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T.P.D.M.V.2.4
4.6.8.7 The method of deciding on the appropriate merging lane layout,
which will provide the required capacity to cater for the design
flows, is described in the following paragraphs. It should be
remembered however that the layout thus recommended may need
adjusting through outside influences such as a policy decision
to provide the same number of lanes throughout the whole main
line route, for example.
4.6.8.8 Design year flows in vehicles per hour should be obtained for
the main line and the entry link* The worst combination of
these flows, allowing for possible different timing of peaks, is
then used as a base. The base flows are then adjusted for nonstandard
combinations of traffic composition and gradient as
indicated in Table 4.6.8.1. The gradient should be measured
over a one kilometre section of the main line, centred on the
merge nose.
Table 4.6.8.1
Percentage Corrections to Predicted Flow for
Non-standard Composition and Gradient
Gradient on Main Line
Percentage of Heavy Vehicles on Link
or Main Line - Being Considered
5JJ)1520253£35.4£
Downhill, Level or 1$ Up - 8 - 4 0 4-4 4-8 +12 +16 +20
155-2$ Uphill +2 +6 +10 +14 +18 +22 +26 +30
2% - 3% Uphill +12 +16 +20 +24 +28 +32 +36 +40
4.6.8*9 The design year flows in v.p.h., adjusted as above, are then
plotted on Diagram No. 4.6.8.5 to identify a flow region, which
is indicated alphabetically.
4.6.8.10 Having identified the flow region to which the adjusted design
year flows correspond, Table 4.6.8.2 should be consulted to find
out which merging layout types are suitable to provide the
required capacity. The layout types most likely to be
appropriate are indicated with an asterisk. Where flow
combinations are close to boundaries between different flow
regions, the layout types indicated for the adjacent flow region
should also be considered.