Minimum horizontal radius

What I used to believe

Towards the end of 2012 I added a blog post on the basic formula for calculating horizontal radius (here). At that time I thought that the geometric design of roads was pretty much straight-forward. With individual design parameters such as minimum radius I thought there would probably beone established, well-founded formula or procedure, and that this formula would be used in most countries. This in turn would mean that most countries would havesimilar values for minimum horizontal radius.

What I believe now

– is that the conventional formula for horizontal radius is not the only one around, that it is not necessarily correct, and that even if the conventional formula is correct it is not necessarily properly applied. I produced a short research paper on this, with the conclusions that:

  • The use of different terms in different standards for what is probably the same thing can lead to confusion.
  • It seems likely that values for minimum horizontal radius are both over-complicated and over-simple. Over-complicated since there should be no need for what appears to be a wide range of values in different countries if radius varies with sidefriction and superelevation and there are only a handful of limiting values for each, and Over-simplified in that design standards from different countries rarely offer values for different vehicle types and different road surfaces.

I also begin to suspect that most of the geometric road design guidelines published today are questionable.

You can take a look at the research paper using this link to the ISSUU website.


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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