It's probably London!

The colour gradient measures, merely speaking, the average speed at which an individual can move away from a given point using public transport, according to the authors of this model. Basically, in orange areas, you might as well walk :)

If you want to know more about how it's calculated and how the researchers aim to measure the quality of public transport, have a look at the source article:

https://www.authorea.com/users/92108/articles/199720-measuring-the-quality-of-public-transports-in-cities