Source: adapted from Metrobits.org, World Metro Database.
There are approximately 200 subway (metro) systems in operation around the world, with several carrying more than 1 billion passengers per year. The construction and setting of subway systems have accelerated in recent years, particularly in cities in developing economies where mass transit becomes a clear strategy to improve urban mobility and mitigate congestion. The level of ridership is linked with several geographical and economic considerations. Some cities with high ridership such as Moscow, Beijing, and Shanghai have a tradition of subsidized public transportation and transit-oriented urban planning.
Many high-density cities have a level of subway ridership proportional to their population (London, Hong Kong, Osaka, Paris, Seoul, and Tokyo) indicating a consistent level of ridership where public transit accounts between 25% and 50% of commuting. Cities, where ridership is proportionally less than the population, have either a significant portion of their population too poor to afford public transit or wealthy enough to prefer the automobile.