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T.P.D.M.V.2.4
4.3.9.4 For shadow islands the width of the right turning lane will vary
but it should be an absolute minimum of 3m for new junctions and
2*5m for improvements to existing junctions* Where it is
desirable to shelter vehicles turning right from the minor road,
widths up to a maximum of 5m should be used. However, on high
speed roads, widths greater than 3*5m should be avoided so as
not to encourage overtaking. The right turning lane should be
introduced by means of a 45 degree splay, at the end of the
taper, except at left/right staggered junctions. At left/right
staggered junctions the deceleration lengths will lie side by
side and the starting points of the deceleration lengths should
be joined by a straight line as in Diagram No* 4.3.16.10.
4.3.9.5 At dual carriageway junctions and single carriageway junctions
with physical islands the width of the central reserve will be
made up of the 3.5m wide right turning lane plus the width of
the physical median. With a minimum median width of 1.5m, a 5m
wide central reserve will be formed. Whilst such a width would
shelter small vehicles turning right from the minor road, it is
recommended that the central reserve be locally widened to 10m
if all but the longest vehicles are to be accommodated. This
10m would include the metre strips which, it is recommended,
should be introduced locally if not present through the major
route.
4.3*9.6 Central islands should normally be developed to their maximum
widths symmetrically about the centre line of the major road at
the tapers shown below :
Design speed (ktn/h) 100 85 70 60 50
Single carriageway taper 1 in 30 25 20 20 20
(physical and shadow
islands)
Dual carriageway tapers 1 in 50 45 40 40 40
It is perfectly acceptable to develop central islands
asymmetrically however in order to avoid utilities for example,
and in the case of climbing lanes or sections on sharp curves
asymmetric development may be essential.
4.3*9*7 At physical island layouts on single carriageway roads central
islands should be introduced by hatched markings until a width
of 1.5m has been developed.
4.3.9.8 Adjacent to the right turning lanes 'are the through traffic
lanes. At shadow island junctions the through lane should be
between 3.0m and 3.65m wide (exclusive of hard strips if
present). At physical islands on single lane carriageways the
through lane should be 4.0m wide, which with two hard strips