Civil Engineering Reference
In-Depth Information
Fig. 14.4
Example of
observer positions in relation
to the field of calculation
according to CIE and CEN,
for a three-lane dual
carriageway.
To p
: for L
av
,U
o
and TI;
bottom
: for Ul
l
obs
obs
obs
obs
obs
obs
ANSI/IES recommend the use of an observer moving transversally in each traffic
lane. As shown in Fig.
14.5
, for each of the two rows of calculation points of a
traffic lane, the observer is positioned in line with the row of calculation points. The
advantage is that in this way the observer always looks under exactly 1
ⓦ
, the viewing
angle for which the road surface reflection tables are also measured. However, this
method does not give a picture of what one observer momentarily sees when viewing
across the carriageway. As already shown in the previous Section, ANSI/IES also
recognise a longitudinally-moving observer as shown in Fig.
14.5
where the different
longitudinal observer positions are indicated by grey dots. The worst value obtained
for each of the calculated quality parameters is the determining value that should
satisfy the relevant specifications.
As shown above, different Standards sometimes employ different conventions for
the observer positions and calculation points to be taken into account, each with
certain advantages and disadvantages. It is extremely important that specifications
for road lighting installations should clearly specify which convention method has
to be followed. Most lighting calculation software programs available have a setting
specific for the different calculation conventions.
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