Capacity of a shared lane

The traditional 'adjustment factor' and the new 'lane interaction' methods for estimating the capacity of a shared lane at a signalised intersection are discussed. The adjustment factor method is needed when the shared lane is combined with adjacent lanes into a lane group with the same departure characteristics. The lane interaction method as implemented in the sidra program employs a direct and explicit method to predict individual lane capacities without using adjustment factors. The method allows for differences in the departure characteristics of shared lanes (eg, opposed turns and through traffic) and the adjacent lanes (eg, through traffic only) by treating the intervals of lane blockage as effective red (lost time). This improves the prediction of not only the queue lengths and delays but also the short lane and opposed turn capacities. A generalised model of lane blockage is employed for predicting the number of departures before being blocked. A full intersection example is given with output from sidra to explain the lane interaction method. Using this example, the relation between the lane interaction and adjustment factor methods are explained. It is shown that substantial differences in capacity and performance predictions and timing solutions may result from the simplifying assumptions used to derive turn equivalents/ adjustment factors. The adjustment factor method has been relevant to the simple manual analysis methods of the past. Computerised analysis techniques have now found widespread use in the traffic engineering profession, and the method of modelling individual lane capacities explicitly and directly can be adopted without difficulty (a).