There is increased interest in building modern roundabouts in Europe, Australia and North America because of the safety and operational benefits that can be realized from this type of intersection. With this interest comes the increasing need to be able to estimate the capacity, evaluate the performance and analyze the emission levels that a roundabout will be likely to provide once it is built. In the early 2000s, a National Cooperative Highway Research Program (NCHRP) study was conducted that, among other tasks, developed single-lane roundabout capacity estimation equations. These equations, presented in the Highway Capacity Manual 2000 and 2010 (referred to as HCM 2000 and 2010 hereafter), can be calibrated using locally determined values for the critical gap and follow-up time. Other models, such as the German Highway Capacity Manual (referred to as GHCM) and SR45 models are also widely used in estimating capacity around the world. However, some studies have indicated that the existing models could under or overestimate roundabout capacity because some important factors, such as headway distribution type, proportion of exiting vehicles and heavy goods vehicles, are not taken into account.