Determining Louisiana’s Roundabout Capacity

Codjoe, Julius; Thapa, Raju; Ayernor, Sam-Mark Ansah; Loker, Matthew · 2021 · ROSA P / Louisiana. Department of Transportation and Development

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Summary

This study addresses the need to calibrate roundabout capacity models for Louisiana, as national standards from the Highway Capacity Manual (HCM) may not accurately reflect local driver behavior. The research was motivated by the observation that driver behavior is intrinsic to specific regions and cannot be modified by design, necessitating local calibration to ensure optimal roundabout performance. Additionally, the study aimed to validate the Environment Factor (EF) currently used by the Louisiana Department of Transportation and Development (DOTD) in SIDRA software, which had not been verified against field conditions. The researchers collected field data from 41 approaches across 17 single-lane roundabouts throughout Louisiana. Using maximum likelihood estimation, they determined gap acceptance parameters, specifically follow-up time and critical headway. The study also explored correlations between these parameters and geometric features such as inscribed circle diameter, splitter width, and lane width. Delays, queue lengths, and levels of service were calculated using HCM 6th Edition equations. Furthermore, a SIDRA analysis was conducted to determine the appropriate EF by comparing software outputs with field-observed capacities. The results indicated that Louisiana drivers have an average follow-up time of 3.36 seconds and a critical headway of 4.76 seconds, compared to national HCM 6 averages of 2.60 seconds and 4.70 seconds, respectively. No significant differences in gap acceptance were found across DOTD districts or road functional classes, although arterials exhibited the lowest values. Geometric features did not show strong enough correlations to be included in the local model. The locally calibrated model, which adjusted the HCM 6 intercept based on local follow-up times, performed closer to the HCM 2010 model than the HCM 6 model, suggesting the latter overestimates capacity in Louisiana. The SIDRA analysis revealed that an EF of 1.06 minimized the root mean squared error for approach capacities for roundabouts with an average age of eight years. The study concludes that Louisiana roundabouts likely have slightly lower capacities than national averages due to higher follow-up times. Based on the EF of 1.06 for eight-year-old roundabouts, the authors recommend an EF of 1.10 for the Build Year in SIDRA analyses, rather than the previously used default of 1.2. This recommendation accounts for the tendency of EF values to decrease over time. The findings provide DOTD with a locally validated capacity model and SIDRA parameter, improving the accuracy of roundabout design and performance predictions in the state. Future research is recommended to develop innovative methods for directly collecting field-observed delays and queue lengths to further validate these parameters.

Key finding

Louisiana drivers exhibit higher follow-up times and critical headways than national averages, leading to a recommended SIDRA environment factor of 1.1 for single-lane roundabout designs.

Methodology

field_study

Sample size: 17

Provenance

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

Topics

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• intersection design
• traffic density