Roundabouts : geometric design and roundabout safety

Abstract

The objectives that are kept in mind while designing roundabouts are operational performance and design consistency. The former affects the level of service of roundabouts, while the latter affects the roundabouts safety. The geometric design of roundabout makes it safer if it compels the traffic to enter and circulate the roundabout with less than the specified design speed. The speed of each vehicle path is controlled by the vehicle path radii, which in turn the radii give a specific design speed. Traditionally, the vehicle path radii were done by drawing freehand each vehicle path, but nowadays computer programs can be utilised. The sensitivity of the geometric elements of the roundabouts influences the vehicle trajectory and speed, which affects the operational performance and traffic safety. In situ speeds were taken at five case studies, where a comparison between the design speed and the actual average speed was carried out for five case studies. The crash reports were also gathered in order to assist in optimizing the safety of all vehicle occupants, pedestrians and cyclists. Comparisons between the United Kingdom, American and Australian Roundabout design standards were also undertaken. The results obtained present the analysis between the design speed and the actual vehicle speed measured at these roundabouts.