Evaluating the performance and safety effectiveness of roundabouts.
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Summary
This report evaluates the performance and safety effectiveness of roundabouts within Michigan, commissioned by the Michigan Department of Transportation (MDOT) to inform future design and budget decisions. The study was motivated by the increasing popularity of roundabouts in the United States and the need for Michigan-specific data to quantify their safety benefits, operational impacts, and return on investment, particularly regarding local climate conditions and geometric configurations. The methodology involved a comprehensive four-phase approach. First, the researchers conducted a literature review and a best practices review of other municipalities. Second, they identified roundabouts across the state and conducted site visits to a select group, collecting operational data (traffic counts, speed studies) and conflict data (surrogate safety measures). Third, they gathered historical crash data from the Michigan State Police database for the period between January 1, 2001, and December 31, 2010, for both treatment sites and untreated reference sites. Finally, the analysis utilized a simple before-and-after comparison alongside a rigorous Empirical Bayes (EB) methodology. The EB analysis accounted for regression-to-the-mean and traffic volume changes to develop Safety Performance Functions (SPFs) and Crash Modification Factors (CMFs). Operational impacts were assessed using RODEL software, and economic benefits were calculated using FHWA crash cost values. The findings demonstrated significant safety improvements. The EB analysis yielded specific CMFs for various intersection types converted to roundabouts; for instance, converting signalized intersections to three-lane roundabouts resulted in a CMF of 0.783 for all crashes and 0.300 for injury crashes, indicating substantial reductions. The study established the first Michigan-specific SPFs and CMFs for roundabouts. Site visits identified specific operational issues, such as lane discipline in multi-lane roundabouts and vehicle yielding failures, which were rated using the Collision Risk Assessment Method. Economic analysis revealed average cost savings derived from reduced crash frequencies and delays, with calculated times of return for different roundabout types. The significance of this study lies in its provision of localized, statistically robust data for MDOT. By developing state-specific SPFs and CMFs, the report allows for more accurate safety predictions for future projects. The identification of specific operational issues and their risk ratings supports targeted mitigation strategies. Furthermore, the economic analysis provides evidence of the cost-effectiveness of roundabout conversions, supporting their continued implementation as a viable alternative to signalized and stop-controlled intersections in Michigan.
Key finding
Empirical Bayes analysis demonstrated significant crash reductions across Michigan roundabouts, with Crash Modification Factors for all sites showing a 34.6% reduction in total crashes and a 41.7% reduction in injury crashes compared to pre-conversion conditions.
Methodology
field_study
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).
| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | rosap | — | — | 2 | 2026-05-23 |
| archive | success | — | — | — | 1 | 2026-05-23 |
| extract | success | cached | — | — | 2 | 2026-06-10 |
| clean | success | — | — | — | 1 | 2026-06-01 |
| chunk | success | — | — | — | 1 | 2026-06-01 |
| embed | success | — | — | — | 1 | 2026-06-02 |
| enrich | success | — | — | — | 1 | 2026-05-23 |
| promote | success | — | — | — | 1 | 2026-05-23 |
| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 3 | 2026-06-10 |
| tag | success | vector_similarity | — | — | 19 | 2026-06-11 |
| verify | success | — | — | — | 2 | 2026-06-10 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.
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- Empirical Findings: crash risk outcomes