TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 13
2.3.2
Multi-lane Roundabouts
Multi-lane Roundabouts (Figure 2.7) require designers to achieve multi-lane entries and exits on each
arm and allow two cars to travel on the circulatory carriageway.
Figure 2.7: Multi-lane Roundabout
NOTES:
Lane markings on circulatory carriageway may or may not be required depending on roundabout size
Road markings to be designed in accordance with the TSM
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 14
2.4
Grade Separated Junctions
A grade separated junction is a separation of the levels at which major and minor roads cross each
other in order to reduce traffic conflicts. A grade separated junction involves the use of an at-grade
junction at the commencement or termination of slip roads.
e)
There are two main types of grade separated junction: Dumb-bell roundabout and
Half-Cloverleaf. These junction types are mandatory on Motorways and Type 1
Dual Carriageways and are described in the following sections. Grade
Separated
Junction types are described in further detail in Chapter 7.
2.4.1
Dumb-Bell Roundabout
The dumb-bell roundabout junction comprises two roundabouts situated either side of the major road.
The minor road crosses over or under the major road at one location using a bridge
–
see Figure 2.8.
A Departure from Standards shall be required for a half diamond dumb-bell junction where access
onto and off of the major road is provided in one direction only.
Figure 2.
8: “Dumb
-
bell” Grade Separated Junctions
2.4.2
Half-Cloverleaf
A half-cloverleaf is used where site conditions are such that a dumb-bell roundabout junction cannot
be provided and the use of all four quadrants is not possible for the provision of a full interchange in
accordance with DN-GEO-03041 (see Figure 2.9). The at-grade junction element should normally
utilise two roundabouts. The half-cloverleaf requires the use of only 2 quadrants, which if possible
should be chosen so as to minimise any right turn movements where priority junctions are provided.
Consideration should be given to future improvement when considering this layout.