Review of Roundabout Design Standards and the Development of a Roundabout Design Guideline for Sri Lanka

In Sri Lanka, the geometry of roundabouts has a considerable influence on their operation. The increase in road traffic and the large number of long vehicles that ply on the roads have contributed to the malfunctioning of some of these roundabouts. The objectives of this study were to review the roundabout design guidelines, identify issues in the existing roundabouts and prepare a roundabout design guideline appropriate for Sri Lanka. Twenty four roundabouts located in some of the major cities were selected for the study. The design parameters of the geometries of the selected roundabouts were compared with those recommended in five major design guidelines. Data on the main geometric parameters of each roundabout were collected through field measurements and calibrated satellite images. The dimensions and the design parameters of these roundabouts were compared with those stated in a few international roundabout guidelines. A swept path analysis was carried out on selected roundabout layouts for single unit trucks to determine the adequacy of their entry widths, circulation widths, exit widths and operational speeds. The entry radiiwere higher than those recommended in the international standards. The design parameters of the existing roundabouts that required improvements were identified and suitable values for the selected design parameters have been proposed. Most of the roundabouts meet the minimum requirements specified in the international guidelines for the centre island diameter (100%), circulation width (92%), and exit radius (92%).