TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 105
7.
Layout of Grade Separated Junctions
7.1
General Principles
The main objective of grade separated junction design is to provide a junction which is safe for the
forecast traffic flows. Certain layouts are not recommended for safety reasons and shall not be used.
These are:
a)
grade separated junctions on single carriageways, Type 2 and 3
dual carriageways
(see DN-GEO-03031 and Chapter 8 of this standard dealing with compact grade
separated junctions);
b)
grade separation on dual carriageways within 0.5 km of a changeover to single
carriageway standard, measured from the end of the merge taper to the beginning
of the right hand lane hatching that removes the offside lane or lanes (see the
Traffic Signs Manual and Chapter 5 of this standard);
c)
offside merges and diverges;
d)
priority junctions, particularly those
with right turning movements, on an otherwise
grade separated route.
Full grade separation is not permitted on single carriageway roads and Type 2 and 3 dual
carriageways due to driver perception and therefore reduced safety and shall be avoided in design.
However, DN-GEO-03031 permits the introduction of compact grade separation on these road types.
Refer to Table 6.1 of DN-GEO-03031.
Junction and Interchange design is an iterative process which is a key part of the overall design
process for schemes. Figure 7.1 is a flowchart for junction and interchange design. Figure 7.2 outlines
the connector road design process.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 107
7.2
Design Process
The first stage is to decide on an initial network and junction strategy, including the connections to be
made, for example whether the junction should be omni-directional.
Having made those starting decisions, it is possible to derive hourly flows to be used in the design
process following the guidance in the TII Project Appraisal Guidelines. An examination of these flows,
applied to the network strategy adopted, will enable a decision to be taken (or confirmed) that the
route should be Motorway or All- Purpose national road. Reference to DN-GEO-03036 will give a
starting point on the level of carriageway provision for the links on the network assumed.
The next stage, and the first step that could lead to iteration, is to assess the likely lane provision on
the mainline and the connector roads. If the basic scheme cannot be tailored to cope with demands,
including those likely to arise when maintenance work needs to be undertaken, then network and
junction strategy will need to be reviewed and alternatives investigated; for example
–
reducing the
number of junction accesses or using link roads. Link roads reduce the frequency of direct access
points along the mainline in order to eliminate sub-standard weaving lengths thus promoting free flow
to minimise the potential for collisions and to preserve the high capacity of the mainline. They can also
be used where it is unsafe or not possible to make direct connections. Link roads can be useful for
maintenance and diversions.
The following stage may also lead to iteration. This is to determine the merge and diverge facilities
and to check that weaving sections at or above the desirable minimum length can be provided. If these
cannot be achieved, then the junction strategy should be reviewed.
The next stage is to check that desirable geometric standards can be achieved with the junction
spacing, and any lane gains or drops proposed, and that an effective and economic signing system
can be provided. Again the strategy may have to be adjusted. Figure 7.2 is a flowchart showing the
connector road design process. It refers to the particular paragraphs, figures and tables of this
standard applicable to connector road design and to DN-GEO-03036. Interchanges may be also be
justified and where required shall be designed in accordance with DN-GEO-03041.
If the junction and interchange
designs pass these stages, the scheme can then be taken to the next
stage in its preparation which is likely to be a cost/benefit assessment. Analysis may not be sufficiently
fine to evaluate the performance of individual junction elements. The best means of ensuring that a
junction is effective is to carry out the operational check outlined above and in Figure 7.1. With the
exception of the interchange these junctions have merge and diverge slip roads which, where
absolutely necessary, may be signalised at their junction with the side road or roundabout.
7.3
Layout Options
7.3.1
General
The most efficient form of grade separation is that which presents the driver with the minimum number
of clear unambiguous decision points as they drive through the junction and in merging and diverging.
Additionally, on a Motorway or Type 1 Dual Carriageway national road that is generally grade
separated, consistency of design for successive junctions is an important consideration involving the
adoption of the same Design Speed. This need for consistency also applies to the signing and road
markings to be adopted particularly at the boundary of responsibility between different road
authorities.