6. General Criteria for Calculating the Capacities of Alternative Types of Roundabouts

Roundabouts are an increasingly common form of junction worldwide and their effective design requires detailed analysis of maximum vehicle throughput capacities. Since the 1970s, a series of models have been developed worldwide for an estimation of the capacity of roundabouts, almost all of which have relied upon extensive empirical data due to the complexity of the physical and behavioral processes affecting roundabout entry capacities. However, given the different fundamental principles (and particularly the geographical origins) of models, it is important to have a clear understanding of their limitations and their applicability within new contexts. These models are based on three main methodologies: empirical, gap acceptance and simulation. Due to their limitations, each of these methodologies on their own cannot completely explain the complex behavioral and physical processes involved at roundabout entries, hence all models require strong semi-empirical or fully-empirical bases using data obtained from their countries of origin.