Evaluation of Alternative Intersections and Interchanges: Volume 1 — Roundabout Capacity and Rollover Analysis for Heavy Vehicles
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Summary
This study addresses the growing implementation of roundabouts on high-speed roads in Indiana, where the presence of heavy vehicles raises concerns regarding rollover safety and operational capacity. While roundabouts generally reduce crash severity, their performance on high-speed arterials with significant truck traffic is not well understood. The research specifically investigates heavy vehicle rollover propensity and entry capacity, examining factors such as circulatory superelevation, aggressive driver behavior, roundabout readability, and nighttime conditions. The researchers developed a generalized rollover model for heavy vehicles and applied it to field-observed semi-trailer speeds and paths at newly built roundabouts in Indiana. They introduced a metric, delta v, representing the difference between the critical rollover speed and the actual vehicle speed to estimate proximity to overturning. For capacity analysis, the study estimated critical and follow-up headways using binary probit models and mixed-logit models. Data were collected from roundabouts on both low- and high-speed state roads, analyzing driver gap-acceptance behaviors under various conditions, including daytime versus nighttime and single-lane versus dual-lane configurations. The findings indicate that the rollover propensity at the studied roundabouts was low, with no excessive risk detected even on high-speed roads. High approach speeds associated with aggressive driving did not significantly increase rollover propensity, and nighttime conditions did not elevate risk; drivers tended to be more cautious at night. The benefit of inward-sloped circulatory superelevation was deemed too small to justify changes to design practice. Regarding capacity, heavy vehicles significantly reduced entry capacity by increasing critical headways. Truck drivers accepted a critical headway 1.1 seconds longer than passenger car drivers. Nighttime conditions further reduced capacity by adding 0.6 seconds to the critical headway. Additionally, drivers on rural dual-lane roundabouts accepted headways 0.6 seconds longer than those on urban single-lane roundabouts. Consequently, single-lane roundabouts on low-speed roads in Indiana exhibited 30% higher capacity than national averages, while dual-lane roundabouts on high-speed roads showed 15% reduced capacity. The study concludes that current design policies for roundabouts do not require modification based on rollover risks, as safety margins remain adequate. However, the estimated critical and follow-up headways provide localized parameters that should replace default national values for capacity and level-of-service evaluations in Indiana. The authors note that findings are based on low to medium traffic volumes and recommend further study under heavier traffic conditions to refine these capacity estimates.
Key finding
Heavy vehicles increased critical headways by 1.1 seconds compared to passenger cars, resulting in reduced entry capacity, while nighttime conditions and dual-lane rural roundabouts each added 0.6 seconds to critical headways.
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