Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 184

13.1.6 Twin-Central Arrangement

The twin-central arrangement is intended for dual carriageways, the luminaires be-
ing mounted on T-shaped masts in the middle of the central reserve. Since this is
essentially two single-sided arrangements placed back to back, the condition that
the luminaire mounting height should be equal to or greater than the effective car-
riageway width applies. The luminance of the road surface in the lanes nearest to the
luminaires, viz. the 'fast' lanes, will usually be higher than that on the 'slow' lanes.
With the twin-central arrangement both the road and kerb-side parts of the lumi-
naire light output contribute to the luminance on the road surface, provided that is
that the central reserve is not too wide. This is in contrast to the opposite arrangement,
where it is mainly the road-side part of the luminaire light output that provides the
luminance. The twin-central arrangement is therefore the more efficient of the two.
It should be noted, however, that the opposite arrangement may give slightly better
lighting under wet-weather conditions; provided, that is, that the central reserve is
rather narrow and does not contain obstructions. The visual guidance obtained with
the central arrangement is good.

13.1.7 Irregular Arrangement

On squares and non-linear street layouts a more flexible positioning of the luminaires
is often wanted. This is possible, but special care should then also be given to the
orientation of the luminaires to avoid glare in those directions where it must be
avoided, such as windows of houses and shops.

13.1.8 Catenary Installation

Catenary lighting installations use closely-spaced luminaires suspended from a cable
(the catenary) stretched between widely-spaced columns along the central reservation
between two carriageways (Fig. 13.5 ).
The special catenary luminaires employed, which are orientated such that the
longitudinal luminaire axis is parallel to the road axis, shed most of their light across
the road rather than along it. Thanks to this feature, catenary installations can light a
wider road than can conventional installations. Good overall uniformity of U o = 0.4,
can in fact be obtained with a luminaire mounting height that is less than 0.5 times the
effective width of a carriageway. Because the luminaires are viewed almost axially
by the motorists, the amount of discomfort and disability glare produced is small.
Moreover the fluctuation in the glare stimulus experienced by the driver by virtue of
his movement along the road is less disturbing under catenary lighting than under any
of the conventional lighting arrangements. Visual guidance for motorists (as will be