Minimum horizontal radius (again)

I recently posted another update to my technical note on minimum horizontal radius (you can download a copy from Academia (here) and from ResearchGate (here)). Afterwards I came across a presentation by Dr. John Rolt, given in 2013 (?), on “Geometric design and safety for low volume rural roads (LVRR)” (ref. 2211). Dr. Rolt has some powerful points to make about various aspects of road design, including on horizontal radius, which I found particularly interesting. For example, after briefly covering the standard formula for calculating minimum horizontal radius, Dr. Rolt says:

One might therefore expect that curvature has been adequately accounted for by design and that the safety of horizontal curves should not be much different from the safety of straight road sections. But this is grossly untrue. Ample data show that on horizontal curves crashes are much more frequent (perhaps by a factor of 3, on the average) than on straight road sections.

And

No empirical information about crash occurrence has been used to develop the design procedure for horizontal curves

And

What is perversely called the smallest ‘safe radius’ is the radius  associated with the highest crash rate

Dr. Rolt seems to have a refreshingly sceptical a view of at least one aspect of current highway design practice. His presentation also referred to two other topics which I will take up in the next two posts.

References

2211 – UK, Dr. Rolt of the UK’s TRL, “Geometric design and safety for LVRR”, presented at the 6th Africa Transportation Technology Transfer (T2) conference in Botswana, 2013

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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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