Uncertainty in highway design (2)

A previous post (here) referred to the concept of uncertainty in highway design and gave an example of how geometric design theory could allow for it. The example, from documentation from South Africa, referred to “design domain”. This concept concept “recognizes that there is a range of values, which could be adopted for a particular design parameter within absolute upper and lower limits” (Ref. 148).

Another example can be found in highway geometric design documents from Austroads, and also from other organisations in Australia. As its website says, Austroads (link) is “the association of Australian and New Zealand road transport and traffic authorities. Austroads promotes improved Australian and New Zealand transport outcomes by providing expert technical input to national policy development on road and road transport issues”. Austroads produces the “Austroads Guide to Road Design” (AGRD). The latest version was released in 2009.

Example 2: Australia – Extended Design Domain

(Ref. 1590) says that the Normal DesignDomain (NDD) :

“can be thought of as a range of values that a design parameter may take and be justified in an engineering sense and therefore can have a reasonable level of defence if questioned….”

 Extended Design Domain (EDD) in contrast is

“the use of criteria below the lower bound of the NDD criteria to address project constraints whilst ensuring that the objectives of the project and safety are still met”.

Note the comment that EDD only extends the lower bound of the NDD criteria. The following figure, taken from (Ref. 1590), illustrates the concept of EDD, and itself comes from theAGRD (Austroads Guide to Road Design) :

uncertainty 02_1

Regarding the application of EDD, (Ref. 1539) says that in the latest version of the AGRD, “definitive parameters have been provided for the designer to use in constrained locations for new works and for restoration type projects on existing roads”

Quoting AGRD part 2,(Ref.1590) says that “EDD extends the lower bound of the Design Domain that can be used for a new road, based on what can be justified and defended, on engineering grounds, in certain circumstances”. (Ref. 1590) also says that “There is less scope to apply EDD on high volume roads… (and) EDD can only be applied to existing roads with no crash history where the existing pavement design and other site constraints may prohibit the upgrade to current design standards”.

I haven’t yet got a copy of the AGRD part 3, but details in (Ref. 1539) at least allow one comparison to be made. The following table and chart compare one value for minimum horizontal radius from the new Austroads guide with a benchmark set of values from a Swiss geometric design document (ref. 732).

uncertainty 02_2


uncertainty 02_3


The Austroads approach does not seem to mention any modification of upper limits to design values (the figure only shows EDD modifyin g the lower values). Also, the graphic (fig.2.1 from AGRD part 2) suggests that the lower value for EDD can be reduced even more where traffic flows are low. I don’t yet know if the design is based on forecast traffic flows, and if so, for what forecast year.


148 – South Africa, “Geometric design guide”, CSIR; 2002

732 – Switzerland, VSS “VSS 640-080 Projektierung, Grundlagen (basics of road design)”, 1991

1539 – Australia: Barton O’Callaghan, “New National Guide to Road Design”; 4th International Symposium on Highway Geometric Design; 2010

1590 – Australia, “VicRoads Supplement Part 2 – Design considerations”, VicRoads, 2012 (VicRoads is the road and traffic authority for the state of Victoria, Australia).


About roadnotes

Robert Bartlett is an international consultant with over 30 years of professional experience as a highway and traffic engineer with leading companies and organisations in several countries, including Germany, China (Hong Kong), Qatar and the UK. Specialised in urban studies, transport and the use of GIS, research has included new ideas on subjects such as the study of social justice using GIS, the dimensions of vehicles, and comparative geometrics (highways and transport).
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