The UK’s principal guideline on the design of roads is the “Design Manual for Roads and Bridges” (DMRB). Wikipedia says the DMRB “is a series of 15 volumes that provide standards, advice notes and other documents relating to the design, assessment and operation of trunk roads, including motorways in the United Kingdom, and, with some amendments, the Republic of Ireland”.
Volume 6 (road geometry) includes a section on highway link design. This essentially relates design speed to geometric parameters such as horizontal radius, stopping sight distance etc. Volume 6 basically consists of a document called TD 9/93.
For me, one (the) problem with TD 9/93 is that it ventures into a realm where design speed doesnt increase in steps of 10 km/hr, as:80 … 70 … 60 …
but in design speed bands, such as 100 … 85 … 70 …
TD 9/93 says “The Design Speeds are arranged in bands, ie. 120, 100, 85, etc., within which suffixes A and B indicate the higher and lower categories of each band”. So we have for an urban road speed limit of 40 miles/hr a design speed of 70A km/h (TD 9/93 table 2).
The document includes a figure (see below) which is meant to help in the selection of design speed for rural roads. The speed selection is related to layout constraint and alignment constraint. Alignment constraint is itself related partly to “bendiness” which is measured in degrees / km.
To me the figure is representative of the whole of TD 9/93 – it seems over-complicated and in part, not very comprehensible. It might be better to produce a new version of the document based on the approach used in other countries. The Austroads document (ref. 1887) might be a good place to start.
1887 – multi-country “AGRD part 3: Geometric design”; Austroads, 2010