The demographic transition theory focuses on changes over time in the causes of mortality affecting certain populations, such as health conditions and disease patterns. A decline in death rates and an increase in life expectancy have been observed in recent history, implying that societies are undergoing a transition from one equilibrium (high birth and death rates) to another (low birth and death rates). This transition is associated with a rapid surge of the population as well as urbanization and can be divided into four phases:
- Phase I. This phase (high stationery) is characterized by high birth and death rates, implying that the total population is stable or grows slowly. Mortality is high, with the prevalence of communicable diseases that have not yet been mitigated by modern medicine. Famine is also common amid uncertain food supplies and poor diets, making people more susceptible to disease. Poor hygiene, lack of clean water, and inadequate sewage disposal also contribute to high mortality rates. Fertility is high since there is no or little family planning (contraception), as parents have as many children as possible because few survive to become adults. Fertility is also encouraged by the dominant socioeconomic structure in rural societies, with many children needed to work the land, and by religious beliefs and cultural traditions, which also encourage large families. Birth and death rates fluctuate with the ebb and flows of events such as wars, epidemics, or droughts. This situation characterized Europe up to the 19th century and developing economies up to the early 20th century.
- Phase II. This phase marked the beginning of the demographic transition (early expanding) with a rapid drop in death rates while birth rates remained high. The main drivers behind the drop in death rates were improvements in medicine, sanitation, and water supply. Improvements in food production in terms of quality and quantity, as well as the capacity to store and transport food over long distances, also played a significant role. The human diet became more stable and diversified. A decline in child mortality enabled more people to reach adulthood. Birth rates declined much more slowly. The outcome is a significant upward trend in the total population. This situation characterized developed economies in the mid-19th century, and only the least developed economies could be considered in this phase by the late 20th century.
- Phase III. This phase marks a significant drop in birth rates, while mortality rates tend to level off (late expansion). Industrialization and urbanization mean a reduced need for labor and are also associated with changes in socioeconomic preferences, in which consumption takes a more salient role. Also, higher costs of living in urban areas are incentives for smaller families, with family planning (contraception) widely available. The total population is approaching peak growth. Then, in the latter part of this phase, birth rates approach death rates. In the late 19th and early 20th centuries, this meant a phase of international migration from Europe to North and South America, as well as to Australia, while in the mid to late 20th century it meant large-scale urbanization in developing countries.
- Phase IV. This phase implies that a new equilibrium has been reached between birth and death rates with a stabilization of the population (low stationarity). Modern (post-industrial) societies are characterized by low death rates because modern medicine is widely available, though death rates may even show a slight increase as populations age. They are also characterized by low birth rates, as each child involves a substantial capital investment before reaching adulthood (healthcare and education). Most, if not all, of the developed economies had reached this phase by the late 20th century, and several developing economies, such as China and Brazil, are close to achieving it. An additional phase (phase 5), where death rates remain higher than birth rates, is possible, implying a decline of the population, at least until birth rates bounce back. Countries such as Japan, Germany, and Italy have reached such a stage.
The demographic transition has several ramifications for transportation:
- It coincides with the emergence of international transportation services in the late 19th century (liners), which supported migration flows, particularly across the Atlantic. It is thus a force that accelerated the emergence of global transport systems.
- It coincides with the rapid urbanization of developed economies in the first half of the 20th century and of developing countries in the second half of the 20th century, and with the related growth in urban transportation.
- It provides a rationale behind the pressures urban and global transport systems are facing due to population growth.
- It highlights significant ongoing and forthcoming demographic changes in developed economies that will affect material demand levels as well as transport preferences.