The influence of geometric parameters on the entry capacity of two-geometry roundabouts

Two-geometry roundabout is an alternative type of modern roundabout with variable circulatory roadway width defined by its’ elliptical outer carriageway edge and a circular inner carriageway edge. The research presented in this paper aimed to determine the influence of variable circulatory roadway width on the right carriageway edge design and their joint impact on two-geometry roundabouts’ entry capacity. The research was based on computer simulations: 45 four-legged single-lane roundabout schemes were designed in the Autodesk AutoCAD 2021 software. The design was based on computer simulations of vehicle movement and the resulting swept paths of a tractor with a semitrailer generated by the Autodesk Vehicle Tracking 2020 software. The entry capacity of the designed roundabouts was investigated according to the regression Swiss Bovy model. This model was chosen because it considers the influence of conflicting traffic on the circulatory roadway, i.e., the distance between exiting and entering traffic flows along the center of the circulatory roadway (b i ) on entry capacity. The simulation results show that the central island radius and parameter (b i ) are proportional to the elliptical outer carriageway edges’ major and minor axis lengths. This means that according to the regression Swiss Bovy model, the entry capacity of two-geometry roundabouts is proportional to major and minor axes lengths and inversely proportional to the relation between the minor and major axis length. At the same time, the two-geometry roundabouts’ entry capacity is lower compared to the standard four-legged single-lane roundabouts with an outer radius that is equal to the two-geometry roundabouts’ major axis.