(Ocean Robbins) The population density of Manhattan is about 70,000 people per square mile, so it’s a good thing that all those people don’t rely on cars to get around. Instead, most use the New York City subway system, which served almost six million riders a day in 2016. The system, which first opened in 1904, is the primary infrastructure that binds the millions of New Yorkers to their sites of work, worship, and leisure.