Changes in Geometric Design Standards on Interurban Undivided Roads

This paper outlines how geometric design standards for undivided interurban roads have changed over time in response to financial, safety and environmental constraints. Initially, the dimensions of each road element were selected to provide a safe road layout and a desirable level of service. For example, the minimum sight distances were based on conservative reaction times and friction factors for each design speed, while horizontal curve radii were selected on assumed safe stability requirements. In restricted areas or difficult terrain, applying these standards increased costs. With better information on speed / flow relationships and on the safety of geometric elements, design standards were amended and reductions were permitted in difficult conditions. Combinations of elements were also taken into account. However, geometric standards still mainly consist of individual design elements which the designer must fit to the terrain, the intersections and the adjacent land use. To illustrate the principal factors which have modified geometric standards for undivided roads in all countries, the different versions of the Irish geometric design guidelines are described. Originally based on US standards, they are now almost identical to the current UK standards. The Irish design guidelines are used to show how geometric standards evolved in response to better knowledge on safety, increased traffic capacities, environmental considerations and to changes in design and construction practices. The principal changes in cross section, alignment and intersection elements are described. Also described are the different intermediate road types used to fill the large gap in capacity between undivided two lane roads and divided four lane roads and the changes for roads in sensitive locations. It is pointed out that geometric design standards have not always been based on actual driver behaviour, speed limits or enforcement regimes. Also, that designers seldom depart from standards due to legal concerns which can result in suboptimum road layouts. In an era of “safety & health” regulations, design standards do not aim at eliminating road fatalities but implicitly assume an “acceptable” number of fatalities and injury accidents per year. Consumer choice is not considered. Modifications to design standards for undivided roads, including the use of intelligent transport systems to achieve balanced networks without reconstruction, and climate change considerations are suggested to give more sustainable standards.