The SIDRA INTERSECTION software uses gap-acceptance methodology for roundabout capacity estimation where gap-acceptance parameters are estimated from the roundabout geometry. This means that a combined (hybrid) geometry and gap-acceptance modelling approach is used to take into account the effect of roundabout geometry on driver behaviour through gap-acceptance modelling.
This approach acknowledges the importance of roundabout geometry but affirms that roundabout geometry alone is not good enough and modelling of driver behaviour is needed for roundabout capacity estimation, just as it is needed for modelling the capacity of any other intersection type (signals, sign control).
There has been a lot of debate about the relative benefits of models based on “gap acceptance theory” vs “empirical models” in the past, with claims that gap-acceptance modelling does not work for roundabouts. Much of this debate has been misleading due to simplistic model categorization based on the suggestion that these modelling approaches are mutually exclusive. Opinion has also been expressed that it does not matter which one of these modelling approaches is used. This has overlooked the fact that SIDRA INTERSECTION has used the gap- acceptance method not only for capacity but also for performance estimation including unique equations for back of queue and stop rate based on modelling of gap-acceptance cycles.
A detailed table comparing the fundamental aspects of the different roundabout capacity models used in Australia, USA and UK is available here. The features of the roundabout capacity models compared include methodology, model level of detail considering entry and circulating lanes, parameters used in the model to represent driver behaviour and roundabout geometry, model calibration methods and many other aspects of modelling.
Numerous publications on roundabout capacity models are available for download via our Publications page.
A unique method for all gap acceptance situations
Gap Acceptance Capacity model using Movement Class parameters
The gap acceptance capacity model in the latest version of SIDRA INTERSECTION has been enhanced by applying the critical gap and follow-up headway parameters to each Movement Class separately.
For lanes that include more than one Movement Class, the shared lane capacity equation is applied using the Movement Class capacity values.
The method applies to all gap acceptance situations (roundabouts, sign control, permitted turns and Right Turn On Red at signals, bypass lanes by yield, merging at exit short lanes).
Most advanced software for roundabouts
SIDRA INTERSECTION is the most advanced software for roundabouts with many powerful features. It is the most widely used software tool in the USA for roundabout capacity and performance analysis.
Learn more by following the links below: