The recently released HCM 2010 provides new capacity models for single-lane and multilane roundabouts, whichgenerally predict lower capacity compared to other popular models previously used in the US. Acknowledging thatboth roundabout design practices and the public’s use of roundabouts are still maturing in the US, the HCM 2010recommends local calibration of the capacity models to best reflect local behavior and specifies a calibrationprocedure using two gap acceptance parameters: the critical headway and the follow-up headway.Using this calibration procedure in a case study, this paper developed a calibrated HCM model based on the driverbehavior at three single-lane roundabouts in Carmel, Indiana. It was found that both gap acceptance parameters inCarmel are significantly lower than the default values in HCM 2010. For validation purposes, the calibrated HCMmodel was compared to the actual 1-minute flow rates collected in the field during the congested periods. It wasconcluded that the calibrated HCM model is consistent with the field conditions for the low to medium circulatingflow rates, and overestimate the capacity for the high circulating flow rates. Other findings of this paper include theimpact of the exiting traffic, the circulating flow rates, the degree of saturation, and the amount of the gapacceptance data on the calibration results.
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