Short Lane Model

Modified on: 2015-02-23 14:30:22 +1100

The basis of the new short lane model in SIDRA INTERSECTION 6 has been described in the User Guide. The following should be considered when comparing the results given by SIDRA INTERSECTION Versions 5.1 and 6.0 in relation to the different short lane models used in these versions.

Definition of short lane capacity

In Version 6, the short lane model includes the effect of short lanes on adjacent full-length lanes. This is deliberate and overcomes a shortcoming of the Version 5.1 short lane model.

The definition of short lane capacity has changed between Version 6 and previous versions. In previous versions, capacity was based on the use of short lane space, and the short lane saturation flow rate was derived from the capacity value (it was not a saturation flow rate measured at the stop line unlike full-length lanes). When the short lane queue exceeded the available short lane space, a degree of saturation of exactly 1.000 was given based on a flow rate which gave an average back of queue that fitted in the short lane space. When this occurred, an “excess flow” was added to the adjacent lane. This flow rate represented the queue overflowing into the adjacent full-length (Through or other) lane. However, this short lane queue did not have any effect on the saturation flow rate of the adjacent lane.

In Version 6, the saturation flow rates both for short lanes and adjacent lanes represent counts of vehicles departing from the queue at the signal stop line during the green period. The basis of the short lane saturation flow model is to determine the reduced saturation flowratesat the stop-linesof 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). Reduction in the stop line saturation flow rates may or may not occur according to queuing conditions.

Definition of delay and queue length for short lanes

In Version 5.1, the definition of delay and queue length for short lanes differed from full-length lanes (they were defined as values corresponding to queues that fitted the restricted short lane space). This resulted in underestimation of delay and queue length values for short lanes.

When very heavy flows used a short lane, it was necessary to switch the short lane specification to the adjacent lane for realistic delay and queue length estimation. The Version 6 short lane model does this switching automatically.

Network model requirements

It was necessary to improve the areas discussed in Points 1 and 2, especially for the purpose of a good network model that requires (i) stop line saturation flow rates for signal platooning and (ii) 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.

Thus, the model in Version 6 removes various shortcoming of the short lane model used in previous versions. These shortcomings were explained in the User Guides of previous versions.

Changes during Version 6 updates

Short lane model improvements were also introduced in several Version 6 updates. Those introduced in later updates include the following.

Version 6.0.20 (released on 7 April 2014): “Short lane capacity calculations for approaches with multiple short lanes and for unsignalised cases have been improved.”.

Version 6.0.22 (released on 14 May 2014): “Improvements have been made to the short lane capacity model in relation to cases with continuous movements, very short lane lengths and some signalised cases with two green periods.”.

Lane Summary report

Model results will be better understood by inspecting the Lane Summary report (lane-by-lane results) rather than the Movement Summary report since individual movements use several lanes with different characteristics especially when using a combination of short lanes and full-length lanes. As a result, it is not easy to see why the movement performance results change.

Changes in signal timings

When capacity models change, the signal timings change in response. The changes in signal timings contribute to further differences in delay, LOS, queue length, etc.

Non-linear characteristic of the fundamental performance - degree of saturation relationship

Significant differences may result in capacity, degree of saturation, delay, and level of service, queue length, and so on. These are as expected given the short lane model changes, especially when near-capacity conditions apply (degree of saturation, x around 1.0). For example, if the degree of saturation, x is increased slightly from 0.95 to 1.05 (11% increase), delay is likely to increase substantially, e.g. from 60 s to 110 s (83%). When the intersection is operating near capacity, the changes in performance (delay, queue length, etc) are large as a result of small changes in the degree of saturation, i.e. the sensitivity to capacity changes is high, due the non-linear characteristic of the fundamental performance - degree of saturation relationship. Use of the Sensitivity Analysis facility of SIDRA INTERSECTION (changing the Basic Saturation Flow parameter) will indicate the level of sensitivity for specific cases.

Shared lane model with Free Queues

The use of the shared lane model with Free Queues is recommended for slip/bypass lanes at signals where queue space for vehicles using the slip/bypass lane is very short (entry to the slip lane is very close to the signal stop line). As a rough guide, use the shared lane model with Free Queues if short lane is less than 30 m / 100 ft as discussed in the User Guide.

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