Population movements within a country can be looked at from two points of view: movements from one self-contained region to another; and movements between areas of different population density. It is not uncommon to observe both movements to more prosperous regions and at the same time a movement from the countryside to the towns. These movements often give rise to serious problems; for instance, as more and more people try to live in urban areas it becomes necessary for some of the surplus population to spread into neighbouring regions of lower population density. As a consequence, not only must more housing, drainage and other services be provided in these regions, but additional jobs must also be found; this brings about a change in the economic structure of the regions, which in turn leads to other demands, in particular on the local system of schools and technical colleges which have to provide the longer training usually called for by urban forms of employment. These problems add force to the demands for regional policies which seek to put all regions on a self-sustaining basis and so slow down, or even reverse, the former regional drifts.
According to some estimates, 75,000 people are leaving the rural areas of the Third World every day to set up home in the towns and cities. Rapid urbanization is bound to have radical effects on a nation's population and on its economy. More of a consensus exists at the world level with respect to spatial distribution of population and internal migration than on any other demographic topic: almost all the developing countries consider the current geographic distribution of their population partially or wholly unacceptable. The major urban centres in most developing countries have already become too large, and are growing too rapidly compared with the smaller cities, towns and rural areas. The costs of management of large, over-congested metropolises (in addition to relative concentration of public investment there), have become phenomenal, hampering general development goals and increasing the geographical inequalities which already exist.
The expected growth in urbanization in the less developed countries is the result not only of the natural increase in urban population but also of massive migrations of population to these areas in search of employment. It is estimated that the largest cities will grow more rapidly than the smaller cities and some of these are likely to reach proportions which are totally unfamiliar to town planners. In 1950, only four of the fifteen largest cities were in the less developed countries, but this number rose to seven by 1975. It is projected that twelve of the fifteen largest cities will be in the less developed countries by the year 2000. There were only six cities with populations of 5 million and over in 1950 and their combined population was only 47 million; in 1980, there were 26 such cities and their combined population was 252 million. Projections indicate that this number will rise to approximately 60 with an estimated population of nearly 650 million by the year 2000.