TRANSYT 16, TRANSYT 15 or TRANSYT 14: Subject to an ARCADY licence, you can specify your roundabout geometries directly in TRANSYT, allowing partial-signalled, fully signalled or unsignalled roundabout models to be compared without touching ARCADY.
TRANSYT 13 or earlier: If you are modelling a give-way within a partially signalled roundabout within TRANSYT, you will need to obtain give-way coefficients (TRANSYT maximum flow and slope coefficients) from somewhere outside of TRANSYT.
These parameters represent the slope and intercept of a priority/give-way stream capacity relationship and these are exactly the numbers which ARCADY provides in its output. So accurate give-way parameters (“Card Type 30” in TRANSYT 12) can be obtained by modelling the give-way part(s) of the roundabout using ARCADY. You do not need to model the whole of the roundabout, or bother about the flows – ARCADY only needs to know the geometry of the entry arm in question in order to calculate the slope and intercept of the capacity relationship.
For grade-separated/very large roundabouts, circulating flows for the central 30 minutes are required in addition to the normal geometric parameters (other flows can be any value).
Where the roundabout entry has lanes marked for different movements, this, in itself, causes no problem. However, if the entry is starved of traffic to any extent (by vehicles being stuck in the queue unable to reach the give-way line) then ARCADY can be optimistic about capacity. Note that starvation has to be substantial for significant overestimation – even if a single vehicle can reach the give-way to form a ‘queue’ in the least-used lane, is enough for the entry to reach full capacity. Where there is sustained starvation, modelling just part of the give-way would be one way of compensating. However, as far as using the ARCADY results in TRANSYT is concerned, a good estimate should suffice for most situations since the priority junction would not normally be one of the critical factors in determining good signal timings; important junctions would normally be signalled.