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T.P.D.M.V.2.4
4.6.8 Merging Lanes
4.6.8.1 The recommended standards for merging lanes are based on a main
line operating speed of 80 km/h, which seems the most
appropriate for those roads in the Territory which will utilise
grade separated junctions. The recommended layouts may be
adopted for lower or higher speeds however with suitable
adjustment to the proposed standards.
4.6.8.2 In normal circumstances the direct entry, i.e. where the merging
lane joins the main road via a straight taper, is preferred.
Situations will occur however where a more generous layout,
including a parallel merging lane plus taper, will be justified
by difficult site conditions as discussed in paragraph 4.6.8.5.
Both layouts are depicted in Diagram No. 4.6.8*1.
4.6.8.3 For the direct entry merging lane a taper of 1:30 is
recommended. For single lane entries however this should be
increased to 1:40 wherever possible.
4.6.8.4 The length of merge nose should be a minimum of 60m to provide
adequate sideways visibility for merging traffic.
4.6.8.5 As stated in paragraph 4.6,8.2 an additional length of parallel
merging lane may be required in the following types of
conditions :-
(i)
(ii)
Where the main line is on a significant left hand
curve. In this case visibility is limited and the
direct entry is not so effective as the angle of
convergence is too great. By providing a length of
parallel lane, drivers are able to observe
conditions on the main line by using their driving
mirrors rather than turning their heads.
Where the main line and hence the merging lane is on
a steep upgrade. Here the problem is one of
matching speeds and a greater distance is required
by merging vehicles in which to accelerate*
(iii) Where the main line is on a steep down gradient
there may also be problems associated with the high
speed of vehicles in the near side lane.
In these situations an additional merging lane 3-7m wide and a
minimum of 100m long with a 70m taper should be provided.
4.6.8.6 At higher entry flows two merging lanes may be required and the
resulting layout may be of the direct or shadow island type.
Both types of layout are depicted; Diagram No. 4.6.8.2
illustrating the situation with a consistent number of lanes' on
the main line, and Diagram Bo. 4.6.8.3 showing the case where
there is a pick up of one lane after the junction.