U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Today, tunnels are lit to high enough levels to ensure that traffic, both during day and nighttime, can approach, pass through, and leave a tunnel, at the designated speed, with a degree of safety and comfort not less than that along adjacent stretches of open
The luminaires that are used to accomplish this typically have one of three types of distribution and effect on objects. The distribution effects are shown in Figures 34 a,b, and c.
Figure 34a. Symmetrical (bisymmetrical) light
Figure 34b. Counter-beam light distribution.
Figure 34c. Pro-beam light distribution.
In figure 34a, light is symmetrically distributed, particularly when linear sources are used. Although a uniform luminance is produced throughout the tunnel, relatively low contrast values are generated.
In figure 34b, light is asymmetrically distributed, with the strongest part of the beam directed toward the approaching driver. This type of lighting provides high pavement luminance and low object luminance, creating negative contrast.
In figure 34c, light is asymmetrically distributed, with the strongest part of the beam directed away from the approaching driver, in the direction of traffic flow. This type of lighting provides high object luminance and low pavement luminance, creating positive contrast.
The panel was most interested in learning about Europe's experience with the different distributions. The French experts suggested that targets disappear under pro-beam and do not use it. In fact, while all of the countries visited have experimented with using pro-beam for threshold lighting in tunnels, none of them use it.
The Swiss have evaluated counter-beam, pro-beam, and symmetric lighting systems. Field measurements and lighting calculations have indicated that, if counter-beam yields a 100 percent light level, then symmetric with the same lumen output yields 70 percent, and pro-beam with the same lumen output yields about 30 percent. This is (less specular) the gain in yield is smaller. The Swiss discovered some problems with counter-beam installations where there is a lot of large truck traffic. Counter-beam lighting is preferred in Switzerland.
Until now, the Belgians have only used symmetrical lighting. They have, however, conducted extensive experiments and found that the best angle (with the vertical) for the main beam in a counter-beam system is 56 degrees.
The Dutch use counter-beam lighting because it is more cost-effective than sunscreens.
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