Population per square mile of a given area. For example, the population density of the United States is approximately 69 persons per square mile overall. High population density levels created crowding. The tendency of large numbers of people to live in urban area creates congestion. For example, if the United States had a population of about 300 million people, it would not suffer from crowding; in fact much of its territory would still remain empty. On the east coast of the United State, the area from Boston to Washington, DC has been termed megalopolis because of its high population density. High population density contributes to increased energy utilization for transportation and pollution.