TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 65
6.5
Provision for Non-Motorised Users (NMUs)
Provision must be made for the specific requirements of NMUs in the design of roundabouts.
Roundabouts are one of the safest forms of at grade junctions for general motorised traffic, however
they pose safety concerns for cyclists due to high vehicular speeds, particularly leaving the
roundabout.
The preferred crossing facility at a roundabout is a bend out crossing located between 10 and 15
metres from the circulatory carriageway of the roundabout. Where the cycle facility crosses roads with
flows of greater than 4,000 AADT, the crossing should include a central island. The central island
should be a minimum of 3.5 metres in width, however it is possible to reduce the width of the central
island to 2.5 metres should the cycle facility not form part of a greenway and where the number of
cyclists using tandem bikes, recumbents or trailers will be very low.
Figure 6.3 demonstrates the design requirement associated with the provision of off
–
road cycle
facilities at a roundabout. The indicative layout shown is for a two-way cycle facility provided on one
side of the road along the major road.
Figure 6.3: Roundabout Junction
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 67
The inscribed circle diameter of a Multi-lane Roundabout catering for single carriageway and dual
carriageway approaches shall not exceed 100m.
The provision of a five or more arm roundabout is not recommended on national roads and if provided
must be applied for as a Departure from Standard.
Where consideration is being given to adding a fifth-arm to a four-arm roundabout it must be shown
that the introduction of the fifth arm;
a)
shall not have a negative impact on the operation of the roundabout as a whole;
b)
shall not reduce the ultimate capacity of the roundabout;
c)
shall be designed to ensure that vehicular flows are
reasonably balanced between
the arms;
d)
shall not result in high circulatory speeds on the roundabout itself;
e)
shall not result in such close spacing of arms that there can be confusion about a
driver’s intended exit.
Traffic modelling shall be undertak
en by the designer to demonstrate this using TII’s Project Appraisal
Guidelines.
6.6.2
Circulatory Carriageway
The circulatory carriageway of roundabouts shall generally be circular and of constant width. However,
at complex roundabouts, for example where spiral markings are used, the width should be in line with
traffic demand. Tight bends should be avoided as they can increase the likelihood of load shedding
by HGVs. They can also cause
loss of control collisions, particularly for powered two wheelers.
The width of the circulatory carriageway as represented by c in Figure 6.6 must be between 1.0 and
1.2 times the maximum entry width, excluding any overrun area.
At Multi-lane Roundabouts, the width of the circulatory carriageway should not exceed 15m. At Single
Lane Roundabouts, it should not exceed 6m, although an additional overrun area may be required for
small values of inscribed circle diameter, depending on the types of vehicles using the roundabout
(see Figure 6.6).
Short lengths of reverse curve of the inscribed circle, where two consecutive tangential circular arcs
curve in opposite directions, should be avoided between entry and adjacent exits. This can be
achieved by linking the curves with a short straight section. Reducing the size of the inscribed circle
diameter can also eliminate the problem. Where there is a considerable distance between the entry
and the next exit, such as at three-arm roundabouts, reverse curvature is acceptable (see Figure 6.5).