## 1 Introduction
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 approach (hereinafter referred to as A-1) implies the application of short, straight lines (5 to 10 m long) inserted between the adjacent arcs. By applying these short lines, the vehicle path is simulated in the time it takes for the driver to turn the vehicle's steering wheel when changing the direction of movement, which results in a realistic representation of the vehicle's movement trajectories [9]. The second approach to the construction of the path (hereinafter referred to as A-2) implies the application of consecutive circular arcs without straight lines inserted between them, following the principles of hairpin bend geometry [9; 11]. This approach is simpler and faster than the A-1, as fewer geometric elements are required for the construction of the path, and it is in line with the field test results discussed in [12], which have shown that at a low driving speed (under 20 km/h) the steering gear of long, articulated vehicles can follow a steering path lacking transitions without significant departures. In this paper, the investigation of the FACPP deviations for paths constructed according to the two aforementioned approaches is presented. The method of the computer simulation of vehicle movement using the Autodesk Vehicle Tracking 2020 computer software was used to define the swept paths. The reliability of chosen software was evaluated and discussed in the paper [13], where a comparison between the swept path widths determined using a vehicle movement simulation and the swept path widths determined using a precise GNSS device at the test site was given. The results of this investigation showed that from a statistical point of view, these swept path widths did not differ. For the investigation presented in this paper, a tractor with a semi-trailer was chosen as a design vehicle. This long, articulated vehicle was selected because these types of vehicles face issues while maneuvering the roundabout due to their large turning envelope requirements [8; 9]. The investigation was conducted on 25 single-lane roundabout schemes with different outer radii (13-25 m), created in AutoCAD software. The investigation aimed to define the applicability of approach A-2 in the design of a right carriageway edge on roundabouts because its construction is simpler and faster than the construction according to approach A-1. The results showed that the A-2 approach is applicable in the design of the right carriageway edge at single-lane roundabouts for chosen design vehicle, but the following should be noted: compared to the A-1 approach, the design vehicle body and chassis turning envelopes derived from the A-2 approach is more offset from the roundabout centers. This results in larger roundabout entry and exit widths as well as the need for larger lateral clearances for entries and exits on roundabouts where heavy pedestrian traffic on sidewalks adjacent to the roundabout is expected. Wider entrances are unfavorable in terms of traffic safety because they enable higher entry speeds [8; 14]. At the same time, wider roundabout exits enable higher roundabout exit speeds, which could help minimize the likelihood of congestion and crashes at the exits and the potential for trailers to track over the right carriageway edge [8]. An offset of the left-hand-side design vehicle body and chassis trajectories on the circulatory roadway could have a positive impact on the vehicle path deflection around the central island, which is favorable in terms of vehicle speed reduction [8; 9; 15; 16]. The paper is organized as follows. Input parameters and methods used in the investigation are presented in Section 2, and in Section 3, the results of the investigation are given.