Fig. 2: Correlations between subway length and urban socioeconomic parameters for all world regions.
From: Global urban subway development, construction material stocks, and embodied carbon emissions
(a) population, (b) GDP, and (c) built-up area. The results are presented in four quadrants based on median values (as dashed lines) of subway length, population, GDP, and built-up area for all cities. The regression trend lines of Africa and Oceania are not added because they only have a small number of cities with subways.
