Evaluation of safety and mobility of two-lane roundabouts : final report.

Parikh, Gordon; Hourdos, John · 2017 · ROSA P / Minnesota. Dept. of Transportation

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Summary

This study addresses the safety challenges associated with two-lane roundabouts, specifically focusing on the high incidence of property-damage-only crashes caused by driver errors. While roundabouts generally reduce fatal and severe-injury crashes compared to signalized intersections, multilane configurations often fail to provide similar benefits for less severe crashes. The research aims to identify specific design features and traffic control interventions that mitigate yielding violations and turn violations, which are primary contributors to these collisions. The study builds upon previous findings from a single site in Richfield, Minnesota, by expanding data collection to four additional sites to determine if effective solutions can be generalized. The researchers selected four roundabouts in Minnesota: two fully two-lane (2x2) sites in Lakeville and Mankato, and two partially two-lane (half-2x2) sites in St. Cloud and Mankato. Data collection involved deploying video cameras to record traffic over several months. Unlike previous studies that relied on manual video review, this project utilized automated computer vision software (TrafficIntelligence) to extract vehicle trajectories, speeds, and turning movements. This automation allowed for precise identification of violations, including failures to yield and improper turns (e.g., right turns from the inner lane). The study also analyzed the impact of specific design elements, such as upstream lane geometry and overhead lane designation signs, and evaluated before-and-after effects of traffic control interventions at the Mankato sites. The results indicate that turn violation rates are significantly influenced by geometric design and signage. Specifically, approaches with single-lane upstream roads exhibited fewer left-from-outer-lane violations, suggesting drivers instinctively align with the curb. Furthermore, the presence of overhead lane designation signs at the Mankato roundabouts correlated with substantially lower turn violation rates compared to sites without such signage. However, the study found no successful intervention for reducing yield violations. Despite changes to traffic controls at the Mankato sites, failure-to-yield rates remained high or unchanged. Yield violations were most frequent when drivers failed to yield to vehicles in the inner lane of the circulating traffic. The TH-22 northbound approach in Mankato showed significantly lower yield violation rates, but this was attributed to having only one lane of cross traffic to yield to, rather than a specific design intervention. The significance of this research lies in its confirmation that specific design features can mitigate certain driver errors in multilane roundabouts. The findings suggest that overhead lane designation signs and single-lane upstream approaches are effective strategies for reducing turn violations. Conversely, the study highlights a persistent challenge in addressing yield violations, as no tested design or traffic control measure successfully reduced their frequency. These conclusions provide actionable insights for transportation agencies aiming to balance mobility and safety in multilane roundabout designs, emphasizing the need for further research into interventions that specifically target yielding behavior.

Key finding

Overhead lane designation signs and single-lane upstream approaches reduce turn violations, but traffic control interventions failed to significantly reduce yield violations.

Methodology

field_study

Sample size: 4

Provenance

The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_rosap on 2026-05-23 (6 acquisition events logged).

Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• intersection design

Information type

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• Empirical Findings: crash risk outcomes