The demographic transition theory focuses on changes over time in the causes of mortality affecting certain populations, such as health conditions and disease patterns. Through recent history a decline in death rates and an increase of life expectancy has been observed, implying that societies go through 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 communicative diseases that have not yet been mitigated by modern medicine. Famine is also common with uncertain food supplies and poor diet, making people more susceptible to diseases. Poor hygiene, no clean water, or 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 with religious beliefs and cultural traditions also inciting 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 related to improved 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 were much slower to decline. 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 (late expanding). Industrialization and urbanization mean less need for labor and are also associated with changes in socioeconomic preferences where 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 reaching peak growth rates. Then, in the latter part of this phase birth rates become similar to death rates. In the late 19th and early 20th century, this implied for Europe a phase of international migration to North and South America, as well as Australia, while in the mid to late 20th century this implied 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 stationery). Modern (post-industrial) societies are characterized by low death rates as modern medicine is widely available and may even show a slight increase in death rates with the aging of the population. 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 have 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 is coincident with the setting of international transportation services in the late 19th century (liners) since they supported migration flows, particularly over the Atlantic. It is thus a force that accelerated the emergence of global transport systems.
- It is coincident with the fast urbanization of developed economies in the first half of the 20th century and in developing countries in the second half of the 20th century.
- It provides a rationale behind the pressures urban and global transport systems are facing due to population growth.
- It underlines significant forthcoming demographic changes in developed economies that will have an impact on the level of material demand as well as transport preferences.