Source: Urban population data from United Nations, World Urbanization Prospects: The 2007 Revision Population Database. Kenworthy J, and F. Laube (2001) The Millennium Cities Database for Sustainable Transport, International Union of Public Transport (UITP), Brussels and Institute for Sustainability and Technology Policy (ISTP), Perth.
The above map provides a typology of the sustainability level of urban passenger transportation systems from a sample of 64 cities across the world. The classification is mainly based on the assumption that a high level of reliance on the automobile, coupled with low-density levels is less sustainable than lower levels of automobile dependency and higher densities. Five major classes can be identified:
- Class A. Comparatively, most sustainable transport. Very low automobile dependency, with public transport, walking, and cycling more prominent than car use, which involves very low gasoline use per capita.
- Class B. Comparatively more sustainable transport. Low automobile dependency, public transport, walking, and cycling equal with cars, low gasoline use.
- Class C. Comparatively sustainable transport. Moderate automobile dependency, an important role for public transport walking and cycling, moderate gasoline use.
- Class D. Comparatively less sustainable transport. High automobile dependency, a minor role for public transport (important support for peak hour movements), walking and cycling, high gasoline use.
- Class E. Urban passenger transport systems that are comparatively the least sustainable. They involve a very high level of automobile dependency, a minimal role for public transit, walking, cycling, and very high gasoline use.
The typology has a distinct geography. North American and Australian cities are dominantly in class E with a few denser cities (e.g. New York, Toronto) in class D, which is mostly related to automobile dependency. European cities rank higher on the sustainable passenger transportation scale, mainly due to the higher role of public transit and denser urban settings. This characteristic is also shared with many Latin American cities. East Asian cities are generally on top of the urban passenger sustainability scale. However, rapid urban development and motorization in most large Chinese cities have resulted in a relative decline in their sustainability level. Southeast Asian cities having a high level of automobile dependence are in a similar situation.
Efficient and productive cities are not necessarily linked to high levels of energy consumption per capita. The first three classes of cities (A, B, and C) are characterized by high densities and rely on public transit for mobility. In comparison, the last two classes (D and E) have lower densities and rely on the automobile for most urban movements.
A shortcoming of the above classification is that it considers only one dimension of urban transportation; the mobility of passengers. Cities are also subject to intense freight flows since they are locations of production, consumption, and distribution. Freight distribution should thus be considered an important component of urban sustainability. The assumption that sustainability relies heavily on public transit use can be subject to debate as the energy efficiency of vehicles has improved significantly, particularly with ongoing electrification.