Visual Performance and Sense of Confidence - Road Lighting: Fundamentals, Technology and Application - page 279

Table 20.2 Safety rating numbers (SRN) used for subjective tunnel entrance appraisals.

(Adrian 1982 )
SRN Experienced sensation Description

1 Black hole Completely unacceptable

3 Inadequate Entrance too dark, not sufficient for a safe entry

5 Fair Lower limit of just-sufficient levels for safe entry

7 Good Satisfactory, driver feels safe when entering

9 Excellent Very good viewing conditions

has already been explained in the previous chapter, it may lead to a hesitant driving
behaviour, such as unnecessary slowing down or even braking, and a change of lateral
position away from the tunnel walls. Subjective appraisals of existing tunnels have,
indeed, shown that different tunnel entrances and different surroundings are appraised
rather differently by motorists approaching tunnels. Many of these appraisals were
conducted without any obstacles in the tunnel, so that the sense of confidence was
appraised without information on the actual effect on visual performance provided
by the lighting.
Methodical research into this aspect for real-life tunnels is in practice rather dif-
ficult. It has, however, been done employing laboratory-scale simulations. The most
extensive simulation study made use of two large photographic slides simulating one
tunnel scene typical for a mountain tunnel and one for an urban tunnel (Adrian 1982 ).
These slides were illuminated from behind by a light box to produce surrounding
luminances up to the highest values occurring in practice and measured in terms of
L seq . The tunnel entrance itself could be varied in luminance independently, and was
measured as well: L th . For a range of different L seq and L th values, observers were
asked to assess the subjective safety of each situation with the aid of the appraisal
scale, called the Safety Rating Numbers (SRN) scale, given in Table 20.2 . These
appraisals are also a good measure for the sense of confidence of drivers during their
approach to a tunnel. Figure 20.13 shows the results of the appraisals.
From the figure it is evident that high threshold-zone levels of clearly more than
300 cd/m 2 are required to provide conditions that result in at least an appraisal
“good” (SRN

7). To compare these results with the results on visual performance
as dealt with earlier in this section, Fig. 20.14 superimposes the appraisal results
onto the L th -L seq relationship of Fig. 20.13 that is based on visual performance. For
the L seq range varying from 100 to 300 cd/m 2 , symmetrical lighting installations
coincide with subjective appraisals “fair” to “good” (SRN 5-7) and counter-beam
systems from “insufficient” to “just sufficient” (SRN 4-6). For approximately the
same visual performance, the sense of confidence provided by counter-beam systems
is thus clearly lower than that provided by symmetrical systems.

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