## 1 Introduction  
Roundabouts are intersections with a one-way circulatory roadway around a central island. In most countries, the vehicles entering the roundabout should give right-of-way to vehicles in the circulatory lane. Consequently, roundabout operational functionality is directly dependent on the traffic conditions in the circulating traffic flow and indirectly dependent on the geometric design of the roundabout. According to [1; 2; 3], single-lane roundabouts are a very good solution for the following transportation engineering demands: for reduced intersection dimensions; for intersections with five or more legs; when there is even distribution of traffic on the intersection legs; for traffic volumes under 25,000 veh/day; for a decrease in the waiting time at the intersection; as a measure to calm traffic, especially in urban areas; and for lowering levels of traffic noise, emissions of harmful gases, and the risk of accidents (due to the low driving speeds of approaching and circulating traffic, and fewer conflict points than at standard intersections). Additionally, single-lane roundabouts are increasingly popular solutions in suburban areas due to their design that provides an easy transition in road category and type (from a dual to a single roadway, i.e., from rural to the urban environment), and the fact that they do not require the installation, maintenance, and operation of signal lights. Roundabout design is an iterative process consisting of a (1) preliminary geometry design, (2) geometry performance checks, which include the design vehicle swept path, fastest path, and visibility analysis, and (3) assessments of intersection functionality through capacity analysis. Roundabout geometry is primarily influenced by the design vehicle swept path. The design vehicle is a vehicle identified as the least maneuverable vehicle expected to use the intersection [4]--a vehicle of a certain type and dimensions that characterize a group of vehicles and fully correspond to the legal regulations on vehicle dimensions [5], or international recommendations [6]. Design vehicles are often chosen depending on the position of the intersection in the road network and the composition and category of expected vehicles (trucks in industrial zones, buses in urban and suburban areas, etc.) [7]. After confirming the preliminary design through the geometry performance checks, the iterative design process shifts to capacity analysis, to assure that the roundabout geometry satisfies traffic performance criteria. If the capacity value is too low or too high, the geometry needs to be redesigned and the capacity analysis repeated [8]. Numerous models have been created over years to assist road designers and traffic analysts in the assessments of intersection functionality. These models can be broadly classified into three categories: (1) probabilistic, analytical, gap-acceptance models, (2) deterministic, empirical regression, geometric models, and (3) time-dependent, microsimulation models. The final selection of the most appropriate roundabout design is usually based on the results of the analytical or regression model for entry capacity assessment [9; 10; 11; 12; 13]. The analytical models were created based on combining the main postulates of traffic flow theory and field-measured driver behaviors. This approach resulted in an analytic formulation of the relationship between field measurements and theoretical performance parameters [14]. They consider traffic flow composition and conflict between vehicles in the circulatory roadway and vehicles entering the roundabout [15]. Probability theory is used to estimate to what extent the vehicles entering a roundabout will be able to use an acceptable gap between two consecutive vehicles in the circulating traffic stream [9; 16]. They do not directly quantify the relationship between the capacity and geometric parameters of the roundabout [11; 13]. On the other hand, regression models were generated from extensive traffic data collected at roundabout entries.