where no tolls are charged–a first in the U.S. The tunnel also boasts other firsts, such as the use of a new TBM, never before used in this country, in its construction.

Additionally, the Port Tunnel design and operation incorporate best practices culled from tunnels around the world, including the installation of Fire prevention boards, security cameras and ventilation system jet fans.

The tunnel measures approximately 4,200ft (1,280m) long and 120ft (36.6m) below the surface at its deepest point. It has two dedicated lanes in each direction, connecting Miami International Airport and Interstate 95 directly to Port Miami. The twin-tunnel allows port traffic to bypass the streets of downtown Miami, and alleviates congestion of the nearly 16,000 vehicles that travel to and from the seaport through downtown streets each weekday.

Given the complexity of this tunnel and its curved, winding geometry, one of the greatest challenges was the need to support drivers’ visual perceptions – both day and night-from the point of tunnel entry to the point of tunnel exit.

As such, a continuous linear fluorescent design was chosen for the Interior Zone lighting while a higher output, HID point-source design was chosen for the Threshold and Transition Zones.

Along with (374) 100-W, 250-W and 400-W ceiling mounted high-pressure sodium fixtures installed to supplement the Threshold/Transition Zones, there are also (1,037) 75-W linear fluorescent Interior Zone and passageway lights along the upper sidewalls of the tunnel, most of which remain on during daytime hours. The tunnel lighting control system automatically adjusts the amount of lighting needed, at a given time, based on the ambient light levels just outside the tunnel portals at any given time.

A monument to infrastructure improvement in Miami, the tunnel also has symbolic meaning in the lighting industry: It may well end up being the last major tunnel to use non-LED, conventional light sources.

There is certainly no argument that LED lighting systems have established a strong foothold in the lighting marketplace over the last several years. The transportation industry is no exception, with new LED cobra head-style luminaires replacing traditional HID cobra head luminaires on our roadways and new linear LED light fixtures replacing conventional linear fluorescent fixtures in our rail transit facilities.

However, LED technology has taken a while longer to penetrate the very specific market of vehicular tunnel lighting in North America. Although both the New York/New Jersey Holland and Lincoln Tunnels have recently been retrofit with LED lighting, and an LED Lighting RFP is currently out for Boston’s Central Artery Tunnels, very few tunnels in North America to date have been illuminated with LED light fixtures.

This is largely because cost-effective LED lighting that met our stringent tunnel lighting standards was, until recently, simply unattainable. With rapid advances in LED technology over recent years, owners and specifiers of North American tunnel lighting projects are finally adopting LED lighting systems as the norm. In fact, a recent Lux Fit/Yole Development "LED in Road and Street Lighting Report" stated, "…LED luminaire growth will be driven firstly by tunnel lighting, and then relayed into highway, road, residential and amenity lighting applications starting in 2014."

Highways are becoming more congested and Provincial and State Departments of Transportation (DOTs) are continually looking for innovative ways to improve traffic flow. One way DOTs are now helping to ease congestion is by expanding the use of tunnels and long underpasses in large interchange projects. With less right-of-way to work with, and high costs to maintain elevated bridge structures, it often makes more sense to "go under" (rather than over) the congestion points.

With these tunnels and long underpasses comes the issue of proper illumination for safe driving. DOTs use recommended lighting practices and guidelines for highway lighting, which include specific criteria for tunnels, set by either the IES or the American Association of State Highway and Transportation Officials (AASHTO). However, both IES and AASHTO point out that these recommended practices and guidelines are not a mandate, and DOTs are free to make their own decisions about lighting criteria. Accordingly, effectively illuminating tunnels is often times a challenge.

This article will examine key considerations of tunnel lighting applications, as well as what LEDs bring to the table and how, ultimately, traditional lighting technologies will be phased out in tunnel lighting. Considerations

There are many tunnel lighting design factors to be considered. The following information is based directly from the recommendations of ANSI/IES RP-22 Recommended Practice for Tunnel Lighting. These guidelines provide information to assist engineers and designers in determining lighting needs, recommending solutions and evaluating resulting visibility at vehicular tunnel approaches and interiors. A well-designed tunnel lighting system facilitates safe operation of vehicular traffic throughout the tunnel, both day and night. The key is designing the lighting system so the drivers’ "visual perceptions" will not be adversely affected when approaching, entering, traveling through, and exiting the tunnel.

