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T.P.D.M.V.2.4
Fig. 5 shows the equivalent
construction for roundabouts
with curvtd weaving
sections* A f D f is a line in
the centre and parallel to
the weaving section. BO is
constructed as with Fig. %
and 0 is measured as the
acute angle between BC and
the tangent to A t D r at the
point of intersection
Fig. 6 shows the construction
for short weaving
sections. BC is constructed
as in Figs* % and 5t JK is
the line in the following
exit mid way between the
nearside kerb and the median
and offside edge of any
median Island* OH is the
tangent to JK at the point
where this line intersects
the outer edge of the
circulatory carriageway. BC
and GH intersect at L. 0 is
then defined as 0 s 90 -
(angle GLB)/2. GLB is
measured on the side facing
away from the central
island. If GLB is greater
than or equal to 180* 0 a 0
CENTRAL ISLAND / /
Fig. 5
Entry path radius : is not one of the geometric parameters Included In the
capacity equation, but has a major influence on safety. The following
procedure may be adopted to measure the entry path radius :~
It is assumed that :~
(i)
(il)
The entering vehicle is 2» wide and will be taking the straight
ahead movement at a 4 arm roundabout or across the head of the
Tee at a 3 arn roundabout;
There is no other traffic on the approach and on the circulatory
carriageway;
(ill) The driver will negotiate the site constraints with minimum
deflections and road markings will be Ignored;
(iv) The vehicle is first considered at a point not less than 50m
from the "Give Way* line;