Terrain classification is an important preliminary step in selecting values for highway geometrics. For example (ref. 294) says, “the geometric design elements of a road depend on the transverse terrain of land through which the road passes”. The following figure, taken from a document on the Asian highway network (ref. 757), shows how figures for elements such as design speed, shoulder width, horizontal curve radius can vary with the class of terrain.
Figure 1: terrain classification and geometrics
How terrain is classified
In reference works on highway engineering , terrain classification has been described by means of text descriptions, by reference to ground contours, and in terms of cross-slope.
1) The AASHTO (USA) publication “A policy on the geometric design of highways and streets” (ref. 713, 831) has said much the same thing since 1994. It refers to three types of terrain classified – level, rolling and mountainous. Mountainous terrain is described as:
In mountainous terrain, longitudinal and transverse changes in the elevation of the ground with respect to the road or street are abrupt, and benching and side hill excavation are frequently needed to obtain acceptable horizontal and vertical alignment.
Terrain classiﬁcations pertain to the general character of a speciﬁc route corridor. Routes in valleys, passes, or mountainous areas that have all the characteristics of roads or streets traversing level or rolling terrain should be classiﬁed as level or rolling.
2) A World Bank publication on the Design and appraisal of rural transport infrastructure (ref. 919) refers to three terrain types (rolling, very flat, very steep (mountainous)). The publication also quotes from the 1988 (UK) TRL publication “Overseas road note 6” which describes mountainous terrain as:
Mountainous (greater than 25 five-meter ground contours per km). Rugged, hilly and mountainous with substantial restrictions in both horizontal and vertical alignment.
3) One India guideline (ref. 293) says that:
“terrain is classified by the general slope of the country across the highway alignment”
“While classifying a terrain, short isolated stretches of varying terrain should not be taken into consideration”
The following table presents terrain classes as described in a number of different highway engineering standards and guidelines. Depending on the guideline, there are from 2 to 4 terrain classes. The different guidelines use different terms for what are probably the same things.
Quantification of terrain classes
A number of the guidelines quoted in table 1 quantify the different classes of terrain in terms of the cross slope (in %) of the terrain.
It seems that terrain classification is an important input to highway geometric design. It also seems that there is less agreement on how to classify terrain than there should be. With “modern” tools such as computer-aided design and geographic information systems (CAD and GIS) there is a need for a much more rigorous definition of terrain classes. This definition should include an indication of scale.
In 1993 the UK’s TLR published a textbook on terrain, the “Terrain evaluation manual”. It includes a section on terrain classification methods and procedures, and is said to be especially relevant to the construction and planning of low cost roads in developing countries. Maybe this book could provide a starting point for developing an improved terrain classification.
293 – Indian Roads Congress, “Geometric design standards for rural (non-urban) roads; India, 2000
294 – Tanzania, “Road geometric design manual, 2011 edition”, Ministry of Works, Dar es Salaam 2012
508 – Nepal, DoLIDAR, “Nepal Rural Roads Standards 2012-1st revision”; Nepal 2012
713 – AASHTO “A policy on geometric design of highways and streets”, USA 1994
726 – UNECE “Trans European Motorway standards and recommended practice third edition”, Poland 2002
763 – Malaysia, “A guide on geometric design of roads”, 1986
757 – UNESCAP, “Intergovermental agreement on the Asian Highway Network”, 2003
831 – AASHTO “A policy on the geometric design of highways and streets”, USA 2011
846 – Ministry of Works and Human Settlement, Royal Government of Bhutan “Guidelines on road classification system and delineation of construction and maintenance responsibilities” Thimpu, Bhutan, 2009
855 – Ethiopia, geometric design manual, Ethiopian Roads Authority, 2002
890 – Spain, Ministerio de Fomento, “Norma 3.1-IC Trazado, de la Instrucción de Carreteras”, 2000892 –
919 – World Bank technical paper 496, “Design and appraisal of rural transport infrastructure”