Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 183

of the road where curves are involved. For these reasons, it is sometimes advisable
to employ instead a single-sided arrangement with increased mounting height, the
higher cost of the taller columns perhaps being offset by the savings in trench cutting
and cabling (one run of cable instead of two) that such an arrangement brings with it.
The staggered arrangement does, however, exhibit advantages over the single-sided
one during periods of wet weather. This is because the shiny patterns then occurring
cover the whole road better than do the bright mirror-strip patterns created with the
single-sided arrangement, thus minimizing dark patches.

13.1.4 Opposite Arrangement

This arrangement, with the luminaires placed opposite one another, is usually em-
ployed on relatively wide roads. The road width can be up to 2-2.5 times the mounting
height. As with the single-sided and staggered arrangements, this arrangement too
employs both mast and wall-mounted luminaires.
When an opposite arrangement is employed on a dual carriageway having a central
reserve wider than about one-third of the carriageway width, it effectively becomes
two independent single-sided arrangements and must be treated as such. The same, of
course, holds true when obstructions (e.g. trees, anti-glare screens) are placed along
the central reserve. In the case of curved roads, visual guidance may deteriorate
because from a distance it is not clear whether a given luminaire is positioned on the
left or on the right-hand side of the road, thus making the run of the road ahead less
clear.

13.1.5 Central Arrangement

In the central arrangement the luminaires are hung from so-called span wires strung
across the road. (This arrangement should not be confused with the catenary arrange-
ment, which will be described later in this chapter). The spanwire arrangement is
normally used at rather low mounting heights (around 6-8 m) for narrow roads in
built-up areas when there is no possibility of employing road-side lighting columns.
The suspension cables are then simply strung between buildings on either side of
the road. A road width of some two times the mounting height can be adequately lit
with this arrangement. This arrangement may also offer a solution to the problem
of where to site the luminaires such that their light output will not be shielded by
roadside trees.
The principle of using cables instead of columns to support the luminaires can,
of course, be applied to any of the three arrangements considered above.