TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 75
The sharpness of flare, S, is defined by the relationship:
S = 1.6 [e-v] / l'
It is a measure of the rate at which extra width is developed in the entry flare. The value of S depends
on the available land-take and the capacity required. Values of S greater than unity correspond to
sharp flares and smaller values (0 ≤ S ≤ 1) to gradual flares. Long gradual flares are most efficient as
they make better use of the extra width but sharp flares have a smaller potential of land take. Sharp
flares can still give significant increases in capacity and may be appropriate where there is cyclist /
pedestrian crossing demand.
The entry width and the average effective flare length are related. The capacity of a wide entry
combined with a short flare can be similar to that of a narrow entry combined with a long flare. There
are many intermediate combinations of e and l' that will have the same capacity.
Although entry width and sharpness of flare (which is a function of flare length and widening) have the
largest effect on capacity, other variables such as entry angle and entry radius are also important.
When capacity is at a premium, small changes in these variables can sometimes provide a bigger
increase in capacity than making a large change in a single variable.
6.6.11
Entry Angle
The entry angle,
ϕ
, serves as a geometric proxy for the conflict angle between entering and circulating
traffic streams. There are two different methods for its measurement, depending on the size of the
roundabout.
For a large roundabout where the arms are well separated, the angle measured is in effect that
between the projected path of an entering vehicle and the path of a circulating vehicle (see Figure
6.11). To determine the entry angle:
a)
construct the curve EF as the locus of the mid- point between the nearside kerb
and the median line (or the edge of any channelising island or central reserve);
b)
construct BC as the tangent to
EF at the yield line;
c)
construct the curve AD as the locus of the mid- point of (the used section of) the
circulatory carriageway (a proxy for the average direction of travel for traffic
circulating past the arm);
d)
the entry angle,
ϕ
, is the acute angle between BC and the tangent to AD.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 76
Figure 6.11: Entry Angle at a Larger Roundabout
For Single Lane Roundabouts, the entry angle is measured as shown in Figure 6.12. This construction
is also used when there is insufficient separation between entry and adjacent exit to be able to define
the path of the circulating vehicle clearly. In this case, circulating traffic which leaves at the following
exit will be influenced by the angle at which that arm joins the roundabout. The angle between the
projected entry and exit paths is measured and then halved to find
ϕ
:
a)
construct line BC as in Figure 6.11;
b)
construct the curve JK in the next exit as the locus of points midway between the
nearside kerb and the median line (or the edge of any channelising island or
central reserve);
c)
construct the line GH as the equivalent of line BC i.e. the tangent to the curve JK
at the point where JK intersects the border of the inscribed circle;
d)
the lines BC and GH intersect at L. The entry angle,
ϕ
, is half of angle HLB.
ϕ
= [angle HLB]/2
Note that if angle GLB exceeds 180 degrees,
ϕ
is defined as zero.
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 77
Figure 6.12: Entry Angle at a Smaller Roundabout
If it is not clear which of the two methods should be used, the following procedure should be
implemented. All three vehicle paths (entry, exit and circulatory carriageway medians) should be
constructed, and the entry and exit paths projected towards the roundabout centre. The choice of
construction for
ϕ
depends on where these projections meet: if the meeting point is closer to the centre
of the roundabout than the arc of the circulatory carriageway median, then the construction shown in
Figure 6.11 should be used; if they meet outside that area, then the construction illustrated in Figure
6.12 should be used. In the limiting case where all three medians intersect at a point, the circulatory
carriageway median approximately bisects the angle between the other two medians, so that the two
methods become equivalent.
The entry angle,
ϕ
, shall lie between 20 and 60 degrees. Low entry angles force drivers to look over
their shoulders or use their mirrors to merge with circulating traffic. Large entry angles tend to have
lower capacity and may produce excessive entry deflection which can lead to sharp braking at entries,
accompanied by shunt collisions, especially when approach speeds are high.
6.6.12
Entry Kerb Radius
The entry kerb radius, r, is the radius of curvature of the nearside kerb line over the distance from 25m
ahead of the yield line to 10m downstream of it (see Figure 6.13). It is the radius of the best fit circular
curve over a length of 25m.
The entry kerb radius should be not less than 10m. (Except at Single Lane Roundabouts), If the
approach is intended for regular use by HGVs, the value should be not less than 20m. However, entry
kerb radii of 100m or more will tend to result in inadequate entry deflection and should not be used.
Although entry capacity can be increased by increasing the entry kerb radius, once its value reaches
20m, further increases only result in very small capacity improvements. Reducing the entry kerb radius
below 15m reduces capacity.