This map represents public transit efficiency and how it varies from place to place, measured in terms of gain from pure walking
We measure public transit efficiency by the amount of places you can reach within a reasonable commute time of 45 minutes.
From a given starting point and without public transit, one would walk to his workplace, so the area where commute would be within 45 minutes would be roughly a circle:
Note that in the current version of the transit engine, we do not take into account natural obstacles such as rivers, water bodies, hills, etc
Using public transportation, more places are available within 45 minutes, and the area will be expanded:
The efficiency of public transit is computed here as the ratio between the area reached with public transit compared to the one simply walking.
So as an example, in the second picture, public transit allows the commuter to reach twice as many workplaces within 45 minutes as in the first picture. The public transit efficiency will be 2x
In short, we're doing a full 45-minutes-isochronous computation starting from each point of the map, and comparing how much land gets covered. That gives us a score for the starting point, and plotting all those scores we get a transit efficiency map.
This computation takes a lot of time (currently up to 10h for the Paris region for instance).
The map only shows the efficiency of public transit agencies who made their data openly available.