The Trivariate Flow Map Problem.

Yesterday I asked on Twitter how to solve a problem using trivariate flow maps. Of course, explaining it all with 140 characters was quite impossible.

Here’s the problem (due to confidentiality, the values aren’t the true ones, but you’ll get the idea).

So, the problem is to visualize the flow of movement from and to a region in a certain territory, the use of public transportation and the possible clients that aren’t taking it. There’s 8 regions so we will have 8 arrows.

For now, here’s the variable:
Total movements : Number of movements from the study area to the other regions.
Use of public transit : Of all the movements from and to the study area, the ratio that are made using public transportation (%)

This is what we have (click to enlarge):

This map tells us that between 30 and 40K people are going from region 6 to region 2 and than 1 in 4 are sing public transit. A little less is going to region 1, but almost 1 in 2 movement is made with public transit. Which is good. Region 4 is interesting. There’s 10-20K people going there, but less that 1 in 5 is using public transportation. There’s room for improvement. Maybe.

There’s many reason that can influence car ridership instead of public transit. On of them is the number of stop you have to made. If you move from you home to work and in the evening from work to home, this is what we call a pendulum movement. Those people are easier to convince to use (or at least try) public transit. Once they know the schedule and route to take.

On the other hand, if you go normally from home, then drop a kid to school, the go to work, the in the evening pick kid from school, go to the grocery store, drop the kid to soccer practice before coming home, it’s more difficult. So, we have identified those making pendulum movements.

Now, how can I add to the same arrows (ideally), a symbology that can represent the ratio of car users that are making pendulum movement?

For now, I think of using a bivariate color legend:

The arrow will be colored using this chart so the darker the color, the more pendulum movements are made on this axis. If also, the use of public transit is low (yellow or red), the more clients we could hope to convince…

What do you think?

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