Unconventional roundabout geometries for large vehicles or space constraints

This paper investigates roundabouts with unconventional geometry (non-circular) that have been proposed during the last decade to address special constraints of space and traffic conditions. The paper presents an unconventional solution we call “two-geometry.” Two-geometry roundabouts have a circular central island surrounded by an oval circulatory roadway. The combination of inner circular and outer oval shapes allow a reduction in carriageway width on the longer section and enhancement of path deflection. The same geometric combination is useful for enlarging the carriageway width corresponding to lower radii of curvature where high-occupancy vehicles require additional turning space. Roundabouts with unconventional geometries are also investigated for better accommodating oversize/overweight large trucks (OSOW) on routes necessary to key industry and the economy. Strategies including paved areas behind curbs, enlarging truck aprons, use of minimum slopes and mountable curbs are presented. Future research is suggested for both data observation and capacity model calibration.