4.6.1 Design Vehicle and Vehicle Swept Paths
The selection of an appropriate design vehicle (and checking vehicle) is discussed in Design Vehicles and Turning Path Templates Guide (Austroads 2013c). As with other types of intersection, the design vehicle and consequently the swept path requirements may be different for the various paths through a roundabout. For example, the straight-through movement at a particular roundabout may have to cater for 25 m B‑doubles whereas the left and right-turning movements may only need to cater for single unit trucks. This can occur because:
- Particular types of heavy vehicles can be restricted to certain routes or from entering certain areas and consequently their turning movements at a roundabout are restricted.
- The volumes of a particular type of heavy vehicle are extremely low on particular turning movements. In this case encroachment areas may be provided, which allow a smaller width of circulating carriageway to be used.
Because travel through roundabouts involves complex reverse-turn movements, particular care is needed in the use of simple turning path templates, – refer to Design Vehicles and Turning Path Templates Guide (Austroads 2013c) to achieve a satisfactory layout. A more accurate result is obtained by using a computer plot of the design vehicle’s swept path on an assumed travel path through the critical turning movements. Computer programs such as VPATH, AUTOTURN, and AUTOTRACK are examples. These programs may also be used to check the ability of a roundabout to cater for any checking vehicles or over-dimensional vehicles which may need to be accommodated.