Comparing values for minimum horizontal radius

Switzerland has a well-developed set of highway design notes, prepared by the VSS. The VSS is an association of specialists in highways and transportation, which is involved in research and standardisation in these two fields. As the country is also seen as neutral, these standards could provide neutral, benchmark values for comparing values from differentcountries on specific highway geometric parameters, against which values from other countries can be compared.

In order to compare different countries’ values for minimum horizontal radius I have taken as benchmark values the Swiss figures from the 1991 VSS publication (ref. 732)


1. Compare Swiss values with values from the new German guidelines

Germany’s fgsv (road and transport research society) began publishing its new series of documents on highway design in 2008. Documents such as (ref. 1615) provide some values ofthe suggested minimum horizontal radius. I have taken other figures from (ref. 1617) .

The first chart below shows the 1991 Swiss values in red, and some values from Germany in black (the different curves for Germany relate to different road types). As I wrote in an earlier post on this topic (here):

The German values are mostlywithin 10% or even 5% of the Swiss values (ok, I know some of the figures are based on different values for superelevation). I am not sure that 5% doesn’t represent such a minimum difference that perhaps there is no advantage in having different values for different road types (Germany).

minradius 01


2. Compare Swiss values with recent values from other developed countries

The second chart compares the Swiss values for minimum horizontal radius against values taken from more recent standards from Australia (~ Austroads, see ref. 1539), Norway (see ref. 1198 and ref. 1601) and the UK (ref. 255). These were also discussed in more detail in recent blog posts on “uncertainty” (see for example, here). All three of these countries have extensive official documentation on highway design, based on research going back many years.

The chart suggests that the Austroads values (thick black line) are also very similar to the Swiss values (which are shown in red), but the UK and Norwegian values appear to be very different. Indeed the top UK line, which represents desirable minimum values, would appear to be far too cautious, and likely to lead to over-expensive roads.

minradius 02

3. Compare Swiss values with recent values from developing countries

Since about 2005 quite a number of developing countries have published new highway design guidelines. These “instant standards” were often prepared wth the help of European consultants.

The third chart again shows the Swiss 1991 values in red, this time compared with values taken from the new guidelines issued for Albania (2007, ref 1155), Georgia (2009, ref. 1389) and Nigeria (2013, ref. 1505).

As the chart shows, all the curves for the three developing countries show very similar values to the Swiss benchmark curve.

minradius 03


With the exception of the data from UK and Norwegian standards, all the other curves for minimum horizontal radius (Germany, Australia, Albania, Georgia, Nigeria) show quite similar values to the Swiss values. This is slightly strange, since the theories behind the German and Austroads values (or at least, the way they tweak the standard formula) look to be quite different. The same cannot be said for the Albanian, Georgian and Nigerian standards because we don’t know how these were developed.

Maybe the following ideas could be considered:

  • If a standard does not include a path which allows you to trace back to the underling theory and assumptions behind a given set of figures, then the standard should be rejected (this probably applies to most of today’s standards)
  • If “instant standards” for developing countries consistently show similar values on different aspects of highway geometrics (such as horizontal radius) then maybe these countries could save some money and buy copies of an established set of standards instead of developing their own.
  • With the various “instant standards”, multi-country standards and country standards there are probably too many standards around the world
  • Comparing values from different countries against benchmark standards can be used as a means to pick out “odd” values. For example, the UK values look to be too cautious, resulting in more expensive roads (and so to more expensive roads for countries such as the Irish Republic and Malta, which base their own standards on the UK ones).


255 – UK, “DMRB 6 section 1 part 1/ TD 9/93 highway link design”, Highways Agency; 2002

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

1155 – Albania, “ARDM 2 Road design manual vol. 2 / geometric design”, 2007

1198 – Norway, “Hb-017 Veg og gateutforming (Road and street design)”, Statens vegvesen; 2013

1389 – Georgia, “SST Gzebi:2009 / Georgia road design standards”; 2009

1505 – Nigeria, “Highway manual part 1 Design / vol. I: geometric design”, Federal Ministry of Works; 2013

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

1601 – Norway, “Hb_265 – Premisser for geometrisk utforming av veger (premises for geometric design of roads), Statens vegvesen; 2013

1615 – Germany, “Richtlinien für die Anlage von Autobahnen”, fgsv; 2008

1617 – Germany, Wolf et al, “Strassenplanung”, Werner Verlag, 2013


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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