Whilst traffic performance and safety are key factors, many other considerations may affect the type of interchange that is adopted. Such factors include, for example, topography, practicality of acquiring particular areas of land, and the presence of watercourses (Section 2.2.1). Whilst land acquisition can sometimes be an issue in rural areas, this difficulty is more likely to occur in urban areas where the cost and disruption associated with land acquisition is high.
In many cases, the topography and the relative grading of the roads will dictate the most appropriate form of interchange. Generally, one of three conditions can arise:
- the influence of topography dominates and the design is fitted to it
- the topography does not favour either of the roads
- the alignment and grade-line controls of one road are sufficiently important to subordinate those of the other.
In general, the design that fits the topography will be the most pleasing and the most economical. If this situation does not prevail, the factors discussed in Section 3.2.2 are to be considered in making the decision on the relative grades of the roads.
Other factors which can influence the type of interchange include:
- Requirement for oversize vehicle manoeuvres
- staging of construction
- environmental factors (including noise and visual impacts)
- requirements for proposed or future mass transit systems.
Further information on the types of interchanges can be found in AGTM Part 6 (Austroads 2013a).