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Road duplication or ‘dualling’ involves changing a single carriageway road to a dual carriageway road by building a second separate carriageway, usually alongside the first.

Road duplication provides a safety benefit through provision of a central median barrier or strip of land (median or central reservation), thereby reducing the chances of head-on crashes.

Divided roads range from high speed, access-controlled roads to moderate speed arterial roads with at-grade intersections. In between, divided roads may consist of a mix of grade-separated and at-grade intersections.

This is costly and requires a large amount of space. Because it is so costly, road duplication projects are often carried out in a staged fashion, in order to make use of limited road funds. Duplication is typically only economically viable at higher traffic flow levels.

The Star Rating Demonstrator is a freely available tool with the iRAP online software, ViDA. With the Star Rating Demonstrator, it is possible to explore the impact that this Safer Roads Treatment has on risk.

Treatment Summary



Treatment life

20 years +

Potential casualty reduction


Case Studies