Adapted from Shlomo Angel and Patrick Lamson-Hall’s NYU Stern Urbanization Project, here and here.
The animation at left shows street grid development from 1801 to 2011, mapping Manhattan’s gradual expansion north. The animation at right shows the population density over time of each census tract in Manhattan. Notice how Manhattan’s population density rises and peaks around 1900 before falling to present levels. Despite Manhattan’s appearance of being denser and more built up with skyscrapers than ever before, the island actually has a lower population density than a century ago.
Before the introduction of subways in the early twentieth century, the difficulties of commuting distances over land and water drove a denser form of urbanism than today. By 1900, the island of Manhattan had over 2.3 million residents in comparison to only 1.6 million in 2020. These people were crowded into dense blocks with upward of half a million people per square mile. The subways had not yet opened, suburban sprawl had not yet arrived, there were no rail connections under the Hudson River, and Manhattan had few or no road connections with the other boroughs and the mainland. This produced an island of remarkable density with the Lower East Side the densest place on earth, while only a few miles north, Harlem remained almost rural.
In 1903, the Williamsburg Bridge over the East River linked the Lower East Side with undeveloped Brooklyn. The trolley lines, subways, and roadway that stretched over the Williamsburg accelerated the development of Brooklyn, first in the higher density parts of Brooklyn closest to Manhattan and later to the distant parts of Brooklyn and Queens with suburban population densities. Suburban growth started earlier than the 1950s image of Levittown, and with the movement of people outwards from Manhattan, the centers of immigrant cultural life shifted, too. In every following year, the Lower East Side lost people, arriving at a density in 2020 only a sixth of what it was in 1900.
Over the following decades, improvements in public transportation and the introduction of the car “smoothed” out the population density. At the same time, Manhattan’s street network expanded to cover the whole of the island from end to end. As the subways made commuting easier, people no longer need to live within walking distance of where they worked. As a result, many industries remained in Manhattan while their workers moved to other boroughs, and later still to the more distant suburbs. As a result, the population densities of Manhattan today are more consistent from one end of the island to the other. Unlike in 1900, Harlem today is about as dense of the Lower East Side because transportation has made one part of the island almost as accessible to work as any other part of the island.
This animation illustrates Manhattan specifically, but Manhattan’s growth and population densities were influenced by larger population and technology changes in the New York region.