TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 108
7.3.2
Siting
The siting of a grade separated junction on a hill top should be avoided if possible as approach
gradients can cause operational problems in the diverge area, even when the percentage of HGVs
is small. Hill top locations could be environmentally damaging to the skyline and might present difficulty
to drivers in comprehending road signs which are silhouetted against the sky. There is also the risk of
drivers being blinded when the sun is low in the sky.
7.3.3
Recommended Layouts for Grade Separated Junctions
Recommended layouts for consideration in order of increasing traffic flow level are:
a)
dumb-bell roundabout
–
junctions with the minor road
are provided by two
roundabouts which are connected by a central link road either under or over the
mainline;
b)
half clover-leaf
–
use of two quadrants and roundabout junctions with the minor
road;
c)
interchange
–
a junction between major roads with
all movements catered for by
free flowing connector roads (Refer to DN-GEO-03041).
7.3.4
Dumb-Bell Roundabout
The dumb-bell roundabout layout has the advantages of reduced cost (only one bridge required) and
less land take than other grade separated junctions.
For the dumb-bell layout, it is possible that the distance between the two roundabouts may be less
than the desirable minimum SSD for the design speed of the connecting link road. In that case, a low
(0.26m) object at the yield line of the next roundabout must be visible from a vehicle as it leaves the
circulatory carriageway of the previous roundabout. Attention must be given to the needs of future
maintenance of the connecting link road to avoid the need for closure of the road. One lane dual
carriageways should, therefore, be avoided and single carriageways would often be preferable.
7.3.5
Half Cloverleaf
A half-cloverleaf is used where site conditions are difficult and the use of all four quadrants is not
possible (see Figures in Chapter 2). A roundabout junction shall be provided at the junctions with the
minor road.
7.3.6
Variants
Variants on the two basic types of grade separated junctions (half cloverleaf and dumb-bell
roundabout) can be provided if:
a)
the junction is 3 way i.e. a T junction;
b)
not all movements need catering for
c)
traffic signals, either full-time
or part time, are included to remove congestion on an
existing grade separated junction. It is recommended that they should only
normally be considered as an alternative
to a priority junction;
7.3.7
Provision for Non-Motorised Users
Generally, non-motorised road users do not need to be catered for along the mainline of a grade
separated interchange. However, should NMU facilities be required to cater for a particular demand,
the NMU’s are to be directed to the at grade junction and away from both
the merge and the diverge
lanes serving the grade separated interchange from the mainline.