NOTE: ARCADY 8, ARCADY 9 and ARCADY 10 include an Entry Lane Simulation feature which allows you to explicitly study the effect of unequal lane usage. The rest of this article refers to EARLIER versions of ARCADY.
The fact that one queue may be shorter than another is necessarily a problem – even one car waiting at a give-way line is enough to ensure that capacity is not being lost. There can however be a problem with some roundabout designs, or flow patterns, which should not be overlooked. Such problems occurs only if there is persistent entry-starvation – the queue in a particular lane is zero or very low when there is queueing in other lane(s). Traffic engineers will be aware that there are a variety of causes of this unfortunate situation (such as restricted exits, heavily uneven flows, inappropriate lane markings), but they all boil down to the fact that spurious “entry capacity” has been provided – in other words, there is potential capacity which cannot be used for a significant proportion of the modelled time period. In cases where zero (or near zero) occupancy of lane(s) is expected to occur at the give-way line, a trivial solution is to ignore the unused part of the roadway, and measure the geometric parameters based on the used part of the roadway. If entry starvation occurs, but the flow on the lightly used lane(s) cannot be described as “near zero” then there are two possibilities:
1. The lightly used part of the approach contains only vehicles turning left (i.e. at the first exit from the roundabout). Again, delete from the model the lightly used part of the roadway and the left-turning traffic using it.
2. The lightly used part of the approach contains straight-ahead or right-turning traffic. This is not so straightforward to handle. The aim would still be to delete the lightly used part of the roadway, but the traffic using it cannot be deleted because that would affect the derived circulating flows, which in turn affects all other entry flows. Several “dummy arm” based solutions have been suggested, but they all have significant deficiencies. A capacity adjustment to the entire approach arm by means of an “intercept correction” is one method of handling this situation. If this correction cannot be measured directly, it will need to be derived by scaling down the full approach capacity by an “appropriate amount” to account for the lost capacity due to entry starvation. Unfortunately, there is no global method of choosing an appropriate amount – it varies from case to case, and will always be a matter of judgement. Therefore caution must always be applied if considering such methods.
ARCADY 8 includes an Entry Lane Simulation feature which allows you to explicitly study the effect of unequal lane usage. The methods above refer to earlier versions of the software.