When a roundabout is specified as large, grade separated, or terminates in a dual carriageway or motorway ARCADY uses a slightly different equation to calculate the capacity. This equation uses the two new parameters, Qr (Mean Circulating Flow) and SEP (Separation Distance). The mean circulating flow can, in fact have the effect of either increasing or reducing the capacity – it all depends on the instantaneous value of Qc which is calculated within the model from the demand data and geometric data. If the instantaneous value of Qc is high then the effect of a high Qr is lightly to increase the capacity and vice-versa if Qc is low.
The effect on the entry capacity is thought to be due to drivers behaving according to the conditions they become accustomed to encountering at the junction in question, e.g. if they have to contend with high circulating flows on a regular basis they may be more aggressive towards finding a gap in the traffic.