Tunnel lighting design is based on the metric of "luminance," which in this application is the total amount of luminous flux perceived by the driver as reflected off the various tunnel surfaces, at a particular angle, at a fixed distance. Luminance for tunnel lighting is calculated and measured in cd per sq meter.

This is in direct contrast with most conventional roadway lighting designs, which are based on "illuminance," which is the amount of luminous flux incident on a flat surface. Illuminance is typically calculated and measured in lux or footcandles (fc). The metric of luminance far better correlates to the needs of drivers in a tunnel, and, to the fact that drivers’ visual perceptions are vital and must be properly maintained while traveling through the tunnel.

During nighttime conditions, the luminance level in a tunnel should be constant and equivalent to the luminance level on the roadway leading into the tunnel. This makes sense; there would be no difference between light levels inside or outside the tunnel and therefore, theoretically, no difference between drivers’ visual perceptions inside and outside the tunnel.

Conversely in daytime conditions, there is a high level of ambient light outside the tunnel. Therefore, it is necessary to increase the luminance levels at the tunnel entrance to avoid a potential "black hole effect" (suddenly going from bright ambient sunlight to total darkness) and a resulting loss of drivers’ visual perception. When drivers enter a tunnel during the daytime, the concept of visual perceptions effectively drills down to an issue of "visual adaptation"; specifically, how (and how well) eyes adjust and adapt to their surroundings.

There are two distinct components of visual adaptation: spatial adaptation and temporal visual adaptation. Spatial adaptation is essentially the eyes’ adjustment to a driver’s narrowing field of vision as he/she approaches the tunnel entrance. The driver’s field of vision is wide outside the tunnel, basically defined by the field of vision within the vehicle’s front windshield. As the driver approaches the tunnel, this field of vision effectively narrows and becomes limited. It is now defined by a narrower angle that directly corresponds to the tunnel’s entrance portal.

Temporal visual adaptation takes place immediately upon entering the tunnel. If drivers were to suddenly go from a high level of ambient luminance to a very low level of luminance inside the tunnel, their eyes would need sufficient time to adapt.

During this adaptation time, their vehicle travels a particular distance relative to the travel speed. Proper temporal adaptation must take place during this timeframe; otherwise, drivers may lose sight of obstacles and possible obstructions on the roadway, compromising safety.

Tunnel Zones
Based on visual adaptation, an effective tunnel lighting design breaks the entire length of the tunnel down into different lighting zones as referenced in the ANSI/IES RP-22 Document.

The Approach Zone extends from the point at which the tunnel opening becomes the principal feature in the driver’s field of vision (also known as the adaptation point) to the actual entrance of the tunnel.

The area immediately inside the tunnel portal is known as the Threshold Zone. When approaching the tunnel entrance, 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. According to the International Commission on Illumination Document CIE 88:2004, in order to maximize the driver’s visual adaptation at this first part of the tunnel, the Threshold Zone should be highly illuminated over a distance equal to one "safe stopping distance for the tunnel’s design speed, minus the adaptation distance." Naturally, the safe stopping distance will increase as the tunnel’s design speed goes higher.

Regardless, good Threshold Zone lighting will help a driver to better see objects inside the tunnel prior to actually entering the tunnel.

The Transition Zone is the second tunnel section where the level of luminance is gradually reduced from the very high levels required in the Threshold Zone down to a minimal level required in the Interior Zone. The Transition Zone essentially provides an acceptable curve for controlling temporal adaptation. Spatial adaptation has already been completed upon passing through the Threshold Zone.

The tunnel’s next area is the Interior Zone. By time the driver reaches the Interior Zone (and assuming the tunnel is long enough), eye adaptation has become complete. Luminance is reduced to a level that is minimal but still safe for driving. The last tunnel area, the Exit Zone, is less critical in terms of visual perception and may be illuminated slightly higher than the Interior Zone to assist drivers’ adaptation in exiting the tunnel and returning to bright ambient luminance just outside the exit portal.

Providing higher luminance levels at the end of a tunnel depends upon several factors including the tunnel’s physical orientation to the sun, geometry of the tunnel exit and length of the tunnel leading up to the Exit Zone.

