## 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).

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) :

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).

and

Comment

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.

References

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).