Visual Performance, Comfort and Pleasantness for Pedestrians, Cyclists and Residents - Road Lighting: Fundamentals, Technology and Application - page 67

In the latter case, luminaires and columns are treated as visual obstructions. Larger
spacings reduce the number of luminaires required, but demand higher mounting
heights. Higher mounting heights increase the possibility of the lighting columns
becoming clearly visible as dark silhouettes against the high luminance of the sky.
This silhouette effect can be avoided for most directions of view by limiting the
column height to half the height of the surrounding houses. It should also be taken
into account that in residential roads the scale should be “human”. This means that
objects in the street should relate to one's own height (Caminada and van Bommel
1980 ). Taking all these, sometimes conflicting, aspects into account leads to the con-
clusion that the mounting height should be as high as possible but lower than half
the height of the facades of the houses.

5.3 Lighting Level, Uniformity and Glare Restriction

From what has been said above about safety, security and visual orientation, certain
conclusions regarding the minimum-required lighting quality needed to fulfil the
requirements for pedestrians and other slow-moving traffic can be drawn.
As it is both the minimum horizontal illuminance and the adaptation level that
determine the required lighting for safety, the quality is usually specified in terms of
the average horizontal illuminance, E hor, av , (which determines the adaptation state)
together with an uniformity ratio, usually the minimum over average horizontal illu-
minance, E hor, min /E hor, av 1 . An average horizontal illuminance of 2 lx is the absolute
minimum required. With surroundings that are brighter than that of the path of the
pedestrian or cyclist, proportionally higher average horizontal illuminances are re-
quired. For security and for visual guidance purposes, the total space should receive
some light: a measure for this can be the façade illuminance which should have
a value of some 1.5 lx. Here too, brighter surrounds require proportionally higher
values. It has been shown that spacial brightness is also dependent on the spectrum
of the light source used. Higher colour temperature light sources (cooler tinted light)
require lower lighting levels. For the identification of the faces and bodies of persons
in the street a minimum semi-cylindrical illuminance of about 1 lx is required at
all positions on the path of the pedestrian. The curved surface of the semi-cylinder
should be directed towards the person who has to identify the approaching pedestrian.
In practice this often means parallel to the line of the pavement, in both directions.
Of course a too-high degree of disability glare will diminish the visual perfor-
mance of all users of the road or path. While motorists have their main direction
of view towards the road in front of them, and therefore seldom look straight into a

1 The Danes recommend hemispherical illuminance instead of the horizontal illuminance, as the
measure for seeing small obstacles in the path of pedestrians. It is suggested that it takes better
account of the three-dimensional aspect of obstacles. However this is doubtful because the effect of
lighting on the rear side of the hemisphere (and of an obstacle) cannot be seen by the approaching
pedestrian.