TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 78
Figure 6.13: Entry Kerb Radius
6.6.13
Entry Path Radius
The entry path radius is a measure of the deflection to the left imposed on vehicles entering a
roundabout. It is the most important determinant of safety at roundabouts because it governs the
speed of vehicles through the junction and whether drivers are likely to yield to circulating vehicles.
To determine the entry path radius, the shortest path allowed by the geometry is drawn. This is the
smoothest, flattest path that a vehicle can take through the entry, round the central island and through
the exit (in the absence of other traffic) (see Figures 6.14 to 6.17). This path represents the actual
vehicle path and must not include instantaneous transitions between circular curves of different radii,
including straights.
The path is assumed to be 2m wide so that the vehicle following it would maintain a distance of at
least one metre between its centreline and any kerb or edge marking. The path starts 50m in advance
of the yield line.
The smallest radius of this path on entry that occurs as it bends to the left before joining the circulatory
carriageway is called the entry path radius. Note that this is different to, and should not be confused
with, the entry kerb radius as described earlier. The entry path radius can be measured by applying
suitable templates to the curve in the vicinity of the yield line (see Figures 6.14 to 6.17). It is the radius
of the best fit circular curve over a length of 25m.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 81
A method for creating entry deflection at a roundabout is to stagger the arms as shown in Figure 6.18.
This will:
a)
reduce the size
of the roundabout;
b)
minimise land acquisition;
c)
help to provide a clear exit route with sufficient width to avoid conflicts.
Sharp curves on the link road design should not be introduced to increase entry deflection, although
a gentle curve to the right preceding left hand entry deflection may be used.
The design of Single Lane Roundabouts is similar to that for Multi-lane Roundabouts, but the single-
lane entries, circulatory carriageway and exits are retained.
6.6.14
Exits
Exit Width
The exit width is the width of the carriageway on the exit and is measured in a similar manner to the
entry width. It is the distance between the nearside kerb and the exit median (or the edge of any
channelising island or central reserve) where it intersects with the outer edge of the circulatory
carriageway. As with entry width, it is measured normal to the nearside kerb. Values are typically
similar to or slightly less than entry widths.
Where the downstream link is a single carriageway road with a long channelising island, the exit width
of the roundabout should be between 7m and 7.5m and the exit should taper down to a minimum of
6m (see Figure 6.19), allowing traffic to pass a broken down vehicle. Where the link is an all-purpose
two-lane dual carriageway, the exit width should be between 10m and 11m and
the exit should taper
down to two lanes wide.
The width should be reduced in such a way as to avoid exiting vehicles encroaching onto the opposing
lane at the end of the channelising island. The width should reduce at a taper of 1:15 to 1:20. Where
the exit is on an up gradient, the exit width may be maintained for a short distance before tapering in.
This helps reduce intermittent congestion caused by slowly accelerating HGVs by giving other drivers
an opportunity to overtake them. If the exit road is on an up gradient combined with an alignment
which bends to the left, it may be necessary to maintain the exit width over a longer distance to help
ensure that overtaking manoeuvres can be completed before the merge is encountered.