Source: Demographia World Urban Areas: 9th Annual Edition (2013.03).
Urban density is reflective of the mode of habitation elected by the population with high density cities characterized by apartment buildings (or crowded slums), while lower density cities have low rise habitation units and even single family homes in the suburbs. It is also reflective of the reliance on collective forms of urban transportation, with high density cities more prone to have a high modal split in favor of public transit. However, higher densities are less suitable for urban freight distribution since delivery trucks, which are usually supporting most urban deliveries, have difficulties to perform deliveries in high density areas.
There is a clear geographical distribution of urban population densities among metropolitan areas of more than 1 million inhabitants across the world. While most metropolitan areas have an aggregate density between 4,500 and 8,000 people per square km, a high number of cities with a density of less than 3,000 people per square km can be found. There is also a number of very high density cities, most of them in South Asia. North American and Australian cities are among those with the lowest densities, followed by European cities. The rest of the world has on average much higher densities, usually above 5,000 people per square km.