Cities can grow in three directions: in, by crowding; up, into highrises; or out, to the periphery. The outward mode of growth befits a nation with abundant space, a young and expanding population, plentiful energy resources and a dynamic economy. New technologies that have helped disperse people and businesses in metropolitan areas caught on rapidly in the USA. By the mid-1920s, well over half of all families in this country owned an automobile. The West Europeans began to reach that level only after World War II. Americans were commuting between dispersed residences and workplaces long before this kind of human ecology was imaginable in other industrial countries. In place for the better part of a century, the decentralized US cityscape is a fact of American life, not a recent trend that can be reversed.
Low-density development drives up infrastructure bills. Desperate for tax revenue because residential development does not pay for itself, politicians offer tax abatements and other inducements for footloose companies. Homeowner taxes soar to fill the gap, even while street repairs, schools and libraries suffer. Each decade property values crest in rings around the city centres. Those are the golden rings of values - feasting circles of profit for developers, contractors, bankers and real estate agents. Hypermobility of the suburban era – working, sleeping, playing, schooling at locations reached only by long auto rides – breaks down the sense of community and creates sterile environments.
Urban sprawl in the United States is also hard to slow because numerous government policies actively encourage it. For example, the current federal tax policy hits earnings and savings, while interposing preferences for selected economic activities such as home-buying or the bond issues that finance new sports stadiums, industrial parks and malls. This blend of incentives frequently overstimulates the exodus of population and jobs from central cities to outlying areas. More recently, a growing tendency of the federal government to impose on localities scores of unfunded mandates – requiring everything from costly "special ed" to separate storm-water sewers – indirectly abets sprawl. Wealthy jurisdictions may easily absorb these added burdens, but fiscally frail central cities often cannot. Forced to raise their local tax rates and cut essential services, these municipalities frequently end up driving more inhabitants and firms away to greener pastures.
In the UK, urban sprawl has consumed close to two million acres of farmland since the second world war. The Council for the Protection of Rural England estimated in 1993 that the current rate of countryside loss is now over 10,000 acres a year (twice the figure given by the government). At this rate, by 2050, one fifth of England would be urbanized, against the present 15%.
In many developing countries, inter-city and rural-urban migration coupled with the natural growth of the urban population are causing cities to expand faster than the municipality can cope with. Unplanned urban expansion results in the creation of informal settlements and slum areas and causes a twofold problem. First, it decreases fertile agricultural land, and second, the disposal of both sewage and solid waste into irrigation canals creates negative environmental and operational effects on farming activities in the vicinity. For farmers the two main problems are pollution of irrigation water and the blockage of irrigation water pipes and openings.
The private New York Regional Plan Association, in 1929 and 1968 reports, called uncontrolled growth the greatest threat to the New York - New Jersey - Connecticut region. Officials failed to listen. Land was devoured 12 times faster than population grew. Suburbia became the region's engine of growth.