TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 126
7.9
Designing Weaving Sections
The principle of calculating weaving sections is that the length is fixed using Section 7.9.1 and the
width is calculated from the formula outlined later in this Section. This determines the number of lanes
and can indicate the addition of one or two lanes. The formula shows that the minor weaving flow has
an impact on the traffic demand of up to 3 times its numerical value.
An
actual weaving length L
act
less than the desirable minimum must not be entered into the formula.
Weaving lengths for taper layouts must be measured between the end of the merge and start of the
diverge tapers, see Figure 7.11A. For auxiliary lane layouts, the auxiliary lane is ignored and the length
between the end of the notional merge and the start of the notional diverge must be measured as
illustrated in Figure 7.11B. In the case of lane gains and lane drops, the methods set out in Figures
7.12, 7.13 and 7.14 must be used.
In the case of ghost island merges and diverges, the examples in Figure 7.15 show the two points
which must be used for the two connector road lanes to provide the averaged weaving lengths
between junctions. Similar techniques must be applied for diverges.
In the case of wide (5 lane or more) carriageways, there should be no reduction below the desirable
minimum weaving length. A vehicle on a 5-lane carriageway requires at least 1km to cross between
Lanes 5 and 1 safely so as to leave at a diverge and the driver will need advance warning.
For weaving sections on motorways and dual carriageway roads, design flows must be calculated as
per this Chapter. In measuring L
act
it will be necessary to consider whether distance is available to
adequately sign the second junction and allow adequate visibility to the sign from all lanes. To
calculate the number of traffic lanes required for weaving the following equation must be used (see
Figure 7.10):
N =
1
D
(Q
nw
+ Q
w1
+ Q
w2
(2 L
min
/ L
act
+ 1)
)
Where:
N
= Number of traffic lanes
Q
nw
= Total non-weaving flow in vph
Q
w1
= Major weaving flow in vph
Q
w2
= Minor weaving flow in vph
D
= Maximum mainline flow from Section 7.10 in vph per lane
L
min
= Desirable Minimum weaving length for the road
L
act
= Actual weaving length available
TII Publications
DN-GEO-03060
Geometric Design of Junctions (priority junctions, direct accesses, roundabouts, grade
separated and compact grade separated junctions)
April 2017
Page 127
In calculating the number of traffic lanes required a fractional part will inevitably require a decision to
round up or down. If it is possible to vary the position of the junctions and thus increase or decrease
the weaving length, the fractional part will converge approximately to a whole number of lanes and
the decision is simplified. However, if this is not possible the decision becomes more difficult. Where
the fractional part is small and is combined with a low weaving flow rounding down is suggested,
whereas a high fractional part with a high weaving volume suggests rounding up. For example the
addition of a fourth lane would have operational advantages in releasing the two middle lanes for
weaving traffic. Other factors which may influence the decision are:
a)
the number of lanes required for merging and diverging (Sections 7.7 and 7.8);
b)
when the fractional part is about 0.5 the number of lanes shall be rounded up;
c)
on recreational routes there can be a high proportion of drivers who are not local
and therefore behave less efficiently than commuters would
at the same flow
levels;
d)
the consequences of under provision should be borne in mind, as the acquisition of
land at a later date could be costly or impossible;
e)
relevant environmental factors should be taken into account.
Figure 7.10: Terms used in Weaving
7.9.1
Weaving Lengths
Weaving lengths must be measured as shown in Figures 7.11 to 7.15.
For Rural Motorways and Type 1 Dual Carriageway national roads, the Desirable Minimum weaving
length shall be 2 kilometres. However, when the design flow on the main line is not greater than 50%
of the capacity (AADT) for level of service D as given in DN-GEO-03031 for rural motorways, a
weaving length of 1 kilometre can be provided as a Relaxation. The weaving formula must not be
used for weaving lengths above 3 kilometres. The requirements for MSAs on rural motorways are as
for rural motorway junctions.