Uncertainty in highway design (4)

The UK series of documents “Design Manual for Roads and Bridges” (DMRB) is “a series of 15 volumes that provide official standards, advice notes and other documents relating to the design, assessment and operation of trunk roads, including motorways in the United Kingdom” (wikipedia). DMRB 6 section 1 part 1 deals with highway link design, and has notes on minimum horizontal radius.

DMRB 6.1.1 does not deal explicitly with uncertainty or factor of safety, but it does introduce the idea of a range of values. For example on minimum horizontal radius table 3 refers to desirab le minimum values, smaller radii described as one step below these values, and even smaller radii described as two steps below these values. Effectively what this means is that “there are desirable minimum values but if circumstances are difficult you can reduce them, and if circumstances are really difficult you can reduce them even more”.

The text of DMRB 6.1.1 says that:

“This Standard defines a sequence of parameter values in the form of a hierarchy of geometric design criteria related to Design Speeds. This three tier hierarchy enables a flexible approach to be applied to a range of situations where the strict application of Desirable Minimum Standards would lead to disproportionately high construction costs or severe environmental impact upon people, properties or landscapes”.

DMRB 6.1.1 also says

“Numerous accident studies have been carried out, both in this country and abroad, and it has always proved difficult to correlate accident rates with causal factors”


“accident rates are unlikely to be significantly affected by small or even moderate reductions in design parameters”

Values for minimum horizontal radius

The tables and figures below list values for minimum horizontalradius from DMRB 6.1.1, and also (in red) from a Swiss design guide which I have been using as a benchmark.

uncertainty 04_1


uncertainty 04_2


I have three main comments on DMRB 6.1.1. The first is that the document seems to have a somewhat relaxed approach to safety in design (“difficult to correlate accident rates with causal factors”) – maybe the difficulty is simply that the theory is wrong. My second comment is on the document’s suggestion of a figure for an acceptable minimum horizontal radius which is 50% smaller than the desirable minimum radius – compare 1020m and 510m (at 120 km/h). This does not seem to be an acceptable level of precision.

My third comment is that I find the document text so convoluted that it is next to impossible to understand (this is not a language problem, as my mother tongue is English). For example, in the notes on horizontal radius the document says:

“The number of Design Speed steps permitted below the Desirable Minimum are normally as follows:-

  • motorways band A 2 step
  • motorways band B 3 steps
  • all-purpose band A 3 steps
  • all-purpose band B 4 steps

However, for all roads in Design Speed band B in the circumstances listed in Paragraphs 3.5 and 3.6, the scope for Relaxations shall be extended or reduced as described”.

It would be good if the document could be completely re-written in a more understandable style.


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

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


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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