Equipment and Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 304

In principal, such screens can be designed such that, for the locally-occurring
sunlight directions, direct sunlight is prevented from striking the road, so preventing
the creation of disturbing dark-and-bright patterns on the road surface underneath the
screens. However, these so-called sun-tight screens are now scarcely ever employed
because their transmission quickly deteriorates due to the accumulation of dirt on the
many diffusely-reflecting surfaces, and they are difficult to clean. So this leaves the
open, non-sun-tight, daylight screens with their associated disadvantage of producing
dark-bright patterns on the road in front of the driver as illustrated in Fig. 22.3 .To
reduce this problem to some extent it is recommended that the screens be so designed
that the dark-bright pattern falls mainly across the road, as in the example of Fig. 22.3 ,
and not along the road, where it would create also a severe flicker problem. The light
transmission percentage of screens varies with varying sky conditions. It is therefore
important that the average lighting level under the screens be continuously measured
and the results used to control the electric lighting in the first part of the actual tunnel.

22.4 Road and Wall Surface

The reflectance of both the road surface and the lower parts of the walls should be
high. Their luminance is determined by the amount of light directed towards them
from the lamps and luminaires and reflected by them in the direction of the motorist.
Higher reflectances thus means that less light is required to provide the required
luminances. For the road surface an average luminance coefficient, Q 0 , of at least 0.1
is recommended. Furthermore, the road surface should not reflect too diffusely. Such
a road surface minimises backwards reflection towards small objects and the lower
parts of larger objects, so that the silhouette effect, for symmetrical systems and
especially for counter-beam systems, is not disturbed too much. As was described
in Sect. 20.2.2 this backwards reflection from the road surface in fact lowers the
contrast-revealing coefficient, q c .
The walls of the tunnel have to be bright: not only because they form the back-
ground for larger objects in the tunnel but, if bright enough, also act as visual guidance
for motorists both outside and inside the tunnel. Reflectances of 0.60-0.70 are rec-
ommended. In order to maintain the high reflectance, such tunnel walls should have
a surface finish that facilitates cleaning.

22.5 Design Aspects

Common aspects of designing lighting installations based on the luminances of the
road surface are dealt with in Chap. 13 of Part 1. In the present section those aspects
that specifically relate to tunnel lighting design will be discussed.
The foremost and probably also the most important task in designing a tunnel-
lighting installation is the determination of the highest lighting level to be installed