Guidelines for roundabout circulatory and entry widths based on vehicle dynamics

Current guidelines for roundabout circulatory roadway width are based on a static method that does not consider circulatory speed. In addition, the roundabout entry width is based on practical experience. This paper presents a method for determining the circulatory and entry widths based on a two-dimensional vehicle dynamics model that involves a system of differential equations of curvilinear motion. The method considers the interactions between a vehicle and road geometric elements, including tire sideslip, vehicle weight, vehicle speed, and vehicle stability. Three design vehicles are considered: intermediate semitrailer (WB-12), interstate semitrailer (WB-20), and city transit bus (CITY-BUS). Design guidelines for the required circulatory width are established for different circulatory speeds (0-60 km/h) and different inscribed circle diameters (30-80 m). To simplify the guidelines, for each design vehicle and for each inscribed circle diameter (ICD) the regression model of circulatory roadway width as a power function of circulatory speed was fitted. Guidelines for entry width were also established for typical conditions. The results show the efficiency of the proposed method which provides smaller values of circulatory roadway widths than those of current methods. The difference ranges from 0.4 to 0.6 m for CITY-BUS, 0.7-1.0 m for WB-12, and 1.3-2.0 m for WB-20. The proposed guidelines would be useful in case of spatial restrictions.