Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 271

in positive contrast, so that the revealing power is comparable with the symmetrical
tunnel lighting installation: 86.6 % for pro-beam against 87.0 % for symmetrical.
However, it should be noted that with the long low-pressure sodium lamps the creation
of pronounced light distributions in the longitudinal direction is not easy. Smaller
light sources, such as tubular high-pressure sodium lamps or LEDs, would have
provided a more effective symmetrical light distribution. Of course, there remains
the lower luminance yield of pro-beam systems and the consequently higher energy
consumption. The suggestion made by Ito et al. ( 2011 ) of halving the lighting level
with the pro-beam installation, would, if realized, result in a low level of confidence
for drivers approaching the tunnel: they would experience the tunnel entrance as
a black hole with scarcely any visual guidance into the tunnel provided by bright
luminaires.

20.2.3 Influence of Traffic Speed

There is an important link between the traffic speed for which the tunnel is designed
and the value of the lighting levels that need to be provided in the tunnel. In this
section it will be shown that the traffic speed has a direct influence on the lighting
level required in the first part of the tunnel zone, the threshold zone. The higher the
speed, the higher the lighting level has to be. It should be realized that this also has
a consequence for the amount of light required in the other zones: with a higher
threshold zone level, the transition lighting in the transition zone has to start at a
higher level, and it takes longer before it can be reduced to the low interior lighting
level. Quantitative details will be given in later sections.

20.2.3.1 Adaptation State

During his approach to a tunnel, the relatively-dark tunnel entrance and its surround,
occupy an increasingly larger part of a driver's total field of view. As a consequence
of this, the state of adaptation of the driver's eyes is gradually decreasing. However,
at any given moment during the approach his state of adaptation is determined not
only by the luminance distribution in his field of view but also, because of “time lag”,
partly by the luminance distribution experienced shortly before his approach to the
tunnel entrance. On the basis of many different experiments (Schreuder 1964 , 1971 ;
Narisada 1972 ; Narisada and Yoshikawa 1974 ; CIE 1984 ), it has become clear that
the luminance to be provided in the threshold zone of the tunnel is best determined on
the basis of the luminances of the tunnel surroundings at the point where the motorist
has to be able to perceive an object in the tunnel entrance. For an object standing
in the mouth of the tunnel this moment is defined by the 'stop-decision point', this
being the point outside the tunnel at which a driver has to begin braking in order
to come to a standstill at the tunnel entrance. The higher the speed, the farther the
stop-decision point from the tunnel entrance. At that point, the luminances in the