You might want to talk with Mitch and Simon. They're doing a project for Civic Design on how to increase the use of buses. And they might be thinking about how to visualize the benefits of buses vs private vehicles.

It might be interesting to create something like: https://hestia.rc.nau.edu/Visualization.html which was used to model carbon emission in specific cities - http://www.globalcarbonatlas.org/global-carbon-cities might help. Reduction is a specific part of that concept so those tools/maps might be too broad / not specific enough.