A unique short lane capacity model applicable to all types of intersection is used in SIDRA INTERSECTION. The short lane capacity model in current versions of the software differs from old versions (Version 5.1 and older) significantly. A new model was introduced in Version 6.0 to answer the requirements of network modelling and also to overcome various shortcomings of the model used in older versions.
The new short lane capacity model uses the stop-line capacity consistent with full-length lanes instead of the queue-space capacity used in older versions. As a result, the concepts of "excess flow" and the related "special short lane degree of saturation of 1.000" are no longer used, and the practice of reduced queue length, delay and stops for short lanes is eliminated.
The short lane capacity model includes the effect of short lanes on the capacity of adjacent full-length lanes, and satisfies the network modelling requirements of
- stop-line saturation flow rates for second-by-second signal platoon patterns, and
- unrestricted queue lengths to model queue blockage of upstream intersection lanes by queues developed in short lanes, flowed onto adjacent lanes and extended to the upstream intersection.
The short lane model also satisfies the requirements of movement saturation flow rates including short lane effects to be used in signal timing and lane performance (delay, queue length, etc) calculations.
The characteristics of the short lane model are such that the saturation flows affected by short lanes are not fixed values based on the short lane length only, but they also vary depending on signal timings and lane flow rates as well.
According to this model, short lane effect on lane saturation flow rates (therefore capacity and performance estimates) can be on the subject short lane or the adjacent lane or both lanes depending on the queuing conditions.
The basis of the short lane saturation flow model is to determine the reduced saturation flow rates at the stop-lines of the short lane and the adjacent lane due to vehicles coming from the common upstream queue with increased saturation headways after the short lane queues are discharged at the full saturation flow rate while the vehicles queued within the short lane section are cleared.