Generating Control Plans from Volumes

Technical Note #58

By Marga Delgado

April 2021

What do you do when control plan data is not available? You might face this problem when working with future scenarios that have not been fully detailed, or on a first approach from macroscopic to dynamic. In dynamic simulations, you can manage simple intersections with vertical signs for priority (give way and stop signs), but for more complex intersections that should be regulated with traffic lights, you cannot omit this information.

 

Aimsun Next offers a functionality that will estimate a reasonable control plan (fixed or actuated) for a single node or set of nodes, based on the volumes in the approaches and turns – a piece of information that you can input manually from field observations or extract automatically from a macroscopic assignment. Note that when calculating the control plan for several nodes, the offset is not calculated automatically, so after the plans have been computed, you’ll need to make manual adjustments to ensure coordination along a corridor.

 

When you select an existing control plan in “Use Control Plan”, the calculation procedure will not alter the association of signal groups to phases and the sequence of the phases. This is useful when you want to adapt the real control plan of the base scenario to the change of demand in a future scenario. Note that the procedure overwrites the selected control plan, so you should first make a copy of the base control plan and select that copy when you run the calculation.

 

When you select “Generate New Control Plan”in “Use Control Plan”, if signal groups are already defined in a node, they will be used, whereas if the node doesn’t contain any signal groups, they will be automatically created based on a logic that groups non conflicting turns in the same phase, and then splits the movements of each phase into a signal group per approach.

Say we have these two intersections in a model, the control on the one on the left being actuated, and the control on the one on the right being unspecified. If we ran a static assignment, we will see that the N-S axe has less load that the E-W.

In the context menu of the node, you can access the Generate Control Plan from the Volumes option:

Either choose the Assignment Experiment from which to use the Volumes or fill them in the Node Volumes table, and set the rest of parameters for the control plan, for Aimsun Next to generate a new control plan:

Aimsun Next has created 4 signal groups, one per approach, grouped the signal groups that have no conflicts in two phases, calculated the optimal cycle length with the Webster formula, and split it between the phases according to the volume of the turns associated to each phase, which leads to a longer green for the E-W axis.

 

If you know in advance which are the signal groups, you can create them in the node before running the control plan generation, so that the procedure will just have to group them into phases.

Aimsun Next detects that the signal groups already exist and will use them when generating the new control plan.

This signal group configuration leads to this control plan:

Finally, you may have a more complex node for which you know not only the signal groups but also the sequence of the phases, yet you only want to calculate the cycle and phase duration.

 

Let’s say that in the future scenario the intersection on the right gets more lanes and allows left-hand turns. You may want to generate a control plan from scratch or reproduce the same signal groups and same phase sequence as in the intersection on the left. For the latter, you’ll need to create the signal groups and a dummy control plan, which will only provide the sequence of the phases and the signal groups associated with each phase (so the cycle and the duration of the phases that you set is not important).

You need to run a macroscopic assignment for the future scenario to get the forecasted volumes for each turn:

…and run the calculation of a control plan, this time selecting the dummy control plan that you just created instead of “Generate New Control Plan”:

Since you selected the option to generate an actuated plan, Aimsun Next will generate the required detectors, assign them, and calculate the minimum green and max-out for each of the phases you defined based on the volumes assigned to the turns.

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