Geographical Paradoxes behind Urban Transport Problems

Motorization leads to three major countervailing forces (paradoxes):

• Spatial specialization. The differentiation between land uses is a generator of movements as people and freight move from several origins and destinations. Thus, the more complex and specialized the land use patterns, the more complex their associated movements will be. Also, efficient and affordable transportation will enhance the segregation of land uses and favor a growth of traffic.
• Spatial agglomeration. Since cities benefit from agglomeration economies, activities located nearby each-others benefit from increased interactions, which also decrease transport costs. However, the agglomeration of movements in a limited area creates congestion, which increases the costs of movements. This can reach a point where the advantages of agglomeration are overthrown by the costs of congestion.
• Spatial imprint. 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, consumes space and thus has a spatial imprint. While space is the rarest (and consequently the most valuable) in urban areas, transportation requirements are at their highest levels. A compromise is thus sought between the availability of space devoted to transportation and the desired level of mobility.

The above figure illustrates the negative impacts on three simplified urban settings of specialization, agglomeration and road imprint:

• A North American suburb tends to have a high level of specialization as most land uses are monofunctional. The level of agglomeration is low, which implies that many streets are underused and that distances between activities are on average significant. The spatial imprint of transportation is high, especially compared to the level of density, implying a high level of automobile dependency.
• The residential section of an European city 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 spatial imprint of transportation as movements occur on more spatially efficient urban transportation modes.
• Residential areas in a Japanese city share several commonalities with 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 lower spatial imprint of urban transportation.