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T.P.D.M.V«2*4
4.6.2 Types of Grade Separated Junction
4.6*2.1 There are very many types of grade separated junction. The
choice of a particular type in any situation will depend upon
such factors as the relative status of the intersecting roads,
the through route and turning traffic volumes and the site
constraints. Some of the more common types of junction are
shown on Diagram No. 4.6.2.1 and discussed below. The designer
should however consider variations on these types or totally
different layouts to best suit the particular conditions.
4.6.2.2 The Trumpet Interchange is a common layout at a T or Y junction
and utilises a single bridge structure carrying the minor road
above the major road. It is most suitable where the loop
movement is small and additional structures may be required to
provide a better alignment for this movement where higher flows
are involved.
4.6.2.3 The Diamond Interchange is the simplest type of 4-way junction
consisting of a single bridge and four one way ramps. It is
particularly suitable in the Territory as it can be located
within a relatively narrow land area, requiring little extra
width beyond that required for the major road itself. Linked
traffic signals may be required at the junctions between the
slip roads and the minor road.
4.6.2.4 The Roundabout Interchange requires two bridge structures and
requires more land than the Diamond but has higher free flow
capacity. It is hence more suitable when minor road/ramp
conflicts are higher.
4.6.2.5 The Elevated Major Route includes those junctions where the
major route is carried on bridge structure over the minor route*
A basic principle in the design of grade separated interchanges
is that the minor route should be carried above the major route.
This is normally the more economical arrangement and has the
distinct advantage of providing accelerating traffic with a down
gradient and decelerating traffic with an up gradient. In the
existing urban areas of the Territory however it has frequently
been necessary to provide new major routes above the existing
road network. This has lead to numerous junctions with an
elevated major route. The junction between the ramps and the
minor road may be controlled by means of a roundabout or by
signals.
4.6.2.6 The Cloverleaf Interchange is the only single structure 4~way
interchange having no terminal right turns at-grade. Internationally
it is perhaps the most common form of interchange,
but its excessive use of land does not recommend it for adoption
in the Territory. The other major drawback with this type of
layout is the short weaving length between entry and exit ramps.