TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 133
Table 7.6: Connector Road Design Speed
Mainline Design Speed
Rural Motorway
120km/h
Rural Type 1
Dual
Carriageway
120km/h
Rural Type 1
Dual
Carriageway
100km/h
Connector
Road Design
Speed km/h
Interchange Link
85
85
85
Slip Road
70
70
70
Link Road
120 or 100
120 or 100
100 or 85
Dumb-bell Link
Road
70
70
70
Loops
50
50
50
Any transition curves at locations where the design speed changes must be designed to the higher
design speed value.
7.11.4
Horizontal and Vertical Alignment
The geometric standards for horizontal and vertical alignment and stopping sight distance for the
mainline through a grade separated junction and for the connector roads must be provided in
accordance with DN-GEO-03031. The maximum gradient for connection roads shall be 6%.
Low radius connector roads must be widened on curves in accordance with DN-GEO-03031.
7.11.5
Vertical Alignment for Merges and Diverges
Vertical design of merges and diverges (see Figure 7.16) shall provide:
a)
At the start of nose, a constant crossfall shall be maintained across the main
carriageway, the tip of the nose and the slip road carriageway (Section A-A on
Figure 7.16).
b)
At the back of the nose either:
i. a constant crossfall across the main carriageway, the back of the nose and the
slip road carriageway (Section B-B on Figure 7.16) or;
ii. a separate crossfall for the mainline and the slip
road with a single change in
crossfall by a maximum of 3%, located on either edge of the nose or within the
nose (Section B-B on Figure 7.16).
c)
From the back of the nose to the point where both slip road and mainline verge
widths have been fully developed (shown as point V on Figure 7.16), the crossfall
of the un-paved verge shall be a maximum of 5% (Section C-C on Figure 7.16).
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 135
7.12
Loops
In the case of the horizontal curvature and superelevation for loops (as defined in Chapter 1), there is
evidence to suggest that the radii of loops can safely be much less than for curves turning through
lesser angles, provided that adequate warning is given to drivers and clear sight lines are maintained.
For loops the minimum radii may therefore be those given in Table 7.7. Within the loop, successive
radii of the same hand must not reduce in radius. The standards for superelevation for loops are set
out in DN-GEO-03031. Superelevation greater than 7% and up to 10% may be provided as shown in
DN-GEO-03031 but superelevation greater than 7% should be used with caution where there is a risk
of prolonged icy conditions. Where loops leave or join the mainline, crossfall alongside the nose must
be the minimum required for drainage design as laid down in DN-GEO-03031. Widening on loops
must be as set out in Chapter 5 of this document.
Table 7.7:
Minimum Loop Radii
–
(m)
Motorway / Type 1 Dual Carriageway
(120 km/h)
On/Off Mainline
All-Purpose
On to Mainline
Off Mainline
75
40
50
Research into loops carried out from 1985 to 1994 did not reveal any systemic safety problems.
Collision levels at sites surveyed were generally low with approximately a third of the sites having no
personal injury collisions over the study period.
The research looked at whether non-compliance with existing standards gave rise to safety problems
and a variety of non-complying loops were examined. The study examined the following loops which
are shown in Figure 7.17:
Basic merge
A loop that passes through approximately 270
o
where traffic merges with the mainline flow. This Basic Merge,
when combined with the Hook Diverge, forms the layout in Figure 7.20b.
Basic diverge
A loop that passes through approximately 270
o
where traffic diverges from the mainline flow. This Basic
Diverge, when combined with the Hook Merge, forms the layout in Figure 7.20a.
D merge
The loop commences at a T-junction or roundabout and merges with the mainline flow. The angle turned is
typically approximately 180
o
.
D diverge
The loop commences at a diverge from the mainline flow and ends at a T-junction or roundabout. The angle
turned is typically approximately 180
o
.
Hook merge
This layout is classified as a loop and the notional total angle is between 180
o
and 270
o
. This Hook merge,
when combined with the Basic diverge, forms the layout in Figure 7.20a.
Hook diverge
This layout is classified as a loop and the notional total angle is between 180
o
and 270
o
. This Hook diverge,
when combined with the Basic merge, forms the layout in Figure 7.20b.