I don't know who first had the idea of making time-scaled maps, but it makes a lot of sense. In every day life, we often measure distances in time; "How far away is X?" "About 15 minutes on foot..."
So as you can see above, distance is based on average time-travelled. Of course, this has downsides because there are sometimes delays, and there's more variability during peak times. But it's still a very cool concept.