Transportation systems, by their inherent characteristics, have an impact on the environment:
- Network. The structure of transportation networks influences the spatial distribution of emissions. Centralized networks experience a concentration of traffic and emissions at specific locations (hubs), but are using less energy since the average distance is shorter. Dispersed networks may be perceived at the local scale as more environmentally friendly since they tend to be less intensively used, but are consuming much more energy. Therefore, a centralized network has more local externalities while a diffused network has more global externalities.
- Mode. Each transportation mode is characterized by specific emissions. For instance, different transport modes have different energy requirements (fuels) and levels of efficiency. A car-dependent transport system has different environmental externalities than a transport system leaning on public transit.
- Traffic. When vehicles are in circulation and terminals operating, they generate emissions and consume energy. Vehicles and facilities act as the point sources of emissions.