Empirical Estimation of Capacity for Roundabouts Using Adjusted Gap-Acceptance Parameters for Trucks

Abstract

This study examines the effect of heavy vehicles (trucks) on the entry capacity of roundabouts. Vehicle movements were observed at 11 roundabouts in Vermont, Wisconsin, and Ontario, Canada, and gap-acceptance parameters were estimated for cars and trucks separately. Consistent with previous studies, it was found that the critical headway and the follow-up time were longer for trucks than for cars. It was also found that the follow-up times for truck-involved vehicle-following cases were associated with the central island diameter and the entry angle. The gap-acceptance parameters for all entering vehicles were adjusted to a volume-weighted average of the gap-acceptance parameters for cars and trucks. The capacity was estimated with the existing capacity models with the adjusted gap-acceptance parameters and compared with the observed capacity at three roundabouts. It was found that the rate of reduction in the observed capacity with an increase in the circulating flow was lower at the roundabouts with a higher truck percentage. Also, the capacity models with the adjusted gap-acceptance parameters estimated the capacity more accurately than did the models with the unadjusted parameters. The study underscores the importance of considering the effect of trucks on capacity for the roundabouts with a high truck volume.