TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 1
1.
Introduction
1.1
General
This Standard sets out the standards and advice for the geometric design of junctions. The design
principles and geometric parameters which shall be considered by Designers when developing safe,
traffic efficient junction layouts and vehicular accesses onto new and improved National roads are
outlined.
This standard does not cover the design requirements and methodology for the geometric layout of
major interchanges (including the expansion and improvement of existing interchanges and junctions).
This is set out in DN-GEO-03041: The Design of Major Interchanges.
1.2
Scope
This standard sets out the design requirements and methodology which shall be followed when
developing junction and access layouts for Motorways and all-purpose national roads, both dual and
single carriageways, in rural areas, taking into consideration traffic operation, physical elements,
economic and safety factors and the requirements of non-motorised users (NMUs). This standard also
provides guidance on junction and access layouts on regional and local roads which form part of a
national road scheme.
This standard provides recommendations on the siting of junctions with respect to topography and
driver expectations and limitations.
This standard shall not be used for the design of road markings and signage at junctions. Road
markings and signage for junctions on national roads shall be designed in accordance with the
requirements of the Traffic Signs Manual (TSM) issued by the Department of Transport.
1.3
Implementation
This Standard shall be used for the design of junction layouts on all new or improved national roads.
The design of national, regional and local roads which are constructed or improved as part of a
national road scheme shall also be developed in accordance with this standard.
1.4
Relaxations and Departures
The standards contained in this document represent the maximum/minimum levels of provision whose
incorporation in the junction design would achieve a desirable level of performance in average
conditions in terms of traffic safety, operation, economic and environmental effects and sustainability.
In most cases, with care, designs can be achieved which do not utilise the lowest levels of design
parameters given. At some locations on new roads or major improvements, however, it may not be
possible to justify even the lowest levels of design parameters in economic or environmental terms,
due to high costs, low traffic levels, and environmental damage, etc. In such cases, sufficient
advantages might justify either a Relaxation within the standards or, in more constrained locations, a
Departure from the standards. Relaxations and Departures should be assessed in terms of their
effects on the economic worth of the scheme, the environment, and the safety of the road user. Further
details on the use of Relaxations and Departures are as follows.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 2
1.4.1
Relaxations within Standard
In difficult circumstances, the Designer may relax a standard set out in this document, where
specifically provided for within the text (refer to DN-GEO-03031 for all combinations of allowable
relaxations from standard). The Designer shall record the fact that a Relaxation has been used in the
design and the corresponding reasons for its use. The record shall be endorsed by the Designer
responsible for the scheme. The Designer shall report all Relaxations incorporated into the design as
part of the project report at the end of each project management phase (refer to the national roads
Project Management Guidelines).
1.4.2
Departures from Standards
In exceptional situations, Transport Infrastructure Ireland (TII) may be prepared to agree to a
Departure from Standards where the standard, including permitted Relaxations, is not realistically
achievable. Designers faced by such situations and wishing to consider pursuing this course shall
discuss any such option at an early stage in design with TII. Proposals to adopt Departures from
Standard must be submitted by the Designer to TII and formal approval received BEFORE
incorporation into a design layout. Further information can be found in GE-GEN-01005 Departures
from Standards and Specification.
1.5
Definitions
The terminology used in this Standard are defined as follows:
a)
Auxiliary Lane: An additional lane at the side of the mainline carriageway to
provide increased merge or diverge opportunity or additional space for weaving
traffic.
b)
Central reserve: The area which separates the carriageways of a dual
carriageway or Motorway. Note that this includes any offside hard strips.
c)
Channelising Island: A raised kerbed island constructed (a) within the bellmouth
of a priority junction to guide traffic movements into and out of the minor road (b) at
a roundabout junction entry/exit to direct traffic onto
or from a roundabout
circulatory carriageway or (c) at a segregated left turn lane, located between an
entry and exit on the same roundabout arm and shaped
so as to direct and also
separate opposing traffic movements onto and from a roundabout circulatory
carriageway.
d)
Compact Connector Road: In the context of compact grade separated junctions a
two way connector road between the major and minor roads.
e)
Compact Grade Separated Junction: A grade separated junction designed in
accordance with Chapter 8 of this standard.
f)
Connector Road: A collective term for slip roads, link roads, interchange
links and
loop roads.
g)
Cycle Facilities: Refers to all types of measures which improve conditions for
cyclists and include:
i.
Cycleways: a public road or proposed public road reserved for the
exclusive use
of cyclists or cyclists and pedestrians.
ii.
Cycle Track: Part of a road, including part of a footway or part of a roadway, which
is reserved for the use of pedal cycles and from which all mechanically propelled
vehicles, other than mechanically propelled wheelchairs, are
prohibited from
entering except for the purpose of access.