Roundabout - Capacity and Delays

Capacity and Delays

The capacity of a roundabout varies, based on the number of entry and circulating lanes, and also on more subtle geometrical elements, including entry angle and lane width. Also, as with other types of junctions, the operational performance of a roundabout depends heavily on the flow volumes from various approaches. A single-lane roundabout may be expected to handle approximately 20,000 to 26,000 vehicles per day, while a two-lane roundabout may be expected to handle 40,000 to 50,000 vehicles per day.

Under many traffic conditions, an unsignalised roundabout may operate with less delay to users than traffic signal control or all-way stop control. Unlike all-way stop intersections, a roundabout does not require a complete stop by all entering vehicles, which reduces both individual delay and delays resulting from vehicle queues. A roundabout may also operate much more efficiently than a signalised junction because drivers are able to proceed when traffic is clear without the delay incurred while waiting for the traffic signal to change. These advantages also reduce air pollution from many idling vehicles waiting for traffic lights to change.

Roundabouts can increase delays in locations where traffic would otherwise not be required to stop, however, for example, at the junction of a high-volume and a low-volume road, traffic on the busier road would normally not have to stop if the junction were signalised, because the traffic signals would provide a green signal to the busier road the majority of the time. When the volumes on the approach roadways are relatively balanced, a roundabout can reduce delay because each approach would otherwise encounter a red signal greater than half of the time if the junction were signalised.

Roundabouts can also reduce delays for pedestrians when compared to traffic signals, because pedestrians are able to cross during any safe gap rather than waiting for the traffic signal to provide the right-of-way to the pedestrian. During peak hours of congestion when large gaps are infrequent, the very slow speed of traffic entering and leaving the roundabout can compensate for the smaller gaps and facilitate pedestrian crossings.

Major research on the capacity of roundabouts has been carried out in several countries including the United Kingdom, Australia, United States, Germany, and France. This research has led to the development of several software packages to help professionals with calculating capacity, delay, and queues at roundabouts. The software packages available in the UK, Australia and USA include ARCADY, Rodel, Highway Capacity Software, and Sidra Intersection.

In the U.K., the Transport Research Laboratory (TRL) developed a mathematical model throughout the 1960s and 1970s to facilitate traffic engineers with the necessary knowledge to engineer and design roundabouts. This was delivered in the form of the ARCADY software. The U.K. TRL roundabout capacity model is also the basis of the Rodel software.

Research on Australian roundabouts was conducted in the 1980s at the Australian Road Research Board (ARRB). The analytical capacity and performance models developed as a result of this research differ from the U.K. TRL model significantly, especially in adopting a lane-based empirical model based on gap-acceptance theory incorporating the effect of roundabout geometric parameters, rather than an approach-based empirical model based on roundabout geometric parameters only. The method was adopted in Austroads guides and implemented in the Sidra Intersection software package. The Australian roundabout capacity model has been under continuous development through revisions implemented in the Sidra Intersection software including a method for the analysis of roundabout metering signals.

Research on U.S. roundabouts sponsored by the Transportation Research Board (TRB) and Federal Highways Administration (FHWA) culminated in a roundabout capacity model being included in the Highway Capacity Manual (HCM) 2010 edition. and the TRB - FHWA Roundabout Informational Guide (NCHRP Report 672) The HCM 2010 model is also a lane-based empirical model, based on gap-acceptance theory.

The roundabout capacity models developed in different countries may give significantly different capacity estimates as they reflect different local driving conditions and employ different modelling principles.