## Cross-fall and superelevation – minimum crossfall

Introduction

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.

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:

1. Traffic flow
2. Material used for the road surface
3. Whether the road is paved or unpaved
4. Traffic lane (inner or outer)
5. Layer of the road pavement
6. Year the design guideline was published
7. Country which published the design guideline
8. The particular guideline (two guidelines from the same country may propose different figures)
9. Type of traffic running along the road surface
10. Type of terrain
11. Design speed
12. Dominant type of weather
13. Module (element) of the road cross-section (e.g. carriageway, shoulder)

The following table gives examples for some of these features:

and

and

Comment

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.

References

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

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

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