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T.P.D.M.V.2.4
4.2.7 Economic Considerations
4.2.7.1 Ignoring decisions which are made for political expediency, the type and scale of junction
provided is ultimately governed by economic considerations. The cost of provision of a
junction in terms of construction costs and land costs etc. can be weighed against the
monetary benefits which accrue from reduced delay and savings in accidents etc. and the
scheme producing the best value of money is the most appropriate scheme.
4.2.7.2 Priority junctions are generally the cheapest in terms of construction cost and are therefore
the most viable at low flows where delays and accident rates are low. At the top end of the
flow range where substantial widening and channelization are required, the additional land
take may prove uneconomical when compared to a more compact traffic signal design.
4.2.7.3 Traffic signals can be relatively economical in the use of land which will normally more
than offset the cost of equipment. High capacity can be achieved through multi lane
approaches, developed over a relatively short distance, thereby minimising the junction area.
4.2.7.4 Conventional roundabouts are always more extravagant in their use of land than traffic
signals with equivalent capacity. Opportunity costs for the alternative land uses will
therefore constitute a major factor. On the other hand small roundabouts do not use large
amounts of land, sometimes less than both signals and priority junctions with large scale
channelization.
4.2.7.5 Grade separated interchanges are far and away the most expensive in terms of construction
cost irrespective of location. However in the Territory, where construction costs are
sometimes dwarfed by land costs, compact grade separated designs may represent an
economic alternative. Grade separated interchanges in rural areas are generally expensive in
terms of land take as the opportunity cost of alternative lane uses may be low. However, in
urban areas compact grade separated schemes have proved more cost effective than
exclusively at-grade schemes requiring greater land take. The simplest example of compact
grade separation is a flyover spanning one or more at-grade junctions, where the flyover is
constructed within the at-grade road reserve, and turning movements are catered for below
the elevated structure.
4.2.7.6 An important aspect of the economic consideration is the feasibility of staged construction.
Economic benefits may be obtained by deferring construction of part of a junction layout
until traffic volumes increase sufficiently to warrant the more extensive design. Some
junction types lend themselves more readily to staged construction than others, though more
often the feasibility of staged construction will depend on the topography and traffic flows
at the particular site. Junctions constructed initially as priority control lend themselves
readily to future conversion into signal control and to a lesser extend small roundabouts.
Planned conversion to a conventional roundabout design would normally require the
uneconomical sterilization of a large area of land in the intervening years. Both
conventional roundabouts and the larger traffic signal controlled junctions lend themselves
to future conversion into grade separated junctions.