ARCADY 8 and above includes an Entry Lane Simulation feature, which does take account of the number of lanes on an approach.
A certain amount of variable lane usage is built into the original empirical ARCADY model. The original research looked at a large amount of data and investigated whether including the number of lanes made the model any more accurate, and found that it did not; the main geometric features such as entry width were far more important.
However, if there is persistent and obvious unequal lane usage (for example if a dedicated left-turn lane is not used because there are no left-turning vehicles), this is NOT included in the model, and must be accounted for by the user. We would recommend the use of ARCADY 8 or above in Lane Simulation mode to investigate such behaviour.