- Causes. Two major factors are contributing to the level of transport activities. Economics refer to the general level of development, income and transport supply. An advanced economy is likely to generate more transportation activities per capita than a developing economy. Land use refers to the spatial structure and location of the transport demand, which indirectly influence travel distance and the modes used to support spatial interactions generated by economic activities.
- Activities. Involve a wide array of factors expressing the usage of transportation infrastructures and all the related services to support the transport system, particularly operations. All these activities have environmental outputs.
- Outputs. The first outcome of transportation activities are emissions of all sorts (carbon monoxide, nitrogen oxides, particulates, etc.). According to the geographical characteristics of the area where emissions are occurring (e.g. wind patterns), ambient pollution levels are created. Once these levels are correlated with population density and levels of activity, a level of exposure to harmful pollutants can be calculated. This exposure is likely to have consequences.
- End results. They include all the health, environmental and welfare effects of the exposure to emissions from transportation activities, which are very difficult to measure.