Multi-criteria comparative assessment of unconventional roundabout designs

Abstract

Roundabout design has been evolving to include versatile types that target specific characteristics of each considered location. They started addressing maneuverability issues and directional splits, among other factors. Therefore, research is required to investigate the functionality and performance gains associated with each new design element. This study was conducted to provide a comparative assessment of three roundabout types (modern, elliptical, and turbo) using microsimulation. The multi-criteria assessment considered three evaluation criteria, namely, safety, mobility, and environmental impacts, for different traffic and pedestrian volumes. The findings indicated that the three roundabout types provided substantial safety and environmental gains when compared to a signalized intersection, while the mobility gains were less significant. Among the three roundabout types, the turbo roundabout outperformed the other two types at low and medium traffic flow levels (≤3000 pcu/h) in terms of safety and mobility, while the elliptical roundabout performed better at high congestion scenarios (>3000 pcu/h). On the other hand, the elliptical roundabout had the worst environmental performance at all traffic volumes. In-depth safety assessment that was conducted using simulated traffic conflicts revealed that 75% of conflicts on roundabouts occurred between approaching vehicles, while only 25% occurred between circulating vehicles. Still, circulating conflicts were considerably more severe. The three roundabouts were especially effective in reducing pedestrian-vehicle conflicts, they also reduced rear end conflicts but to a lesser degree. Moreover, the elliptical roundabout experienced higher severity of conflicts, even when the conflict frequency was lower.