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T.P.D.M.V.2.4
4.6.9 Diverging Lanes
4*6.9.1 As with merging lanes (see para, 4,6.8.1) a main line operating
speed of 80 km/h has been used in arriving at the recommended
standards.
4.6*9«2 The direct diverging lane, i.e. where the diverging lane exits
via a straight taper, is to be preferred under normal site
conditions. The diverge taper shall be 1:20 except that 1:35
should be adopted, to give additional space for diverging
traffic, at single lane links or at two lane links where one
lane is dropped from a three lane main line.
4.6.9.3 The length of diverge nose should be a minimum of 40m.
4.6.9.4 More generous diverging layouts may be required under the
following adverse site conditions :-
(i) If the main line is on a significant right hand
curve, a direct taper would result in a tangential
alignment and would be confusing to drivers.
(ii)
(iii)
If the main line is on a steep upgrade a longer
distance is needed for the diverging manoeuvre, to
enable faster vehicles approaching in the outer
lanes to penetrate through those heavy vehicles
which are moving slowly in the left hand lane.
When the main line is on a steep downgrade a problem
may arise associated with the high speed of vehicles
on the main line left hand lane.
In these situations an extra diverging lane 3*7m wide and at
least 100m long with a 70m taper should be provided.
4.6.9.5 To decide upon the most appropriate diverging lane layout to
accommodate design year flows, a method is employed which is the
same as that used for merging lanes. Igain it should be
emphasised however that the method described in subsequent
paragraphs indicates the most suitable layout for a diverge
considered in isolation. The other junctions along the main
line route should also be considered, so that drivers may be
presented with a consistent set of layout features.
4.6.9.6 The main line and link flows should be assessed in terms of
v.p.h. in the design year and the worst combination of these
flows, taking account of different peak periods, used as a base.
The base flows should then be adjusted for non standard traffic
composition and gradient as shown in Table 4.6.8.1, The
gradient should be measured over a distance of one kilometre,
centred upon the nose of the diverge lane.