![Population Density, Selected Cities | The Geography of Transport Systems](https://i0.wp.com/transportgeography.org/wp-content/uploads/population_density_cities.png?resize=900%2C422&ssl=1)
Density, measured in people per square km, has substantially declined in major metropolitan areas. The period between 1960 and 2020 was characterized by a process of suburbanization and in many cases the expansion of the metropolitan statistical area. While densification can be observed in many urban areas, at the aggregate level of a metropolitan area lower densities are observed.