Capacity assessments are normally an integral part of a Transport Assessment but can be requested by the Highway Authority where they deem the impact of a development proposal on a junction to be material.
The industry standard computer software used for the junction modelling will vary according to the type of junction.
As a guide the following software packages are generally used;
Junctions 9 PICADY Module (TRL) for major/minor priority junctions
Junctions 9 ARCADY Module (TRL) for mini and larger non signalised roundabouts
LINSIG (JCT) for signalised junctions
LINSIG (JCT) or TRANSYT (TRL) for linked signalised junctions and small networks