TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 62
6.
Geometric Design of Roundabouts
6.1
General
This Chapter describes the geometric design of roundabouts for application to new and improved
junctions on national roads.
The cut-off point between roundabout and link design shall be 50m measured from the yield line which
corresponds to the point at which the entry path radius assessment commences. Geometric design in
accordance with this chapter shall be undertaken from this point. However, full SSD in accordance
with DN-GEO-03031 shall be provided to the yield line on the approach roads to the roundabout.
Figure 6.1: Cut-Off Point between Link Design and Roundabout Design
6.2
Siting
A project appraisal should be carried out in accordance with TII current practices when considering
the provision of roundabouts.
Where several roundabouts are to be installed on the same route, they should
be of similar design in
the interests of route consistency and safety to the extent that this is possible with the traffic volumes
concerned.
In addition to its natural function as a junction, a roundabout may usefully:
a)
facilitate a significant change in
road standard, for example, from dual to single
carriageway or from grade separated junction road to at-grade junction road;
b)
facilitate U-turns;
c)
facilitate right turning flows.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 63
The majority of collisions at priority junctions are associated with right turns. The inconvenience of
banning right turns and using left in left out junctions can be mitigated by providing a roundabout
nearby.
For Type 3 Dual Carriageways, roundabouts will be appropriate as major junctions. Two-lane sections
shall start directly at the exit from the roundabout. Similarly, a two-lane section may terminate at a
roundabout with the overtaking lane becoming the right-hand entry lane into the roundabout.
On single carriageway roads where overtaking opportunity is limited, roundabouts may be sited so as
to optimise the length of straight overtaking sections along the route (see DN-GEO-03031).
Where a proposed roundabout may affect the operation of an adjacent junction, or vice versa, the
interactive effects should be examined. Where appropriate, traffic management measures such as
prohibited turns or one-way traffic orders at the adjacent junction may be considered. The effects of
queuing at the roundabout should be examined to check that additional risk is not generated.
Where it is proposed to add an arm to an existing roundabout, the effects shall be examined. If the
proposed arm will adversely affect the roundabouts operation and safety, measures shall be
considered to mitigate and minimise these effects. Adverse effects include the introduction of unequal
flow distribution, roundabout capacity problems and increased risk of collision of vehicular conflicts.
6.3
Landscaping within a Roundabout
Passively safe landscaping is allowable within a national road roundabout and
shall only be provided
with TII approval.
Non-passively safe landscaping and any artwork features constructed on a national road Roundabout
shall not be provided in the direct line of a potential errant vehicle that overruns the roundabout. Figure
6.2 illustrates this principal for a typical single lane roundabout; non-passively safe landscaping or
artwork must not be positioned outside the green shaded area within the roundabout. Designers must
assess the potential path an errant vehicle is most likely to take based on the specific design
parameters of the roundabout. Sight lines shall also be considered when landscaping and artwork
features are being designed.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 64
Figure 6.2: Permitted location for non-passively safe landscaping and artwork within a single lane
roundabout
6.4
Selection of Roundabout Type
The choice of roundabout type is governed by a combination of factors including:
a)
whether the approach roads are single or dual carriageway (or grade separated);
b)
the speed limit on the approach roads;
c)
the level of traffic flow;
d)
other constraints such as land-take.