Urban Roundabouts Safety Analysis: The Case Study of Potenza city (Italy)

Roundabouts are widely appreciated as an effective way to regulate intersections, offering a lot of advantages in terms of functionality and safety. These include traffic calming, reduced delays, noise and environmental pollution mitigation, and the ability of performing U-turns. However, the roundabouts inclusion in existing urban areas is often constrained by limited surface availability, which can preclude the use of appropriate geometric dimensions and consequently lead to vehicle maneuverability issues, particularly for larger vehicles. This paper presents the study of four urban roundabout intersections located in Potenza a small city in the south of Italy. These roundabouts are positioned at the vertices of a rectangular grid that defines an important city square called “Piazza della Costituzione Italiana”. The surrounding area accommodates various commercial establishments, schools, public green areas and the headquarters of the Basilicata Regional Government, all contributing to a considerable traffic volume. The authors analyzed the geometric failures of these roundabouts in accordance with current regulatory standards, focusing on vehicle inscribability issues associated with different vehicle classes. The geometric and functional considered parameters include: road type, number of entry and exit lanes, number of circulating lanes, inscribed circle diameter, central island diameter, width of the mountable apron, width of the entry and exit lanes, width of the circulating lane, deflection path, and deviation angle. Vehicle inscribability analyses were performed using the “Swept Path Analysis” tool in Autodesk Vehicle Tracking®. The analysis’ results highlighted the difficulties encountered by various vehicle types in negotiating roundabout curves and identified geometric deficiencies that could be mitigated through geometric and functional retrofitting measures. In particular, the main geometric issues were found to relate to the reduced width of entry and exit lanes and to the inappropriate alignment of the intersecting branches. These factors negatively affect the maneuverability of heavy vehicles, unlike passenger cars and trucks with a mass of less than 7.5 tonnes. The study revealed that buses are unable to perform U-turns within any of the examined roundabouts. These findings confirm the widespread existence of “worst practices” based on the erroneous assumption that roundabouts can be implemented in urban contexts even when the required geometric and functional conditions are not satisfied. Another significant contribution of this study is its emphasis on the fundamental role played by swept path analysis software in the study and design of urban roundabouts. This is particularly relevant in retrofitting, where design standards become more performance-based and consequently less prescriptive for designers. Only roundabouts designed in full compliance with regulatory and design standards can effectively fulfill their intended roles in traffic regulation and traffic calming, thereby enhancing road safety in urban environments.