# Speed Reduction Capabilities of Two-Geometry Roundabouts
## 1 Introduction
This iterative process can be simplified by designing the circulatory roadway and the right carriageway edge according to the results of the design vehicles' swept path analyses. These analyses are conducted with software for vehicle movement simulation today. This approach is an effective way to determine the optimal roundabout design elements because it incorporates the optimization technique rather than a trial-and-error process [17; 18; 19]. Today, several types of modern roundabouts represent an alternative to standard single-lane or double-lane roundabouts. They differ in one or more design elements, have different impacts on traffic safety, capacity, mobility, and the environment, and are either in usage worldwide (for example, hamburger, turbo, and peanut roundabouts) or still in the development phase (for example, flower, target, and elliptical roundabouts and roundabouts with segregated left-turn slip-lanes) [20; 21; 22]. One such alternative modern roundabout is the so-called two-geometry roundabout. This roundabout has variable circulatory roadway width due to the different roadway edge shapes: the outer edge is elliptical, and the inner edge, which is defined by the shape of a central island, remains circular [22]. This peculiarity differentiates them from elliptical roundabouts, i.e., roundabouts with elliptical outer and inner edges. Its circular central island should provide increased deflection of personal vehicle paths which results in lower vehicle speed [22]. As the number of constructed two-geometry roundabouts is still low, there is not yet enough experimental data collected to make a robust comparison to other modern roundabout types [22]. Therefore, computer simulations must be conducted to determine the optimum geometry of these intersections and their overall applicability in the road network. The main gap to be filled is the influence of the variable circulatory roadway width (which can be significantly wider compared to the standard single-lane roundabouts [22]) on the design of other geometric elements. Additionally, their impact on traffic safety, i.e., the vehicle path deflection around the central island these roundabouts provide, and, consequently, their capability for personal vehicles' speed reduction and consistency should be investigated. In this paper, 40 four-legged single-lane roundabout schemes were designed in the Autodesk AutoCAD 2021 software utilizing computer simulations of vehicle movement and the resulting swept paths generated by the Autodesk Vehicle Tracking 2021 software. The reliability of this software is discussed in the paper [23], where the simulated swept path widths are compared with the ones determined using a precise GNSS device at a test site. The results of this investigation showed that from a statistical point of view, these swept path widths did not differ, so the software can be used for further investigation. The design vehicle selected for the investigation presented in this paper was a tractor with a semi-trailer. This long, articulated vehicle was selected because these types of vehicles face issues while maneuvering the roundabout due to their large turning envelope requirements [19; 24]. The investigation included the determination of the vehicle path deflection provided by the designed central island and the determination of passenger vehicle speed through the designed roundabout schemes based on the following approaches: (1) by measuring the deflection provided by the designed roundabout's geometrical features; (2) by measuring the designed roundabout's geometrical features and calculating the fastest path radii and vehicle speed using the CROW model; (3) by constructing the fastest through paths, measuring the path radii upon entry, around the central island, and upon exit (according to the FHWA model) and then calculating the vehicle speed for each location using the CROW model. The aim was to determine whether the designed two-geometry roundabout elements provide the appropriate vehicle speed reduction.