Geometric design of low-cost roads in developing countries

The geometric process involves selecting the alignment and cross section of a road to meet users' needs. The background and approach to developing a guide for the geometric of roads in developing countries are described. The approach recognizes that road and driving conditions in developing countries often differ from those in the industrialized countries in terms of (a) the traffic mix between commercial vehicles and private cars, as well as between motorized and nonmotorized vehicles; (b) the rate and nature of road accidents; and (c) the level of economic development and its implications for roads. An iterative approach to is proposed including the following steps: (a) identifying traffic flow, terrain type and road function, leading to a choice of design (b) selecting trial alignments; (c) identifying alignment elements that are of a lower geometric standard than that of the chosen class; and (d) estimating approach speeds for the above elements. If they are acceptably consistent, the goes forward to economic evaluation; if not, the alignment may be amended or the standards relaxed with appropriate measures for safety. The thrust of the approach has been to develop a methodology that emphasizes the economic aspects of geometric design. The standards recommended tend to be lower than many of those in common use. Recommended standards include maximum carriageway widths of 6.5 m with shoulders for use by nonmotorized traffic, roads with 5-m carriageways to carry up to 400 vehicles per day where 1.0-m shoulders may be used for passing, roads of 2.5 to 3.0 m for very low flows of traffic, with room for passing, and horizontal radii as low as 15 m. The guide itself was published by the Transport Research Laboratory as Overseas Road Note 6.