Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 38

3.6.2 Snow

When it is snowing, much of the light from the road-lighting luminaires is lost
through absorption in the snow flakes and some is scattered in the direction of the
driver, giving rise to glare. When the road is covered with snow, the average road-
surface luminance increases by a factor of 4 to 5 while, due to the diffuse character of
snow, the uniformity does not deteriorate much (Ekrias et al. 2007 ). If the installation
permits of dimming, the lighting level can be reduced as long as the settled snow
remains clean.

3.6.3 Fog

The poor visibility that occurs on a lighted road during foggy weather can be attributed
to light being absorbed and scattered by the fog-producing water droplets, which
often contain dirt particles, 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'.) Nothing can be done regarding the loss of light. The degree
to which fog scatters light in the direction of the driver's eyes decreases if more of
the light of the installation is radiated across instead of along the road. The visual
guidance obtained from the road-lighting installation can be important under foggy
weather conditions as it enables the motorist to see the run of the road and to detect
areas with denser fog ahead.

3.7 Fixed Road Lighting and Car Lights

Surprising as it may seem, as far as the visibility of objects at certain positions on
the road is concerned, the combination of fixed road lighting and vehicle lights is far
from optimum. But the former is nevertheless an important positive counter measure
for glare from oncoming cars.

3.7.1 Visibility of Objects on the Road

As has been discussed earlier, fixed road lighting aims at making objects visible
as dark silhouettes against the bright road surface (in negative contrast). The lower
the vertical illuminance on the road and the higher the road-surface luminance, the
better the visibility of possible objects. Car headlights do exactly the opposite: they
aim at making possible objects visible by having them stand out brightly against a