Evaluation of the roundabout capacity model in HCM6 edition and HCM 2010 on a multilane roundabout

This study evaluated the performance of the new Highway Capacity Manual 6th Edition (HCM6) roundabout capacity model at a multilane roundabout in Richfield, Minnesota. Traffic flow rates and gap acceptance data were collected during 20 hours of afternoon peak period traffic over four days. The observed critical headways for the left and right lanes was equal to 4.43 and 3.99 s while the observed follow-up headways were 3.05 and 2.96 s, respectively. Roundabout capacity curves for both lanes were built through the collected flow information, as well as through the observed headways and the HCM formula. As compared with the field observed results, the default model in HCM6 overestimated the capacity of the study roundabout. The default model in HCM 2010 closely estimated the capacity for the right lane, but overestimated the capacity of the left lane when the circulating flow rate was high. The HCM6 model was calibrated with the observed critical and follow-up headways, as well as by calibrating the intercept of the model using only the follow-up headway. The fully calibrated model overestimated the capacity of the right lane by 7-10% as the circulating flow decreases and overestimated the capacity of the left lane by 7% and 31%. The partially calibrated model overestimated the capacity of the left lane by 6% and overestimated the capacity of the right lane by at most 10% under low circulating flow rate, but it underestimated the capacity by at most 21% under high circulating flow rate.