An alternative type of modern roundabout with variable circulatory roadway width, the so-called two-geometry roundabout, has an elliptical outer carriageway edge and a circular inner carriageway edge. This variable width results in a roundabout that is less bulky than standard modern roundabouts and therefore more easily placed in locations with space constraints. Previous research has shown that these roundabouts must be equipped with truck aprons to guarantee vehicle trajectory deflection linked width the required personal vehicle speed reduction. The investigation presented in this paper aimed to determine the influence of truck apron design on two-geometry roundabouts’ personal vehicle speed reduction capabilities and relative speed. The investigation was based on computer simulations of vehicle movement and the resulting swept paths of a tractor with a semitrailer generated by the Autodesk Vehicle Tracking 2020 software. 32 four-legged single-lane roundabout schemes were designed in the Autodesk AutoCAD 2021 software by varying lengths of (1) the major axis from 18 to 25 m, with a 1-meter increment, and (2) the minor axis from 0.75 to 0.90 times the major axis length, with an increment of 0.05. Additional eight four-legged single-lane roundabout schemes were designed to represent a standard modern roundabout by varying the outer radius from 18 to 25 m, with a 1-meter increment. The truck aprons were designed to accommodate the 5-meter-wide circulatory lane. The expected driving speed through the roundabouts was estimated by constructing the fastest paths, measuring the path radii, and calculating driving speed according to the FHWA model. The investigation resulted in relative speeds between conflicting traffic streams and consecutive geometric elements, whose consistency can help to improve traffic safety at intersections.

computer simulation; swept path; long vehicle; path deflection; fastest path