Motorization leads to three main paradoxes:
- Spatial specialization. The differentiation between land uses resulting from high specialization levels generates passengers and freight flows. For instance, a central business district focuses on administrative, institutional, and commercial activities, which depend on passengers and freight flows generated by other land uses, such as residential districts. Thus, the more complex and specialized the land use patterns across an urban area, the more complex their associated movements. Also, efficient and affordable transportation will enhance the differentiation of land uses and favors traffic growth.
- Spatial agglomeration. Since cities benefit from agglomeration economies, adjacent activities benefit from increased interactions, which also decrease transport costs. As the level of agglomeration increases, transport externalities tend to decline. However, at some level, agglomeration induces congestion, which increases the costs of mobility. This can reach a point where the advantages of agglomeration are overthrown by the costs of congestion.
- Road footprint. The main goal of transportation is obviously to overcome the friction of distance by providing a level of mobility. However, transportation, like any urban function, has a spatial footprint. While space is scarce (and consequently valuable) in urban areas, transportation requirements are at their highest levels. A compromise is thus sought between the available transportation footprint and the desired level of mobility. While low levels of road footprint may be linked with a prevalence of urban transit, high levels of road footprint are linked with automobile dependency.
The above figure illustrates externalities related to three simplified urban settings of specialization, agglomeration, and road imprint:
- A motorized city, such as in North America, tends to have a high level of specialization as most land uses are mono-functional. The level of agglomeration is low, which implies that many streets are underused and that distances between activities are on average significant. The road footprint is high, especially compared to the level of density, implying a high level of automobile dependency.
- A hybrid city, such as in Europe, is fairly multifunctional with different economic functions sharing the same space. Typically, residential and locally oriented commercial functions are closely integrated. This is linked with a good level of agglomeration, enabling a significant share of movements to occur locally either by walking or by public transit. This characteristic implies a lower level of road footprint as movements occur on more spatially efficient urban transportation modes.
- A transit-oriented city shares several commonalities with East Asian and European cities in terms of the level of specialization. However, higher levels of agglomeration tend to imply higher levels of congestion which is reinforced by a lower road footprint.