Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 191

Fig. 13.11 Relationship
between overall uniformity,
U o , for a twin-central
arrangement and cross factor
CF, for various wet road
surfaces characterized by
S1 wet

0.7

U o

S1 wet = 0.84

0.6

0.5

S1 wet = 2.6

0.4

0.3

S1 wet = 6.2

0.2

0.1

S1 wet = 30.5

0

0.1 1 10

CF

13.4.2 Foggy Weather

The poor visibility that may occur on a lighted road during foggy weather can be
ascribed to light being absorbed and scattered by the fog-producing water droplets
suspended in the atmosphere. Some of this light is lost, and some is reflected towards
the road user in the form of a bright veil of fog that obscures details on the road
ahead. In vehicle lighting, this is commonly known as the “white-wall effect”. Whilst
nothing can be done regarding the loss of light, there are some measures that can be
taken to reduce the disturbing light scatter towards the road user.
The degree to which fog scatters light in a given direction is dependent upon the
angle

that this direction makes with the incident light beam. The relative degree of
scatter for both dense and light fog conditions is shown in Fig. 13.12 .
From the figure it can be seen that scatter drops to a minimum as the angle θ
approaches 90 ⓦ . It therefore follows that any luminaire having a high cross factor
(CF) radiating relatively much light across the road, results in the lowest scattering
from the fog particles towards a motorist with a viewing direction mainly along the
road. As mentioned above, the highest cross factor is that possessed by the catenary
luminaire. The catenary installation is therefore ideally suited for use in situations
where fog is frequently encountered, especially since the visual guidance obtained
with such an installation is also excellent.
At some areas where fog very often occurs, it can be advisable to mount the
luminaires very low down, clearly lower than the eye-height of a motorist. In this
way disturbing scattering from fog particles above eye level is avoided to a large
extent and absorption of light is minimized because of the short path the light has
to travel through the fog from the luminaires towards the road surface. These types
of lighting installations, with their closely-spaced row of bright luminaires, may
strengthen visual guidance and increase visibility of objects under eye height (Gira-
solet et al. 1998 ). Because of the low mounting height involved, it may be difficult to

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