Spatial Form, Pattern and Interaction

The structure of urban land use has an important impact over transport demand and over the capacity of transportation systems to answer such mobility needs. This involves three dimensions influencing the environmental impacts of transportation and land use:

·         Spatial form. Relates to the spatial arrangement of a city, particularly in terms of the setting and orientation of its axis of circulation. This form thus conveys a general structure to urban transportation ranging from centralized to distributed. The dominant influence has been expansion and motorization. The resulting polycentric cities are economically and functionally flexible but consume more energy.

·         Spatial pattern. Relates to the organization of land use in terms of location of major socio-economic functions such as residential, commercial, and industrial uses. The prevailing trend has been a growing specialization, disconnection, and fragmentation between land uses. Also, different types of land use can be incompatible with their proximity to the source of additional externalities. For instance, residential land use is incompatible with the majority of industrial, manufacturing, warehousing, and transport terminal activities. They generate noise and congestion externalities to which residents are highly susceptible. In such a context, buffers, which apply different barriers effects to promote physical separation, can help mitigate incompatible land uses.

·         Spatial interaction. Relates to the nature and the structure of movements generated by urban land uses. The prevailing trend has been a growth in urban interactions in terms of their volume, complexity, and average distance.

The location of activities such as residence, work, retail, production, and distribution is indicative of the required travel demand and the average distance between activities. With specialized land use functions and spatial segregation between economic activities, interactions are proportionally increasing. It is over the matter of density that the relationships between transportation, land use, and the environment can be the most succinctly expressed. The higher the density level, the lower the level of energy consumption per capita, and the relative environmental impacts. A remarkable diversity of urban densities is found around the world, which is reflective of different geographical settings, planning frameworks, and levels of economic development. This complexity is compounded by how density changes in relation to the city center.

Paradoxically, the outward expansion of cities and suburbanization has favored a relatively uniform distribution of land use densities, notably in cities with prior low-density levels. In recent decades, the average density of several large metropolitan areas has declined by at least 25%, implying additional transport requirements to support mobility demands. Further, residence/work separation is becoming more acute as well as the average commuting time and distance. It is consequently increasingly challenging to provide urban transit services at an efficient cost. This underlines that the future of sustainable mobility will require accomodating personal mobility requirements, even if this mobility is considered less sustainable than collective mobility.

An important effect of land use pattern and density on the local environment concerns the heat island effect. It is an outcome of differences in albedo between an urban surface composed of buildings and paved surfaces (roads, parking lots) and the natural landscape. The urban landscape absorbs more heat during the day, which is released during the night and can result in ambient temperatures up to 5 degrees Celsius higher than normal. The land use pattern plays a role in the heat island effect with grid patterns (or other ordered patterns) retaining more heat than other disordered patterns, mostly because buildings and other structures reabsorb the heat emitted by others.

A higher level of integration between transportation and land use, particularly density, often results in increased accessibility levels without necessarily increasing the need for automobile travel. The slow transformation of urban land uses, with annual rates lower than 2%, makes it difficult to establish sound transportation/land use strategies that could have effective impacts over a short time period. As it is generally market forces that shape such changes, it is uncertain which drivers of change would significantly impact the transformation of urban land use.