Visualize public transport coverage

The official [„Nahverkehrsplan“](https://www.berlin.de/senuvk/verkehr/politik_planung/oepnv/nahverkehrsplan/download/nvp2019-2023/NVP_2019-2023_Entwurf_Stand31Juli2018.pdf) (public transport scheme?) for Berlin specifies that 80% of homes in Berlin need to be located within a given distance of a public transport stop where services run at least every 20 minutes (page 100 of the document, page 116 of the PDF).

The official scheme also cites a study to verify that the given goals have (not) been met in the past. We could either try to explicitly validate this information using census data, openstreetmap, public transport APIs or - which might make more sense IMHO - create a map that simply displays those locations within the city for which the requirements made by the „Nahverkehrsplan“ are not met.