## 5 Conclusions  
The investigation presented in this paper focuses on one of the elementary issues in transportation engineering: whether the traffic infrastructure will provide required traffic safety, i.e., how steering path geometry impacts turning movements for long, articulated vehicles such as a tractor with a semi-trailer. Roundabouts are potentially hazardous locations due to the large turning envelopes required by long, articulated vehicles. When designing a roundabout based on the results of the design vehicle swept path analysis, the parameters for the construction of the design vehicle turning envelope are not specified. Therefore, FACPPs need to be constructed solely based on the roundabout outer radius, approach alignments, and the selected shape of the splitter island. Two approaches can be applied when constructing the path for the straight passage. The first one (A-1) implies the application of short, straight lines inserted between the adjacent arcs. The second one (A-2), which is faster and simpler to use, implies the application of the principles of hairpin bend geometry. In this paper, the investigation of the deviations for paths constructed according to these approaches was presented. The results of the investigation can help to identify an adequate vehicle path construction approach and the range and combination of path geometric elements needed for the simulation of design vehicles through movements at single-lane roundabouts with outer radii varying from 13 to 25 m.  
The investigation results showed that the front axle center point paths for straight passage through the roundabout constructed following the principles of hairpin bend geometry, i.e., with consecutive circular arcs, is applicable in the design of the right carriageway edge in single-lane roundabouts for a design vehicle tractor with a semi-trailer, as only 7% of right-hand-side path and resulting right-hand-side design vehicle body and chassis turning envelope deviations exceeded 0.25 m. At the same time, the offset of left-hand-side design vehicle trajectories on the circulatory roadway (which was mostly between 0.05 and 0.15 m) could have a positive impact on the deflection around the central island. Nevertheless, the following should be remembered when utilizing this approach: when compared to the path with short, straight lines inserted between the adjacent arcs, the design vehicle body and chassis turning envelopes are more offset from the roundabout centers. This could result in the following:  
* Up to 0.35 m wider roundabout entrances, which is unfavorable in terms of traffic safety as it enables higher roundabout entry speed;
* Up to a 0.15 m larger central island radius, resulting in larger vehicle path deflection around the central island, which is favorable in terms of speed reduction;
* Up to 0.40 m wider roundabout exits, which enable higher roundabout exit speed and could help minimize the likelihood of congestion and crashes at the exits and the potential for trailers to track over the right carriageway edge;
* The need for lateral clearances larger than 0.30 m on roundabout entries and exits when the roundabout outer radius is smaller than 17.0 m.  
Conceptualization, S.A. and V.D.; methodology, S.A. and M.A.; formal analysis, S.A. and M.A.; investigation, S.A. and T.D.; resources, V.D.; visualization, M.A.; writing--original draft preparation, S.A.; writing--review and editing, M.A., T.D., and V.D.; supervision, V.D. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.