Visual Comfort for Motorists - Road Lighting: Fundamentals, Technology and Application - page 50

Fig. 4.2 Just acceptable
longitudinal uniformity, Ul, l ,
as a function of luminaire
spacing, s, for different values
of average road-surface
luminance, L av . Based on a
driving speed of 50 km/h in a
semi-dynamic road-lighting
simulator. (Walthert 1975 )

0.80

U l

0.75

L av

0.5 cd/m 2

0.70

1 cd/m 2
2 cd/m 2
4 cd/m 2

0.65

0.60

20 30 40 50

s (m)

from a visual performance point of view because the ratio of minimum luminance
to average luminance (viz. U o ) is somewhat better. However, this installation is very
bad from a visual comfort point of view. The problem is the sequence of bright and
dark areas on the road ahead of the motorist, viz. the so-called “zebra effect”. The
longitudinal uniformity Ul, l , defined as the ratio of minimum to maximum luminance
on the line parallel to the road axis, is very poor. As it is important that both vi-
sual performance and visual comfort are good, road-lighting installations should be
designed so that both overall and longitudinal uniformity are acceptable.
The appraisal for longitudinal uniformity is dependent on both the average road-
surface luminance level and the spacing of the luminaires. Figure 4.2 shows that the
higher the average road-surface luminance and the longer the luminaire spacing, the
lower the longitudinal uniformity can be for the same “just acceptable” appraisal for
that longitudinal uniformity.

4.2.2 Luminance Gradient

The road-surface luminance gradient, viz. the rate of change of luminance with
distance covered, also has an influence on visual comfort. If, for the same longitudinal
uniformity value the minimum and maximum values are located close together, the
discomfort effect will be greater than when those locations are farther apart from
each other. With conventional luminaires it is difficult, if not impossible, to produce
such pronounced beams that the luminance gradient becomes a problem in practice.
Since with LED luminaires much more pronounced beams can be produced, an old
and seldom-used concept, introduced by De Boer and Knudsen ( 1963 ) could become
useful for today's circumstances. They defined as a metric for the discomfort aspect
of the luminance gradient the so-called relative maximum luminance slope, S max .
It is defined as the maximum luminance variation found over any 3 m along or 1 m
across the road, expressed as a percentage of the average road-surface luminance.