United States Department of Transportation
i
Accelerating Roundabout Implementation in the United States - Volume II of VII: Assessment of Roundabout Capacity Models for the Highway Capacity Manual
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2015-09-01
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[PDF-1.31 MB]
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Abstract:This volume is second in a series of seven. The other volumes in the series are: Volume I - Evaluation of Rectangular RapidFlashing Beacons (RRFB) at Multilane Roundabouts, Volume III – Assessment of the Environmental Characteristics of Roundabouts, Volume IV – A Review of Fatal and Severe Injury Crashes at Roundabouts, Volume V – Evaluation of Geometric Parameters that Affect Truck Maneuvering and Stability, Volume VI – Investigation of Crosswalk Design and Driver Behaviors, and Volume VII – Human Factor Assessment of Traffic Control Device Effectiveness. These reports document a Federal Highway Administration (FHWA) project to investigate and evaluate several important aspects of roundabout design and operation for the purpose of providing practitioners with better information, leading to more widespread and routine implementation of higher quality roundabouts. This report presents an assessment and development of updated roundabout capacity models for implementation in the major update of the HCM 2010. The roundabout capacity models in the HCM 2010 are based on data collected in 2003 as part of NCHRP Report 572; this task is based on a new set of capacity data collected in 2012. A total of 819 usable minutes of data at single-lane roundabouts, 711 minutes of usable data were obtained for the multilane case for two entry lanes conflicting with two circulating lanes, and 519 minutes of usable data were obtained for the multilane case for two entry lanes conflicting with one circulating lane. Parametric analyses were conducted to extract and review critical headway, follow-up time, and key geometric parameters. The new data confirmed that the HCM 2010 generally underestimates capacity. The most promising models involve recalibrating the HCM 2010 models with an intercept that is calibrated to follow-up time and a slope parameter based on regression of the data. Additional calibration factors such as critical headway or geometric parameters beyond number of lanes were not found to add significant improvement.
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