Evaluating the Performance and Safety Effectiveness of Roundabouts – An Update
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Summary
This study evaluates the performance, safety effectiveness, and economic benefits of roundabouts in Michigan, updating a 2011 assessment by the Michigan Department of Transportation (MDOT). With over 180 roundabouts now installed in the state, the research aims to provide updated guidance for road agencies on converting conventional intersections to roundabouts. The investigation covers driver behavior, safety outcomes, operational efficiency, and environmental impacts. The methodology involved three primary phases. First, field data were collected at 18 roundabout sites to analyze driver speed selection, gap acceptance/rejection, and yielding behavior toward both circulating traffic and pedestrians. Second, a comprehensive safety analysis was conducted using data from all 180 known roundabouts and comparable reference intersections. This included naïve before-after analyses, empirical Bayes (EB) methods to estimate crash modification factors (CMFs), and cross-sectional analyses. Third, operational and economic evaluations assessed delay, level of service, fuel consumption, and benefit-cost ratios for select sites. Key findings indicate that roundabout geometry significantly influences driver behavior. Drivers accepted smaller gaps at multilane, three-legged, and rural roundabouts, while yield rates toward pedestrians and circulating traffic were lowest at interchange locations. Safety analyses revealed that converting intersections to roundabouts significantly increases the total number of crashes but substantially reduces fatal and injury crashes. EB analysis yielded CMFs of 2.10 for total crashes and 0.92 for fatal/injury crashes across all types. Specifically, single-lane roundabouts showed a CMF of 0.60 for fatal/injury crashes, whereas triple-lane roundabouts showed higher crash increases. Cross-sectional analysis confirmed that roundabouts have 58% more total crashes but 27% fewer fatal/injury crashes than conventional intersections. Safety performance functions were developed for three-legged and four-legged roundabouts with varying lane configurations. Operationally, roundabouts reduced user delay by 57–67% and improved level of service. Economic analysis estimated annual savings of $67 per vehicle in delay costs and $2.3 per vehicle in fuel costs. Benefit-cost analyses demonstrated that the benefits of roundabout conversion significantly outweigh construction costs, with positive benefit-cost ratios across evaluated sites. The study concludes that roundabouts are effective in improving safety by reducing crash severity and enhancing operational efficiency, particularly at intersections with high rates of severe crashes. However, special considerations are recommended for roundabouts at interchanges due to lower yielding rates and higher approach speeds.
Key finding
Converting intersections to roundabouts significantly increases total crash frequency but reduces fatal and injury crashes, while also providing substantial operational benefits through reduced delay and fuel consumption.
Methodology
mixed_methods
Sample size: 180
Provenance
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| 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