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T.P.D.M.V.2.4
of the two values is the minimum acceptable length of weaving
section (Lmin).
4.6*10.5 The actual weaving length, particularly in connection with a
weaving section between rather than within interchanges, will
often be greater than the minimum weaving length* The width of
weaving section is hence defined as a function of both the
minimum and actual weaving lengths together with the predicted
and design flows. The number of lanes required in the weaving
section (N) is given by the equation
U - Qnw * Qwt ^ ( 2 x Lpin + ^ Qw2
D Lact D
where Qnw = total non weaving flow (adjusted) v.p.h*
Qw1 s major weaving flow (adjusted) v.p.h.
Qw2 s minor weaving flow (adjusted) v.p.h.
D s design flow per lane in v.p.h* as described in
para. 4.6.10*4
Lmin = minimum weaving length (m)
Lact * actual weaving length (m)
4.6.10.6 The number of lanes within the weaving section as calculated
above will invariably involve a fraction and a decision must be
taken on whether to round up or down. In some cases it may be
possible to vary the actual weaving length in order to bring N
nearer to a whole number and hence simplify the decision* Wiere
the available weaving length is fixed by outside constraints, as
is normally the case, the decision to round up or down becomes
more difficult. Obviously if the fractional part is small with
relatively low weaving volumes and a low design flow, it is not
unreasonable to round down. If the opposite is true, N would be
rounded up. Between these two extremes it is necessary to
exercise judgement taking account of such factors as cost and
availability of lane for an extra lane.
4*6*10.7 It is also important to bear in mind that it is not sufficient
merely to provide the proper number of lanes; the physical
arrangement of the lanes within the weaving section must also be
considered. Care must be taken that use of the lanes by weaving
and non weaving traffic will be in proportion to their relative
volumes so that parts of the roadway do not become underused
while other parts become congested. The possibility of such an
imbalance of flows occurring may well influence the decision on
whether to round up or down, For example an outer lane added to
a weaving section will generally aid only an outer non-weaving
flow (particularly where 3 or more lanes are already proposed}*
This is likely to be of little use if most of the traffic is
weaving and the relevant outer non-weaving flow is small.
4.6*10.8 Finally it is necessary to ensure that the number of lanes in
the weaving section satisfies the requirements of the merging
and diverging sections of the inter change (s) as discussed
earlier.