TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 152
c)
Vertical Curvature (desirable minimum radius with a relaxation of one design
speed step);
d)
Carriageway Width (three options and a one step relaxation);
e)
Curve Widening (three options are permitted for each radius).
8.2.2
Design Speed
Design speeds for the mainline and the minor road shall be determined from DN-GEO-03031.
As a general principle it is intended that the speed of a vehicle through the compact connector road
shall be limited by its speed through the entry and exit junctions with the mainline and the minor road.
For this reason long straight sections within the compact connector road shall be avoided.
The design standards for compact grade separated junctions relate to a single design speed of
approximately 30km/h. Junctions shall only be designed to this design speed, higher or lower
standards shall not be used. A range of standards would result in driver confusion and uncertainty
and problems of perception of the junction with consequent safety implications.
If speeds markedly in excess of this do occur or are anticipated then speed limits within the junction
may need to be considered.
8.2.3
Compact Connector Road
Compact connector roads shall be designed in accordance with Table 8.1 except for the special
conditions when the stopping sight distance may be relaxed further as outlined in Section 8.2.5. The
stopping sight distances in Table 8.1 are not related to the design speed but are based upon the
requirements to provide adequate stopping sight distance through the compact connector road to
show the shape of the junction clearly to the driver leaving the mainline. The design parameters
provided in this section are for the compact connector road only, junctions either end of the connector
road shall be designed in accordance with the relevant chapter of this standard and DN-GEO-03031.
Table 8.1: Design Standards for Compact Connector Roads
Stopping Sight Distances
Desirable Minimum (m)
70
Relaxation of One Design Speed Step (m)
50
Horizontal Curvature
Desirable Minimum Radius (m) with 5%
Superelevation
40
Relaxation of One Design Speed Step(m) with 5%
Superelevation
32
Vertical Curvature
Desirable Minimum Crest K
3.3
Relaxation of One Design Speed Step Crest K
2.3
Absolute Minimum Sag K with lighting
2.3
Absolute Minimum Sag K without lighting
3.2
8.2.4
Horizontal and Vertical Alignment
Horizontal radii shall comply with Table 8.1. In normal circumstances the Desirable Minimum Radius
should be used, however in difficult circumstances e.g. space constraints or environmental issues
exist, a Relaxation of one design speed step may be used at the discretion of the Designer.
Given the low design speed and the nature of the compact connector road, transition curves are not
required within the compact grade separated junction.