TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 72
Figure 6.8: Approach Half Width and Entry Width
Lane widths at the yield line (measured along the normal to the nearside kerb, as for entry width) must
be not less than 3m or greater than 4.5m, with the 4.5m value appropriate at single lane entries and
values of 3 to 3.5m appropriate at Multi-lane entries.
On a single carriageway approach to a roundabout, the entry width, e, must not exceed 10.5m. On a
dual carriageway approach to a Multi-lane roundabout, the entry width must not exceed 15m.
If flaring is provided, tapered lanes should have a minimum width of 2.5m.
On a single carriageway road, where predicted flows are low and increased lane width is not
operationally necessary, a Single Lane Roundabout with single lane entries shall be used. The entry
may need to be closed to carry out any form of maintenance so the design of traffic management for
maintenance should be discussed at an early stage in the design process with the Maintaining
Organisation.
The development of entry lanes must account for the anticipated turning proportions and possible lane
bias, since drivers often have a tendency to use the nearside lane. The use
of lane bifurcation where
one lane widens into two should maximise use of the entry width. The use of very short offside lanes
is not recommended as they tend to be used infrequently in practice with the result that debris collects
on the road surface and forms a safety hazard, particularly for two-wheeled vehicles.
For road improvement schemes on national roads, forecast design year flows sometime after opening
will be considered. This can result in roundabout entries with too many lanes for initial flows,
subsequently leading to operational problems. A layout based on projected flows will determine the
eventual land requirements for the roundabout, but for the early years of operation it may be necessary
for the designer to consider an interim layout. This approach can result in reduced entry widths and
entry lanes.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 73
6.6.8
Alignment of Entry Lanes
The alignment of entry lanes is critical. The kerb line of the channelising island (or central reserve in
the case of a dual carriageway) should lie on an arc which, when projected forward, meets the central
island tangentially (see Figure 6.9).
Figure 6.9: Arc Projected Forwards from the Channelising Island and Tangential to the Central
Island
6.6.9
Design of Multi-lane Entries
On Multi-lane entries, it is important to ensure that entries are configured in order to avoid the situation
where some lanes exceed capacity and others are underused. On flared entries, the queue from an
overused lane may back up and block access to other lanes.
6.6.10
Entry Flaring
Entry flaring is localised widening at the point of entry. Roundabouts usually have flared entries with
the addition of one or two lanes at the yield line to increase capacity. Single lane entries should be
slightly flared to accommodate HGVs. Even a small increase in entry width may increase capacity.
The average effective flare length, l', is the average length over which the entry widens. It is the length
of the curve CF', shown in Figure 6.10.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 74
Figure 6.10: Average Effective Flare Length
Notes:
AB = e (entry width).
GH = v (approach half width at point G which is the best estimate of the start of the flare).
GD is parallel to AH and distance v from AH (v is measured along a line perpendicular to both AH and GD and, therefore, the length of AD
is only equal to v if AB is perpendicular to the median at A).
CF' is parallel to BG and distance ½ BD from the BG.
To determine the average effective flare length, l':
a)
construct curve GD parallel to the median HA (centre line or edge of central
reserve or channelising island) and
distance v from it;
b)
construct curve CF' parallel to curve BG (the nearside kerb) and at a constant
distance of ½ BD from it, with F' the point where CF' intersects line DG;
c)
the length of curve CF' is the average effective flare length l'.
In cases where the line AB is not perpendicular to the median, the length AD will differ slightly from v.
The total length of the entry widening (BG) will be about twice the average effective flare length.
The capacity of an entry can be improved by increasing the average effective flare length. Similar
levels of capacity can be obtained with a variety of flare lengths and entry widths. A minimum average
effective flare length of 25m in rural areas is desirable, but capacity will be the determining factor.
Effective flare lengths greater than 25m may improve the geometric layout but have little effect in
increasing capacity. If the effective flare length exceeds 100m, the design becomes one of link
widening. Where the design speed is high, entry widening should be developed gradually with no
sudden changes in direction.