Road design

Table of Contents

4.11.1 Wide Medians and Streets of Unequal Width

Particular problems in roundabout design occur at locations where one intersecting street is considerably wider than the other and/or where a wide median exists. This situation can occur on local, collector or arterial roads or, as is often the case, where the intersecting streets are not of the same functional classification. Very often a roundabout will not be the appropriate type of treatment in these cases. However, where the volume of traffic on the narrower street is greater than or equal to that on the wider street, and if there are heavy right-turn flows, a roundabout could be suitable.

Where a roundabout is proposed, special care should be taken to ensure that the design is in accordance with the guidelines given in this Guide. In particular, providing sufficient entry curvature for through traffic entering the roundabout is most important. Generally, a low-cost solution that does not require roadworks encroaching onto existing nature strips and/or the median will not be possible.

Figure 4.15 is an example of a roundabout designed for an undivided road crossing a divided road with a wide median. In these situations the central island is not circular and as a result there will be different circulating speeds for different sections of the circulating carriageway. A circular roundabout at this type of location, although quite large, would provide a safer treatment and is therefore desirable, if space permits.

The oval island does not necessarily have to be aligned with the centreline of the intersecting roads. A designer may choose to orientate the oval to favour a predominant right-turning movement or to assist a substantial right-turning movement of heavy vehicles.

Figure 4.15: Roundabout on a road with a very wide median

oblong

Source: Department of Main Roads (2006).
Department of Main Roads (2006) has been superseded and Figure 4.15 has not been carried forward into Department of Transport and Main Roads (2015).