Unlocking the power of Path Assignment Plans
Marga Delgado explains how the the Path Assignment Plan feature in Aimsun Next gives you unparalleled flexibility in reusing previously calculated paths.
Comparing the impact of route choice changes
March 2024 — Technical note #85
Tessa Hayman
Product Specialist
We often want to know the effect on routing of a scheme or a change to a model. Comparing flow in a dynamic model as a view mode can be misleading as any change in travel time will also change the flow in each time interval. Therefore, it is very useful to instead compare the assigned flows from the route choice model (dynamic traffic assignment). This technical note will take you through how you can easily visualize and compare the route choice of two models.
This can help you to:
- Compare the differences between two runs during calibration.
- Compare the effect of a scheme with respect to a baseline condition.
- Define an area of interest for a subnetwork.
- Understand the synergy between two schemes – see https://www.aimsun.com/technical-notes/select-link-analysis-for-schemes/
In a dynamic model, the route choice model (dynamic traffic assignment) assigns a set of routes to each origin destination pair with a set of probabilities, e.g., in interval 1, 75% of trips from origin A to destination B use route 1 and 25% of trips from origin A to destination B use route 2. This information is saved in the path assignment object and file. If we combine all origins and destinations together, we can use this to determine the assigned flow along a link for vehicles generated in a certain interval e.g. the number of vehicles that are generated at 07:00 that use a certain link at a point in their journey.
The advantage of this is that we are then making a fair comparison, as we can compare the number of vehicles that use different routes departing in the same time interval, regardless of when they reach a certain point along the route.
Analyzing path assignment outputs of two models
- Before running each model, ensure that you have ticked the path assignment output and that they are saving into separate path assignment files.
2. Run both replications/results that you want to compare.
3. Go to the path assignment tab of one of the replications/results. Select the time interval, and vehicle type that you want to compare.
4. Click on the Link Analysis button. This generates the assigned flow on every section object in the network for the interval and user class selected.
5. Click on the Action button, select Save Link Analysis and enter a name to identify this configuration. This will allow you to quickly execute the same calculation with the other replication/result.
6. Go to the path assignment tab of the other replication/result. Click on the Action button, select Retrieve Link Analysis and pick the one you just saved. This generates the assigned flow on every section object for the other replication/result.
7. Use the data comparison tool to create a view mode. Select the correct replications to compare, the section object and link analysis flow or volume. Then select the correct link analysis outputs – they are labelled based on the time.
8. You may wish to edit the default view mode to only show where differences are greater than 100 vehicles. In the legend click the top view style to quickly open it. Then change the range from -inf to -100 and 100-inf.
Here is how the final output looks like – the green roads show a reduction of more than 100 vehicles and the red roads show an increase of more than 100 vehicles.
Now that we have identified which roads will experience some significant increase of traffic, we can create a subnetwork (or a microscopic area) around them and run a micro (or a hybrid micro-meso) to do some operational analysis on the impact of the additional vehicles and design some policies to accommodate them.
More technical notes
Comparing the impact of route choice changes
Tessa Hayman explains how to analyze route choice changes between schemes tested using microscopic, mesoscopic and hybrid network loading.
