In order to decide whether a single lane approach is appropriate or not you should look at what is happening at the giveway line. If vehicles at the giveway are queuing side-by-side, and not just behind each other within the acceleration lane, this constitutes two traffic streams and hence it should be modelled as either a ‘one lane + flare’ or ‘two-lane’ situation as appropriate. If the queueing within the acceleration lane is simply one vehicle behind the other, then only one traffic stream exists and hence it is best modelled as a single lane. If the junction does not yet exist professional judgement will be needed to determine the most likely scenario.