Improvements in transportation infrastructure encourage suburbanization, as people become increasingly able to live in a suburb and commute in to the nearby town or city to work. Developments in railways, bus routes and roads are the main improvements that make suburbanization more practical. The increase in the number and size of highways is a particularly significant part of this effect.
Government policies can have a significant effect on the process. In the United States, for instance, policies of the Federal government in the post-World War II era, such as the building of an efficient network of roads, highways and superhighways, and the underwriting of mortgages for suburban one-family homes, had an enormous influence on the pace of suburbanization in that country. In effect, the government was encouraging the transfer of the middle-class population out of the inner cities and into the suburbs, sometimes with devastating effects on the viability of the city centers. Insurance companies also fueled the push out of cities, as in many cases, it redlined inner-city neighborhoods, denying mortgage loans there, and instead offering low rates in the suburban areas. More recently, some urban areas have adopted "green belt" policies which limit growth in the fringe of a city, in order to encourage more growth in the urban core. It began to be realized that a certain amount of population density in the center city is conducive to creating a good, working urban environment.
Race also played a role in American suburbanization. During World War I, the massive migration of African Americans from the South resulted in an even greater residential shift toward suburban areas. The cities became seen as dangerous, crime-infested areas, while the suburbs were seen as safe places to live and raise a family, leading to a social trend known in some parts of the world as white flight. This phenomenon runs counter to much of the rest of the world, where slums mostly exist outside the city, rather than within them. With the increasing population of the older, more established suburban areas, many of the problems which were once seen as purely urban ones have manifested themselves there as well. Some social scientists suggest that the historical processes of suburbanization and decentralization are instances of white privilege that have contributed to contemporary patterns of environmental racism.
Recent developments in communication technology, such as the spread of broadband services, the growth of e-mail and the advent of practical home video conferencing, has enabled more people to work from home rather than commuting. Although this can occur either in the city or in the suburbs, the effect is generally decentralizing, which works against the largest advantage of the center city, which is easier access to information and supplies due to centralization. Similarly, the rise of efficient package express delivery systems, such as (in the United States) Federal Express and UPS, which take advantage of computerization and the availability of an efficient air transportation system, also eliminates some of the advantages that were once to be had from having a business located in the city. The overall effect of these developments is that businesses as well, and not just individuals, now see an advantage to locating in the suburbs, where the cost of buying land, renting space, and running their operations, is cheaper than in the city.
This has led to another recent phenomenon in American suburbs, the advent of edge cities in suburban areas, arising out of clusters of office buildings built around commercial strips and shopping malls. With more and more jobs for suburbanites being located in these areas rather than in the main city core that the suburbs grew out of, traffic patterns, which for decades centered on people commuting into the center city to work in the morning and then returning home in the evening, have become more complex, with the volume of intra-suburban traffic increasing tremendously.
By 2000, half of the US population lived in suburban areas.