Fig. 3: Relationship of between subway ridership, crowdedness, and length with key socioeconomic factors in the selected 25 cities with the world’s busiest subway lines.
From: Global urban subway development, construction material stocks, and embodied carbon emissions

(a) Subway ridership and population density; (b) Subway crowdedness and population density; (c) Subway length per capita and population density; and (d) Subway length per capita and GDP per capita. The five selected Chinese cities are shown in the inner figures of (c) and (d), due to available historical data.