1.8.2 Flexibility and Constructability
Interchanges are one of the most costly parts of the road system, being expensive to construct initially and exceptionally expensive to reconstruct under traffic when expansion to a new configuration is necessary to meet future needs. For these reasons it is most desirable that initial interchange designs should provide flexibility well beyond the target design life. For example, this flexibility could be provided (e.g. built-in) by ensuring that overpass bridges will be long enough for the ‘ultimate stage’ of the interchange and motorway, or that another level could be economically added without requiring significant changes.
Designers should always consider the potential to build flexibility into the design to accommodate possible future changes, particularly when future land use changes and traffic patterns are uncertain. Whilst flexibility is usually more important for system and other major interchanges, designers should take account of the long-term (say 50 years into the future) layouts and configurations that may be required for all interchanges. For example, where sufficient space is available and a satisfactory alignment of the minor road through the initial at-grade intersection with the major road can be achieved, the minor road should be constructed clear of any future bridgeworks. In determining a suitable alignment, consideration should also be given to providing sufficient clearance to likely worksites e.g. allowance for deep excavation for any future bridge construction and clearance for machinery such as pile driving.
Traffic operations during construction must also be considered in the planning and design process. Construction of interchanges may result in long periods of disruption and delay (and hence cost) to road users, particularly if an existing interchange is to be reconstructed. It is desirable that future works can be constructed without interrupting the existing traffic as far as possible so the first stage will generally include the outside elements of the facility.
For staged treatments in rural areas, an intersection treatment may be provided on a duplicated road with the intention of providing an interchange at some future time. The first-stage of an at‑grade intersection should be located to allow the future overpass structure to be constructed on the ultimate alignment without affecting the first-stage construction. In urban areas, considerable thought should be given to the traffic management when converting existing intersections to interchanges. Each step should be documented by a drawing with a description of the construction in progress, and the locations and management of the traffic movements.
It may be possible to provide minimum widths of carriageways to suit initial traffic volumes, and to widen or duplicate as necessary in the future. In some cases, ramps with very low predicted volumes may be omitted. However, if any movement is provided, the ramp for the return movement at the same site also should be provided.
It is desirable that future works are able to be constructed with minimal interruption to traffic, and therefore the first stage will generally include the outside elements of the facility. For example:
- If an interchange with one or more loops is needed in the future, it may be possible to omit the loops in the first stage and use the outer connection ramps as legs of a diamond interchange in the interim. These ramps would be positioned in their final locations allowing the loops to be constructed inside these ramps at a later date.
- Provision could be made in the first-stage development of an interchange for a third level to be ‘easily’ added to the interchange when traffic reached a predetermined volume.
- If a freeway is terminated initially at a proposed cross-road interchange, it is desirable to use the ultimate ramp alignments for the interim intersection treatment to facilitate the later construction of the interchange.