Normally a road surface can be considered to slope in two directions – longitudinally , in the direction of the centreline of the road, and transversely, at right angles to the centreline of the road. This post discusses the transverse slope of a road. Several different terms are used to describe the transverse slope. They include camber, crossfall, superelevation, cant and cross slope. Some definitions (from ref. 138):
- Camber the transverse convexity given to the surface of a carriageway or footway
- Crossfall (or cross slope) The slope, at right-angles to the alignment, of any part of the carriageway.
- Superelevation is the continuous transverse slope normally given to a carriageway at horizontal curves
Basically, the terms all refer to the same thing. In fact one reference (ref. 1037) refers to superelevation, cant and crossfall in the same breath. Depending on which standard you look at, the crossfall can be designed to fall from the edge of a carriageway to the centre median, can vary with each lane across the width of one carriageway on a dual carriageway road, and can vary constantly, with the road surface taking the shape of a parabolic curve.
There are therefore three sub-aspects of crossfall which could be looked at: minimum values, maximum values, and direction of the crossfall. The remainder of this post looks at minimum crossfall.
Suppose we have a one-lane road with travel restricted to one direction. The simplest design would be for the road to have a flat, horizontal surface. However , rainwater will not drain off of this road surface, and there will be the risk of ponding during rain. So the road surface has to have a minimum slope for drainage purposes.
At this point I thought that design guidelines would agree on some figure, say a minimum crossfall of 2.5% – and that would be the end of the matter. But things aren’t that simple. Depending on which design guideline you read, the minimum crossfall can vary depending on features such as:
- Traffic flow
- Material used for the road surface
- Whether the road is paved or unpaved
- Traffic lane (inner or outer)
- Layer of the road pavement
- Year the design guideline was published
- Country which published the design guideline
- The particular guideline (two guidelines from the same country may propose different figures)
- Type of traffic running along the road surface
- Type of terrain
- Design speed
- Dominant type of weather
- Module (element) of the road cross-section (e.g. carriageway, shoulder)
- Highway section type (at-grade road, road in tunnel etc)
- Longitudinal gradient
The following table gives examples for some of these features:
Most or all of the 15 features listed above probably do (that is, should have) an impact on the choice of minimum crossfall. In this case, surely any self-respecting guideline on highway geometrics would include them. In fact it is difficult to see any consistency between different design guidelines as to the features they do / do not include; and what makes life more difficult is that the points which a particular guideline does include are often in different pages and chapters of the document. There is also the problem that in many countries there exists a jungle of guidelines, manuals, and papers, which partially contradict each other, which may or may not be up to date, and for which there is no overview.
Maybe university departments could produce an overview of their country’s guidelines, with an indication of which are still relevant.
137 New Zealand, Transit State highway geometric design manual, section 6: cross section, 2002
138 New Zealand, Glossary of terms, 2005
231 India, Four-laning of highways through PPP, Planning commission, government of India, 2010
308 Ethiopia, Design manual for low volume roads, part-B, Ethiopian Roads Authority, 2011
508 Nepal Rural Roads Standards, 2012
745 UK, LTN 2-08 Cycle infrastructure design, DTp 2008
753 UK, Cycle Infrastructure – quick reference sheet, 2007
757 UNESCAP, Intergovermental agreement on the Asian Highway Network, 2003
831 USA, A policy on the geometric design of highways and streets; AASHTO, 2011
843 Singapore, Civil design criteria for road and rail transit systems, LTA 2010
855 Ethiopia, geometric design manual, ERA, 2002
857 Australia, Rural road design (8th ed.), Austroads 2003
859 Ireland, Design manual for urban roads and streets, Department of Transport, 2013
890 Spain, Norma 3.1-IC Trazado, de la Instrucción de Carreteras, Ministerio de Fomento, 2000
917 USA, Los Angeles bicycle plan, technical design handbook, 2010
919 World Bank Technical Paper No. 496, Design and appraisal of rural transport infrastructure
922 Norway, Hb265: Linjeføringsteori, Statens vegvesen, 2008
1011 Abu Dhabi, ADM Roadway design manual
1024 Austria, Anlagen für den nichtmotorisierten Verkehr, 2010
1037 UK, Traffic engineering design, principles and practice,M. Slinn et al, Elsevier, 2005
1062 Netherlands, Nieuwe Ontwerprichtlijn Autosnelwegen, Ministerie van Verkeer en Waterstaat, 2007
1063 Ethiopia, Geometric design manual, ERA, 2002