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