Sioux Falls has long been used as the standard example in transportation network algorithm studies, and who are we to break that tradition?
Here we present an image-based example on a realistic modelling workflow for the beginner modeler out there. All the data used here can be downloaded at the Tutorials and Sample data page.
As to not upset those who think that Sioux Falls is not a realistic example (you would be right to think so), the example data is also available for the Chicago regional model, which has nearly 40,000 links and almost 1,800 zones.
Got a bigger instance we could use as an example? Send it over!
Creating a project¶
IF all you have to start your project are the layers and matrices (or the demand model) from your original model, then we can import it into AequilibraE. Just Make sure that you followed the layer preparation instructions on Preparing a network.
The project creation resource lives under the project menu.
The first 7 fields for links are mandatory, and one needs to associate the corresponding layer fields to the network fields.
The other fields that will be listed on the left side come from the parameters file (see the manual for that portion for more details), but the user can add more fields from the layer, as all of them are listed on the left side of the screen
In the case of the nodes layer, only two fields are mandatory.
After filling all fields, it is just a matter of saving it!
Opening the project¶
Before we do anything else, we need to open the project.
Individual path computation¶
The first thing we can do with this project is to compute a few arbitrary paths to see if the network is connected and if paths make sense.
Before computing a path, we go to the configuration screen
For the case of Sioux Falls, we need to configure the graph to accept paths going through centroids (all nodes are centroids), but that is generally not the case. For zones with a single connector per zone it is slightly faster to also deselect this option, but use this carefully.
If we select that paths need to be in a separate layer, than every time you compute a path, a new layer with a copy of the links in that path will be created and formatted in a noticeable way. You can also select to have links selected in the layer, but only one path can be shown at at time if you do so.
We can also skim the network to look into general connectivity of the network
To perform skim, we can select to compute a matrix from all nodes to all nodes, or from centroids to centroids, as well as to not allow flows through centroids.
The main controls, however, are the mode to skim, the field we should minimize when computing shortest paths and the fields we should skim when computing those paths.
With the results computed (AEM or OMX), one can display them on the screen.
On the matrix display screen, one can control how many decimal places are shown and whether decimal separators are shown. One can also browse through all the skims in this file by selecting the skim of choice in the drop down menu in the bottom left of the screen.
One can also generate the desire lines and Delaunay lines for the demand matrix provided.
After selecting a matrix, the user can choose to un-check the use all matrices box and select which matrices they want to use (the list of matrices will only show if the option is un-checked).
Make sure to select a zone/node layer and node id that is compatible with your matrix.
The user also needs to choose if they want Delaunay lines
or desire lines
Plotting the flows¶
The tool for plotting link flows you just saw above can be found under the GIS menu
If you have selected the Expert mode in the previous screen, you can also control the overall look of these bands (thickness and separation between AB and BA flows) in the project properties.
And have our map!! ( You need to refresh or pan the map for it to redraw after changing the project variables)
Traffic assignment with skimming¶
Having verified that the network seems to be in order, one can proceed to perform traffic assignment, since we have a demand matrix:
Matrices are provided in both OMX and AEM formats, so you are not required to install openmatrix.
For this example, we only add one traffic class for mode c (car)
To select skims, we need to choose which fields/modes we will skim
And if we want the skim for the last iteration (like we would for time) or if we want it averaged out for all iterations (properly averaged, that is).
The final step is to setup the assignment itself.
Here we select the fields for:
link free flow travel time
We also confirm the Relative gap and maximum number of iterations we want, the assignment algorithm and the output folder. In this case, we again choose to not block flows through centroids for the reason discussed above.
In order to plot the flows as shown above, first we need to add the link flows CSV to the QGIS workspace
Then we join it with the link layer by accessing the link layer’s properties
Now we can revisit the instructions above for Plotting the flows
Gravity model calibration¶
Now that we have the demand model and a fully converged skim, we can calibrate a synthetic gravity model.
We click on Trip distribution in the AequilibraE menu and select the Calibrate Gravity model option
The first thing to do is to load all matrices we will need (skim and demand).
Select which matrix/matrix core is to be used as the impedance matrix
Which one is the observed matrix
We then select which deterrence function we want to use and choose a file output for the model
We can then run the procedure
Inspect the procedure output
The resulting file is of type *.mod, but that is just a YAML (text file).
If one has future matrix vectors (there are some provided with the example dataset), they can either apply the Iterative Proportional Fitting (IPF) procedure available, or apply a gravity model just calibrated. Here we present the latter.
With the menu open, one loads the dataset(s) with the production/origin and attraction/destination vectors
We also load the impedance/skim matrix to be used
We select the production/attraction (origin/destination) vectors
And the impedance matrix to be used
The last input is the gravity model itself, which can be done by loading a model that has been previously calibrated, or by selecting the deterrence function from the drop-down menu and typing the corresponding parameter values.
To run the procedure, simply queue the job (and select the output wile with the screen that will open) and press Run jobs.
The result of this matrix can also be assigned, which is what we will generate the outputs being used in the scenario comparison.
After joining the two assignment results (the original one and the one resulting from the forecast we just did) to the links layer, one can compare scenarios.
When joining the assignment results, make sure to name them in a way you will understand.
The scenario comparison tool is under the GIS menu
The scenario configuration requires the user to set AB/BA flows for the two sets of link flows being compared, as well as the space between AB/BA flows, and band width.
The user can also select to show a composite flow comparison, where common flows are also shown on top of the positive and negative differences, which gives a proper sense of how significative the differences are when compared to the base flows.
As it was the case for stacked bandwidth formatting, expert mode sets project variables as levers to change the map formatting.
And this is what it looks like