## 4 Discussion  
As kinematic and dynamic requirements lose their importance due to low driving speed on roundabouts, a prerequisite for quality roundabout design becomes the definition of the design vehicle movement geometry and its turning envelopes [9]. According to the results of the investigation presented in this paper, the design vehicle tractor with a semi-trailer chassis turning envelopes derived from the A-2 approach were more offset from the roundabout centers when compared to the path constructed via the A-1 approach. Namely, 72% of right-hand-side path and resulting right-hand-side design vehicle body and chassis turning envelope deviations were between 0.00 and 0.15 m, 27% were over 0.15 m, and 7% were over 0.25 m. Deviation frequency for deviations over 0.25 m is inversely proportional to the outer roundabout radius. This shows that, in general, the A-2 approach results in wider entrances and exits compared to the A-1 approach. Wide entrances are unfavorable in terms of traffic safety as they enable higher entry speeds, especially for personal vehicles [8; 14]. On the other hand, wide exits enable higher roundabout exit speeds and could help minimize the likelihood of congestion and crashes at the exits. Wide exits may also be desirable in areas with high truck volumes to provide ease of navigation for trucks and reduce the potential for trailers to track over the outside curb. However, at urban intersections, there is a need to maintain lower speeds through the pedestrian crossing on roundabout exits [8]. The deviations of the left-hand-side design vehicle body and chassis trajectories on the circulatory roadway (cross-sections 4-10) were small, mostly between 0.05 and 0.15 m. Nevertheless, this means that approach A-2 provides an opportunity to design a central island with a larger radius than approach A-1. A central island with a larger radius could have a positive impact on traffic safety. Namely, it provides for greater vehicle path deflection, and consequently, vehicle speed reduction [8; 16]. Speed reduction is a benefit of a well-designed roundabout that usually results in homogeneous driver behaviors and increased traffic safety [15]. The investigation results showed that allowances larger than 0.30 m should be added for lateral clearances when utilizing the A-2 approach for roundabouts with outer radii smaller than 17.0 m. Namely, on these roundabout schemes, deviations larger than 0.30 m were noted for the design vehicle body and chassis envelopes at circulatory roadway entry and exit cross-sections. This could be of concern if approach A-2 is used for roundabouts in locations where heavy pedestrian traffic on sidewalks adjacent to the roundabout is expected, i.e., for urban intersections, as pedestrians could come in conflict with passing vehicle body. In general, in urban areas, the design of roundabouts should be carefully conducted by considering the presence of these vulnerable users [4; 16]. The limitations of this investigation are as follows. The investigation was conducted for single-lane roundabouts whose approach leg axes are placed on the straight that passes through the geometrical center of the roundabout (center of the circular island). In the real world, this ideal position of the approach leg axes is sometimes not possible to achieve due to the spatial constraints, as well as the requested number and the alignment of the approach legs. This results in different locations of points A, B, and C on the FACPPs for straight passage due to the offset of the approach leg axes from the center of the circular island and different angles between the approach legs. Furthermore, long vehicles that pose a challenge in roundabout design are not only tractors with semi-trailers, but also three-axle buses, which are not included in this investigation. Another issue that should be addressed is the influence of the approach A-2 application on personal vehicle speed.