Provision of Sight Distance Around Concrete Barriers and Structures on Freeways and Interchanges

The use of concrete barriers 1.1 m (3.6 ft) high at both median and outer-shoulder edges of freeways is common in Australia. The use of such barriers is also common on interchange ramps on or under structures. Where curved ramps form an overpass of a major road or a road crosses a rail line, concrete barriers 1.4 m (4.6 ft) high and larger are used. The provision of criteria for normal stopping sight distance around these concrete barriers and other structures such as bridge abutments and retaining walls on horizontal curves (and on combinations of horizontal and vertical curves) can lead to a requirement for extremely wide shoulders. Such shoulder widths were often considered uneconomical, and only narrow shoulders were provided. This choice was either based on some artificially low design speed or, even worse, was made by ignoring the sight distance requirements altogether and simply providing what was considered to be practical. This paper discusses new criteria for the provision of sight distance around barriers and structures, as documented in the 2009 release of the Austroads Guide to Road Design series. The new criteria achieve practical, yet justifiable, results.