TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 68
Figure 6.5: Three-arm Roundabout Illustrating Reverse Curvature
There may be situations where the turning proportions are such that one section of the circulatory
carriageway has a relatively low flow, resulting in an unused area of carriageway, usually adjacent to
a channelising island:
a)
For larger roundabouts, the circulatory carriageway can be reduced in width by
extending the channelising island. This method of reducing circulatory width may
also be adopted as an interim measure in the early years of a scheme. At the
same time, the offside entry lane may be taken out of use, for example, by the use
of coloured or textured surfacing or hatched markings.
b)
For smaller roundabouts, increasing the size of the central island is a more
appropriate method of interim circulatory carriageway reduction, preferably by
physical means but alternatively using coloured surfacing or hatched markings.
Hatching should not be used to reduce the entry width in areas adjacent to pedestrian facilities.
6.6.3
Central Island
The central island should be circular and at least 4 metres in diameter. The inscribed circle diameter,
the width of the circulatory carriageway and the central island diameter are interdependent: once any
two of these are established, the remaining measurement is determined automatically.
It should be noted that the swept path for the Design Vehicle may impinge slightly (by up to 0.3m) into
either the inner or outer 1m clearance allowance. Given the anticipated frequency of this type of
vehicle, this is not considered to be particularly significant and the dimensions in Figure 6.6 should
not be increased accordingly.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 69
In order to ensure that light vehicles encounter sufficient entry deflection at Single Lane Roundabouts,
an overrun area (i.e. a raised low profile area around the central island) may be necessary (Figure
6.6). It should be capable of being mounted by the trailers of HGVs, but be unattractive to cars e.g.
by having a slope and/or a textured surface.
The profile dimensions of the overrun area are shown in Figures 6.7a and 6.7b. It should be noted
that these diagrams are not intended to dictate the actual cross sectional shape of an overrun area,
but only to illustrate the dimensions that must not be exceeded. The overrun area does not need to
incorporate any step like projections.
Figure 6.6: Turning Widths Required for Single Lane Roundabouts
(a) = Main central island
(b) = Central overrun area, where provided
(c) = Remaining circulatory carriageway width =
1.0 - 1.2 x maximum entry width
(d) = Vehicle
(e) = 1m clearance minimum
(f) = Inscribed Circle Diameter
Central Island
Diameter (m)
R1(m)
R2(m)
Minimum ICD (m)
4.0
3.0
13.0
28.0
6.0
4.0
13.4
28.8
8.0
5.0
13.9
29.8
10.0
6.0
14.4
30.8
12.0
7.0
15.0
32.0
14.0
8.0
15.6
33.2
16.0
9.0
16.3
34.6
18.0
10.0
17.0
36.0
In these cases no channelising islands should protrude within the inscribed circle diameter.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 70
Figure 6.7a: Profile of overrun area
Figure 6.7b: Detail of overrun area
It should be noted that these diagrams show an indicative cross sectional shape of an overrun area, developed to illustrate the dimensions
that must not be exceeded.
6.6.4
Channelising Islands
Channelising islands shall be used on each arm, located and shaped so as to separate and direct
traffic entering and leaving the roundabout. They shall be raised and kerbed. Markings may also be
used to extend a channelising island on the approach, the exit or the circulatory carriageway. Kerbed
channelising islands can act as pedestrian or cyclist refuges provided that they are large enough to
give adequate safe standing space for accompanied wheelchair users, and pedestrians with
pushchairs or cyclists. Signs and other street furniture can be sited on kerbed islands provided that
there is sufficient room to maintain the required clearances. The provision of directional signage shall
be considered when designing the size of channelising islands.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 71
6.6.5
Entries
A number of variables need to be considered in selecting an entry design which is safe and has
adequate capacity. These variables are:
a)
approach half width (v);
b)
entry width (e);
c)
entry flaring (F);
d)
entry angle (
ϕ
);
e)
entry kerb radius (r):
f)
entry path radius (EPR).
These are described below and shown in Figures 6.8 to 6.19.
6.6.6
Approach Half Width
The approach half width, v, is the width of the approach carriageway, excluding any hatching, in
advance of any entry flare (see Figure 6.8). It is the shortest distance between the median line, or
the edge of the central reserve on dual carriageway roads, and the nearside edge of the road.
Where there is white edge lining or hatching, the measurement should be taken between markings
rather than kerb to kerb.
6.6.7
Entry Width
The entry width, e, is the width of the carriageway at the point of entry. It is measured from the point
A at the right hand end of the yield line along the normal to the nearside kerb (see Figure 6.8). For
capacity assessment, the measurement should be taken as the total width of the lanes which drivers
are likely to use i.e. the effective width, which is normally between any white edge lining or hatching.
Where the alignment of the entry lanes is as described in Section 6.6.8, the entry width and the
effective entry width are the same.
Entry width is a key factor affecting capacity, in conjunction with length and sharpness of flare. One
or two extra lanes may be added to the approach at a roundabout. However, as a general rule not
more than two lanes should be added and no entry should be more than four lanes wide.
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