TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 29
c)
smooth the flow at junctions with
irregular geometry;
d)
improve safety;
e)
improve junction efficiency;
f)
increase clarity to road users of the junction layout.
4.4.1
Existing Priority Junctions
At existing rural priority junctions, the cost of upgrading a simple junction to provide a right turning
facility will vary from site to site. However, upgrading should always be considered where the right
turning flow into the minor road exceeds 120 vehicles per day, a right turning collision problem is
evident, or where vehicles waiting on the major road to turn right inhibit the through flow and create a
hazard.
For existing roads where the flow levels are not great enough to justify the provision of a ghost island
or roundabout and a right turning problem remains, consideration may be given to the use of other
low cost measures such as a nearside passing bay where the road width allows through vehicles to
pass drivers waiting in the centre of the major road to turn right, albeit at a reduced speed. Where
there is not enough road width for a passing bay, then warning arrow markings may be used, with two
directional arrows on one stalk. Such measures shall require a Departure from Standard.
4.4.2
Existing Grade Separated Junctions
At existing grade separate junctions where significant delay is incurred during peak periods as a
result of high exit or entry flows, it may be appropriate to convert the layouts to
non-standard layouts
such as a ghost island diverge layout based on the results of traffic modelling. Use of such layouts
require a Departure from Standards.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 30
Figure 4.3: Junction Analysis Procedure Form
Junction Analysis Procedure
Date:
Project:
Location:
Data Collection
Classification and
Name
AADT
Design
Speed
(Km/h)
Posted
Speed
(Km/h)
Current Year
(______)
Design Year
(______)
Major road
Intersecting Road
Junction Type
New
Existing
Site Visit
Yes: ____
No: ____
Date of Site Visit (if applicable): _______________
Functional Characteristics
Part 1 (General Information for all Intersections)
Collision Analysis
Access Requirements (Including
NMU Requirements and Level of
Usage)
Access Control
Future Development
Vehicle Design Type (Include any
Special Design Vehicle Details)
% HGV’s
Part 2 (Specific Information for More Detailed Analysis)
Junction Layout & Turning
Movement Diagram
Include Layout with all approaches clearly labelled.
Direction of flow and flow ratios to be clearly identified.
Worked example included in Appendix D to this document.
Notable Constraints due to
Upstream/Downstream Junction
Yes: ____
No: ____
Notes:
Proposed Improvements to Other
Roads (that would impact the
traffic movement at this location)
Geometric Characteristics
Road Geometry on all approaches
To be attached in an appendix to this document.
Is design compliant with the
standards
Yes: ____
Desirable Stopping Sight Distances
Achievable Stopping Sight
Distances
Mainline Horizontal Curvature
Profile gradient on mainline
_____________________%
Intersecting Roadway
_________________%
Other Characteristics
Traffic
Management Measures
Impact on Utilities
Impact on Right of Way
Recommendation of Type of
Junction Treatment based on
Functional, Geometric and Other
Characteristics
Scope of Modelling Required
Local Junction Modelling
Micro-simulation Modelling
Designer: ____________________ Date:________________
Approved:____________________ Date:________________
Worked example included in Appendix D.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 31
5.
Geometric Design of Priority Junctions &
Vehicular Access to National roads
5.1
General
This Chapter describes the geometric design of priority junctions and direct accesses for application
to new and improved junctions on national roads both dual and single carriageway roads.
5.2
Siting of priority junctions and direct access to national roads
5.2.1
General
A saving in collisions may be achieved, and an improvement made in operational performance, by
reducing the number of lightly trafficked minor road connections onto major roads. The cost
effectiveness of connecting such routes together with a link road before they join a new major road
should always be investigated.
5.2.2
Horizontal Alignment
Ideally, priority junctions and direct accesses onto national roads shall not be sited where the major
road is on a sharp curve (below Desirable Minimum R in accordance with DN-GEO-03031). However,
where the siting of a priority junction or access on a curve is unavoidable, T-junctions should be sited
with the minor road on the outside of the curve and desirable minimum Stopping Sight Distance (SSD)
in accordance with DN-GEO-03031 can be achieved for a vehicle on the major road approaching the
junction.
Figure 5.1: Junction located on the inside of a sharp curve
New or altered direct accesses or priority junctions shall not be sited at any location where the
desirable minimum SSD envelope of the national road falls outside the paved surface of the road. The
paved surface includes hard strips if present. It also includes the hard shoulder at accesses where the
stop line is positioned at the back of the hard shoulder.
Direct accesses shall also not be sited within overtaking sections of new single carriageway roads as
defined in DN-GEO-03031.