Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 267

The following sections will describe the lighting in the various tunnel zones in
more detail from a point of view of visual performance and sense of confidence.

20.2 Threshold Zone

20.2.1 Daytime Luminances in the Access Zone

As already stated above, the daylight situation in front of the tunnel, viz. in the access
zone, determines for an important part the lighting required in the threshold zone.
Needless to say, the daylight level is not constant; it is continually changing with
the time of day, season and weather conditions. A tunnel-lighting installation has
to be designed so that safe traffic conditions are achieved under the highest lighting
levels regularly occurring. The criterion is often the estimated daylighting level that
is only exceeded for a certain percentage in a year (e.g. 2 %, which is for the 4000
annual daylight hours, 80 h per year). At all moments that the daylight level is lower,
the electric lighting in the tunnel should be partly switched off or dimmed. Blaser
and Dudli ( 1993 ) developed a method that allows the determination of the daytime
luminance situation for different percentage criteria from the mean sunshine time
distribution during the year for the region in question. Their method builds on the
L 20 concept that will be described in a following section.
Of course, it is not the illuminance level that determines the state of the approach-
ing driver's eyes but the luminances in his field of view. These luminance values are
dependent on the daylight illuminance in combination with the direction of view of
the driver and the reflectance of the surfaces. Narisada and Yoshikawa ( 1974 ) found
that drivers fixate for about 80 % of the time on the tunnel entrance itself, so that the
direction of view can safely be taken as being towards the centre of the tunnel en-
trance. The position of the sun relative to the direction of view and thus the direction
of travel, also has an influence. Table 20.1 gives an indication of luminance values for
different driving directions of an approaching motorist corresponding to a daylight
situation of around 100,000 lx horizontal illuminance. The low luminance value for
dark paint illustrates the possibility to decrease the luminances in the field of view of
the approaching driver and thus to lower the required lighting level in the tunnel. This
can be achieved by painting the tunnel portal and its surrounding structures in a dark
colour. Planting trees and shrubs above the tunnel entrance is another possibility.
Sometimes, the construction of a large entrance canopy that shields part of the bright
sky from view, can be effective as well. Figure 20.3 shows an example of this.

20.2.2 Symmetrical, Counter-Beam and Pro-Beam Lighting

Three fundamentally different tunnel-entrance lighting systems can be employed
for the threshold zone: a symmetrical system, a counter-beam system and a pro-
beam system. Which of these systems is employed has an influence on the level of