Unconventional Roundabouts: Third-Generation Insights from the United States and Europe

This paper presents a comparative analysis of unconventional roundabouts, commonly referred to as third-generation roundabouts, based on case studies from the United States and Europe, specifically Croatia and Italy. These intersection designs deviate from traditional circular geometries to overcome limitations in safety, capacity, and spatial integration, especially in constrained or high-demand environments. The study focuses on three major typologies: raindrop, turbo, and two-geometry roundabouts (TGRs), examining their geometric characteristics, operational principles, and context-specific implementations. Based on real-world examples and qualitative assessments, each national section investigates design rationale and performance considerations in relation to local traffic dynamics. The paper contributes to the understanding of adaptive and context-sensitive intersection design, offering a conceptual framework for comparing unconventional roundabout typologies across different regulatory environments. The comparative analysis reveals that each typology responds to specific operational and spatial constraints rather than representing a universally optimal solution. In particular, raindrop and dog-bone roundabouts are most effective in interchange contexts, turbo roundabouts enhance safety and capacity in regulated multilane environments, while TGRs provide greater adaptability in constrained and irregular urban settings. These findings highlight the importance of context-dependent design strategies and support the need for flexible and context-sensitive evaluation frameworks.