Investigating the Deviation Angle Method for Ensuring Deflection at One-Lane Rural Roundabouts

Berloco, Nicola; Colonna, Pasquale; Intini, Paolo; Ranieri, Vittorio · 2018 · Crossref

DOI: 10.7250/bjrbe.2018-13.407

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Summary

This study addresses the lack of uniformity in international roundabout design standards, specifically focusing on methods used to ensure vehicle path deflection for speed control. While various countries employ different geometric parameters—such as deflection radius, entry path radius, or deviation angle—to manage traffic speeds, the deviation angle method is rarely used, appearing primarily in Italian and Swiss guidelines. The research aims to evaluate the effectiveness of the deviation angle method compared to more common approaches, improve the applicability of current standards, and provide practitioners with predictive tools to determine geometric requirements early in the iterative design process. The methodology involved a comprehensive simulation of over 7,000 hypothetical one-lane rural roundabouts using Computer-Aided Design (CAD) software. These designs adhered to Italian geometric standards, varying key parameters including the inscribed circle diameter (ICD), approach lane width, entry angle, and specific radii for entry, exit, and approach legs. The study calculated the deviation angle ($\beta$) for each configuration to determine the minimum geometric conditions required to satisfy the Italian standard’s recommendation of $\beta \ge 45^\circ$. Additionally, a subset of these roundabouts was analyzed using the entry path radius method (based on US guidelines) and the lateral displacement method (based on German guidelines) to facilitate a direct comparison of their geometric constraints and effectiveness. The results indicate that the deviation angle is strongly influenced by the inscribed circle diameter and the entry angle; specifically, larger ICDs and smaller entry angles facilitate higher deviation angles. The study found that the Italian recommendation of $\beta \ge 45^\circ$ is not achievable for mini roundabouts or many compact roundabouts, regardless of the entry angle, and fails for entry angles below 140 degrees. When compared to other methods, the deviation angle approach was found to be more effective at ensuring deflection than the entry path radius method in certain configurations. The analysis revealed that while the entry path radius method is widely used, it may allow for geometries that do not adequately control speed compared to the stricter constraints imposed by the deviation angle method. The significance of this work lies in its validation of the deviation angle method as a robust tool for ensuring vehicle deflection and speed reduction at rural roundabouts. The findings suggest that current Italian standards may need revision to account for the geometric limitations of mini and compact roundabouts, where the 45-degree threshold is unattainable. By establishing clear relationships between geometric parameters and deflection outcomes, the study provides engineers with a framework to predict compliance with deflection requirements before finalizing designs, thereby reducing the iterative nature of roundabout planning and promoting safer, more consistent intersection designs.

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• intersection design