Choosing the appropriate crossfall (slope) for a roundabout’s circulating carriageway can be a challenging task. Some key considerations include:
Adverse crossfall or positive superelevation?: Adverse crossfall (tilts downward from the center) provides good visibility of the island for approaching motorists. It also helps with drainage. However, it increases the risk of single vehicle truck rollover, and single vehicle motorcycle loss-of-control crashes.
Positive superelevation (tilts upward from the center) reduces single vehicle truck rollovers but may hide the roundabout from approaching motorists. It can also have issues for left turning vehicles as they navigate out of the superelevation.
A third alternative exists which is to keep the crown in the centre of the circulating carriageway, however this is difficult to construct and can destabilise vehicles travelling straight through.
Generally, it is appropriate to use crossfall or superelevation of 2.5% to 3%, but each situation is different so check out the Austroads Guide to Road Design Part 4B for further detail.
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