Impact of Geometric Parameters and Operational Speed on Traffic Safety at Roundabouts: A Conflict Analysis Using Microsimulation Models

Klobučar, Mirna; Šurdonja, Sanja; Deluka-Tibljaš, Aleksandra · 2025 · DOAJ

DOI: 10.33492/JRS-D-25-3-2681114

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Summary

This study investigates the relationship between geometric design parameters, operational vehicle speeds, and traffic safety at single-lane, medium-sized urban roundabouts. The research was motivated by the prevalence of underreported crashes in Croatia, which renders historical crash data unreliable for safety assessment. Consequently, the authors sought to evaluate safety through surrogate measures—specifically traffic conflicts—rather than actual collision records. The study focuses on nine roundabouts in Rijeka, Croatia, aiming to determine how specific design elements, such as entry/exit widths, central island size, and circulatory road width, influence vehicle behavior and conflict frequency. The methodology combined field data collection with microsimulation modeling. Traffic volume and speed data were gathered using radar counters over 24-hour periods on weekdays, capturing daily operating speeds (v85) and traffic volumes ranging from 12,000 to 24,000 vehicles per day. Geometric parameters were extracted from design drawings. Using VISSIM software, the researchers created individual microsimulation models for each roundabout, simulating 24-hour traffic flows to generate vehicle trajectories. These trajectories were analyzed using the Surrogate Safety Assessment Model (SSAM) to identify and classify potential conflicts. Conflicts were defined using Time to Collision (TTC) thresholds and categorized by type, such as crossing or rear-end, based on conflict angles and speed differentials. The results indicate that geometric parameters significantly impact both vehicle speeds and conflict types. Wider entry and exit widths were associated with reduced conflict occurrences, likely by facilitating smoother traffic flow. Conversely, larger central islands were linked to a higher frequency of rear-end conflicts, suggesting that while they may enforce lower speeds, they can create bottlenecks or driver hesitation. The width of the traversable apron was found to affect exit speeds, while wider circulatory roads led to an increase in lane-change conflicts. Despite these geometric influences, daily traffic volume remained the most influential factor in determining overall roundabout safety. The study also noted significant variability in speed compliance, with approximately 40% of vehicles exceeding posted speed limits at certain high-volume approaches. The significance of this research lies in its validation of microsimulation and SSAM as effective tools for proactive safety assessment in the absence of reliable crash data. The findings underscore the importance of tailoring roundabout designs to local traffic conditions. Specifically, the study suggests that designers must balance geometric elements to control vehicle speeds without inadvertently increasing specific conflict types, such as rear-end collisions caused by large central islands. By providing evidence-based insights into how specific geometric features affect safety, the study offers practical guidance for optimizing roundabout design to enhance traffic safety in urban environments.

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• intersection design