The key zone
The Threshold Zone is the key issue and driving force in an effective tunnel lighting design. At times, Threshold Zone lighting can actually be somewhat problematic; depending on many factors such as tunnel orientation and traffic design speed, the recommended luminance levels in the Threshold Zone can be as high as 370 cd per sq m. Simply put, this kind of high luminance level requires a lot of light fixtures with high lumen output in the Threshold Zone.

Until very recently — the Port of Miami Tunnel being the latest example–Threshold Zone lighting designs typically used traditional 400-W HID tunnel luminaires, either HPS or MH, of a specific IES distribution type, spaced extremely close together. (Conversely, Interior Zone lighting designs typically used: traditional 100-W-150-W HID tunnel luminaires, of a specific IES distribution type, spaced much farther apart; or conventional 80-W linear fluorescent tunnel luminaires, of a specific IES distribution type, placed end-to-end for a continuous aesthetic.) Light source choices in tunnel lighting design during recent years, as referenced in the ANSI/IES RP- 22-11 document, follow in Table 1.

Until recently, LED tunnel luminaires did not have enough output-either in a cost-effective manner, or at all – to justify using them in the Threshold Zone.

However, with the latest improvements in LED technology and LED tunnel luminaires, higher output LED fixtures for the Threshold Zone are now a realistic option. As such, traditional HID tunnel lighting systems do not figure to be used in tunnel lighting designs much longer.

How LED applies
There are a myriad of advantages to using LED luminaires, but until recently the consensus was that all the typical advantages to using LED fixtures didn’t apply to the world of tunnel lighting. The issue always returns to the Threshold Zone. While recent estimates indicate that LED tunnel lighting typically uses substantially less energy than equivalent HID or fluorescent alternatives in the Interior Zone, the same had not been true of the Threshold Zone.

Up to this year, efficacies of even the most powerful LEDs and fixtures in the marketplace were typically just not powerful enough to replicate the punch of a traditional 400-W HID lamp and fixture in the Threshold Zone. Further, the delta in energy savings between LED fixtures and HID fixtures tended to disappear in a high-output scenario. And though studies performed on exterior lighting systems indicate that some whiter (light) sources appear brighter at low nighttime light levels because of their color temperature, this correlation unfortunately did not apply to higher daytime light levels.

However, with the latest evolution of high-efficacy LED chips over the past year, the principles of effectively lighting tunnel Threshold Zones with LED luminaires have improved dramatically. Efficacies have increased substantially, and yet power consumption has held constant. An example is as follows:

  • Recent 300-W Nominal, 5,000K IES Type II Short LED Tunnel Fixture: 27,000+/- lm (Initial Delivered) driven @ 1050 mA; 309 watts actual power consumption
  • Latest 300-W Nominal, 5,000K IES Type II Short LED Tunnel Fixture: 35,000+/- lm (Initial Delivered) driven @ 1050 mA; 309 watts actual power Consumption For purposes of comparison, the following example is given for a traditional 400-W HPS tunnel fixture:
  • Recent 400W Nominal, 2,200K IES Type II Short HPS Tunnel Fixture: 35,000+/- lm (Initial Delivered, 70 percent Fixture Efficiency); with HPS CWA ballast, 464 watts actual power Consumption

With energy efficient LED lighting now effectively meeting or exceeding established tunnel lighting technologies, there is now a substantial enough increase in the efficiency of LED lighting design and related savings that can be realized while simultaneously meeting our tunnel lighting design standards.

A white paper, "Human Eye Response to LED Light: Scotopic versus Photopic Light and Vision," discusses how the human eye perceives white light more easily in low-light conditions, such as tunnels. This is another advantage to LED luminaires being installed in tunnels – the ambience of white light is perceived as being brighter because colors are more easily distinguished.

The same research suggests that a car driver’s reaction time (mesopic vision) is at least six times higher under white light and the higher color rendering also improves obstacle recognition.

Finally, research has shown a correlation between pupil size and color temperature. Cooler light sources such as LEDs cause pupils to shrink more than warmer sources such as HPS. Smaller pupil size increases visual depth of field and visual acuity. Increased depth and brightness perception can affect safety accordingly.

As a lighting professional who focuses on tunnel illumination, the author is excited about new cutting-edge LED technology and how it will transform tunnel applications. Moving forward, state and federal highway agencies will now be able to reduce energy costs and consumption, and drastically decrease lighting maintenance expenses while ensuring required tunnel illumination and drivers’ safety.