TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 110
Figure 7.3 provides a number of layouts to cater for anticipated flows. Layout A (Figure 7.4.1) provides
a basic parallel merge. Layout B (Figure 7.4.2), Layout C (Figure 7.4.3) and Layout D (Figure 7.4.4)
are required where flows justify a lane gain arrangement. Layout E (Figure 7.4.5) may be considered
as a Departure from Standard where it is not possible to use Layout C (see also Section 7.9).
Ghost island road markings at merges should be designed in accordance with Chapter 7 of the Traffic
Signs Manual.
The minimum width of a ghost island at a merge lane is 2.0 m at its widest point and the minimum
width of a chevron is 0.5m. If the ghost island marking is less than 1.2m wide it will be too narrow to
mark with chevrons. The length of ghost island that is unmarked with a chevron could extend over a
long distance. In order to prevent this problem, the minimum width of a ghost island must be 1.2m at
a distance of 50m from the tip of the ghost island head or tail. It should be noted that ghost island
layouts can require significant length to comply with the standard and this may be reflected in the land
requirement especially where the layout is being provided within an existing road boundary.
Gap finding is assisted when the merging traffic has the opportunity to match the speed of the mainline
traffic. For all connector roads, a near straight at least equal in length to the nose length given in Table
7.1 column (3) for the appropriate Road Class must be provided upstream of the back of the merge
nose. This requirement will enable merging traffic to achieve a matching speed.
Platoons of traffic can enter a merge slip road if junctions upstream are signal-controlled. This traffic
can have a significant effect on the mainline flow especially at peak times when
available gaps in the
mainline traffic flow are few. Turbulence and congestion are the result. Care should be taken to
program such traffic signals with a view to reducing their impact on the mainline flow.
In circumstances where traffic modelling shows significant delays incurred during peak periods as a
result of high entry flows, it may be appropriate to use non-standard layouts that are not included in
this Standard. Use of layouts not included in this Standard shall require a Departure from Standards.