TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 5
jj)
Rural Road: A road outside of built-up areas including:
i. Single Carriageway roads;
ii. All-purpose Dual Carriageway roads; or
iii. Motorways.
kk)
Single Carriageway: Two-lane single carriageway road with lane widths of up to
3.65m.
ll)
Skew or Y-Junction: An at-grade junction of two roads, at which
the minor road
approaches the major road at an oblique angle and terminates at the junction.
mm)
Slip Road: A connector road within a junction between a mainline carriageway and
the local road network, or vice versa, which meets the local road network at-grade.
Traffic using a slip road usually has to yield to traffic already on the mainline or on
the local road network.
nn)
Taper Merge/Diverge: A layout where merging or diverging traffic joins or leaves
the mainline carriageway through an area forming a funnel to or flare from the
mainline carriageway.
oo)
Type 1 Dual Carriageway: A divided all-purpose road with a minimum of two
lanes and hard shoulder in each direction constructed to the geometric standards
of DN-GEO-03031 and CC-SCD-00006.
pp)
Type 1 Single Carriageway: An all-purpose road with a 3.65m lane in each
direction constructed to the geometric standards of DN-GEO-03031 and CC-SCD-
00001.
qq)
Type 2 Dual Carriageway: A divided all-purpose road with two lanes and hard
strip in each direction constructed to the geometric standards of DN-GEO-
03031and CC-SCD-00005.
rr)
Type 2 Single Carriageway: An all-purpose road with a 3.50m lane in each
direction constructed to the geometric standards of DN-GEO-03031 and CC-SCD-
00002.
ss)
Type 3 Dual Carriageway: A divided all-purpose road with two lanes in one
direction of travel and one lane in the other direction, constructed to the geometric
standards of DN-GEO-03031 and Standard Construction Details CC-SCD-00004.
The two-lane section alternates with a one-lane section at intervals of 2km
approximately.
tt)
Type 3 Single Carriageway: An all-purpose road with a 3.00m lane in each
direction constructed to the geometric standards of DN-GEO-03031 and CC-SCD-
00003.
uu)
Upstream: That part of the carriageway(s) where traffic is flowing towards the
section in question.
vv)
Urban Relief Road: An urban road where the primary purpose
of the road is to
facilitate the movement of traffic and avoid congestion or other obstacles to
movement.
ww)
Urban Street: A road within a built-up area where the primary purpose of the road
is to provide direct access to premises.
xx)
Weaving Section: The length of the carriageway between a successive merge or
lane gain and diverge or lane drop, where vehicles leaving the mainline at the
diverge or lane drop have to cross the paths of vehicles that have joined the
mainline at the merge or lane gain.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 7
Table 2.1: Advantages and disadvantages by junction type
Junction Type
Advantages
Disadvantages/ Limitations
Priority Junction
Through traffic on the major road is not
delayed.
Land take and construction cost reduced
relative to more complex junction
layouts.
All turning
movements have potential to
create conflict.
Right turn movements onto and off the
major road can lead to serious collisions.
Not suitable for high flows and turning
movements.
Major road through traffic speeds need to
be controlled.
Roundabout
Simplifies conflicts and provides a clear
indication of priority.
Facilitates right turning flows and U-
turns.
Can facilitate a change in road
standard/cross section.
Major road traffic must yield to traffic from
the right which may cause delays.
Dominant flows on
one approach may
lead to excessive delays on other
approaches.
Grade
Separated
Junctions
Turning vehicles are removed from the
major road.
Major road through traffic is not delayed.
Can facilitate large turning flows.
Land take and construction costs high
relative to less complex junction layouts.
Not suitable for single carriageway roads,
Type 2 and Type 3 Dual Carriageways.
Compact Grade
Separated
Junction
Minimises land take.
Design enforces low traffic speeds
through the junction.
Suitable for
use where high major road
and minor road through traffic.
Can be used on single carriageway
roads and Type 2 and Type 3 Dual
Carriageways.
Not suitable for Motorways
or Type 1 Dual
Carriageways.