TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 116
7.6
Diverges
–
General Principles
Diverging traffic should be able to leave the mainline easily and without impeding the progress of
through traffic.
There is potential for collisions on diverge connector roads if the capacity of the connection to the local
road network is insufficient and causes queuing on the connector road. Drivers leaving the mainline
should have sufficient time to react and brake safely before the end of any queue. The designer must
therefore ensure that the downstream cross-section (designed in accordance with DN-GEO-03036)
and junctions do not cause queues that approach the back of the diverge nose. This will allow drivers
to use the diverge area and length of nose to decelerate in reasonable comfort, as intended.
7.6.1
Choosing a Diverge Layout (Refer to Figure 7.5)
Hourly flows, as determined from the traffic flows section of this Chapter, for the diverge and the
mainline downstream of the back of the diverge nose must be inserted in Figure 7.5 to select a diverge
layout as shown in Figures 7.6.1 to 7.6.4. Where design flows lie close to, or on, a boundary between
the flow regions, the probability of the particular flow actually occurring should be carefully reviewed.
The provision of a layout that differs from that derived from the use of Figure 7.5 is a Departure from
Standard, whether the proposed design is an under or over provision.
The minimum length of a diverge slip road to be provided will normally be dictated by the requirements
in the preceding paragraphs in relation to capacity and the topographical layout of a junction. Where
this is not the case, as for instance at the diverge slip road leading into a service area, then the
minimum length diverge slip provided shall be equal to the Desirable Minimum Stopping Sight
Distance (SSD) for the mainline from the tip of the diverge nose to the stop or yield line at the end of
the connector road.
For all connector roads, a near straight at least equal in length to the nose length given in Table 7.2
column (4) for the appropriate Road Class must be provided downstream of the back of the diverge
nose. This requirement will enable drivers to comprehend the layout ahead and adjust their speed
accordingly.
The minimum length of a diverge slip road from a motorway shall be Desirable Minimum Stopping
Sight Distance (SSD) for the mainline from the tip of the diverge nose to the stop or yield line at the
end of the connector road.
Where the required length of Near Straight cannot be achieved, it may be appropriate to provide an
auxiliary lane instead or in combination. An application must be made for a Departure from Standard.
For diverges, the layout of the edge line must incorporate the radii shown on Figures 7.6.1 to 7.6.4.
In circumstances where traffic modelling shows significant delays incurred during peak periods as a
result of high exit flows, it may be appropriate to use non-standard layouts that are not included in this
Standard. Use of non-standard layouts shall require a Departure from Standards.