## 4 Discussion and Conclusions  
The iterative process of roundabout design requires shifting between geometry design and capacity analysis, to ensure that chosen roundabout geometry satisfies the desired traffic performance criteria. The quality of traffic flow on the roundabout is indirectly dependent on its geometric parameters, and should be estimated during roundabout design through calculation models or simulations. In this investigation, a comparison of the analytical and empirical models for single-lane roundabout entry degree of saturation estimation was performed on 60 designed four-legged roundabout schemes. When designing a single-lane roundabout based on the design vehicle swept path analysis, the choice of geometric parameters and the capacity calculation model influences the roundabout performance evaluation results. The observed models differ in the utilization of the geometric parameters: (1) the analytical model uses only the roundabout outer radius as an input for calculation, while (2) the regression model considers the influence of conflicting traffic on the circulatory roadway defined by the distance between exiting and entering traffic flows along the center of the circulatory lane, i.e., it considers the joint influence of roundabout outer radius, the central island radius, and the angle between the roundabout legs. The results of the geometric and traffic parameters analysis showed that the central island radius and entry capacity are proportional to the outer radius. At the same time, the circulatory roadway width and the entry degree of saturation are inversely proportional to the outer radius. This is in line with the conclusions given in [27], which state that circulatory roadway width significantly influences capacity. As can be seen from our results, the influence of the circulatory roadway width, derived from the design vehicle swept path, on capacity is counterintuitive; narrower circulatory roadways on roundabouts with a larger outer radius enabled higher entry capacity, i.e., lower entry degree of saturation. According to [11], entry width has a positive correlation with entry capacity. According to [27], the distance between the entry and exit has a greater impact on the entry capacity than the entry radius. The results of our investigation show that an increase in entry width is directly linked with an increase in outer radius value and the design of the right roadway edge derived from the design vehicle swept path, and that it decreases the roundabout entry degree of saturation in the regression model. A larger outer radius in the analytical model results in a lower degree of saturation. A larger outer radius and a greater distance between the conflicting points (dependent not only on radius but also on the width of the circulatory roadway and entry and exit widths) in the regression model result in a lower degree of saturation. In general, for all the observed combinations of geometric parameters, the regression model results in lower values of roundabout entry degree of saturation. This difference is more pronounced as the outer radius and angle between the roundabout legs increase. This implies that it is possible to justify the chosen roundabout geometric parameters by choosing the capacity analysis model, which will allow higher traffic flow volumes. Namely, the results of the investigation showed that it is possible to increase the traffic flows that could be processed at a given roundabout from 6% to 15% if the regression model is applied instead of the analytical one during roundabout performance evaluation. From the perspective of the time in which the roundabout is expected to satisfy the desired performance criteria and considering that the average annual increase in motor vehicles' number in Europe is 2% (according to the European Automobile Manufacturers' Association), it could be said that the application of the regression model could extend the expected service life of a roundabout by 3 to 7 years.