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T*.D.M.V.2.4
***3.7 General Layout Requirements
4.3*7.1 The layout should be designed to follow the natural vehicular
paths. Unduly sharp radii or complex paths involving several
changes of direction must be avoided. The general aim is to
achieve a layout which is easily understood by motorists.
4.3-7.2 To achieve this objective, islands, traffic signs and road
markings should be specifically designed to define the paths to
be taken* Cutting, merging and diverging movements can usefully
be separated by physical or painted islands. Numerous small
traffic islands should however be avoided as they are
ineffective and confusing,
^•3*7.3
Allowance should be made for the swept turning paths of long
goods vehicles where they can be reasonably expected to use a
junction* Consideration should also be given to the manoeuvring
characteristics of these vehicles in the design of staggered
junctions.
4.3*7.4 Specific aspects of the various geometric parameters which
combine to produce a satisfactory layout are covered in the
following sections. It should be remembered that the geometric
standards suggested are ideals to be aimed at but should not be
so rigidly applied that a junction becomes out of scale with its
surroundings, environmentally damaging or exorbitantly
expensive* Several of the standards are related to design
speed. Where junction design refers to a new junction on an
existing road or improvement to an existing junction the
measured 85 percentile speed should be used, rounded up to the
next highest design speed step. Where the design is concerned
with a new road, the design speed of the road should be adopted.
Suffixes A and B attached to design speeds are defined in
Chapter 3, paragraph 3.3*2.3 as :
Design Speed A represents a high standard alignment with only
occasional low radius curves and Design Speed B represents a
heavily constrained alignment where low radius curves have been
frequently adopted because of difficult topography or dense
development.