The ecology of urban emissions.

Vehicle emissions depend directly on urban driving patterns which are an integral part of a wider range of urban features including density of settlement, car ownership, status of public transport, etc. Thus the conditions vehicles experience and their consequent emissions are directly related to the urban fabric. A methodology of sampling an urban area is developed by defining homogeneous areas within the city in terms of their activity intensity, modal split and social/economic status. These homogeneous areas are used as a basis for sampling an urban area and defining the variation in driving patterns both spatially and temporally. This illustrates that changes in traffic conditions are based primarly on a decrease in intersection-based traffic events from central to outer areas, and secondarily on a general decrease in the number of vehicle based traffic events. These progressively freer flowing driving patterns, are associated directly with a variety of wider land-use and transport features, which also vary systematically according to centrality. Thus urban structure is directly related to vehicle emissions with an increase in emission rate as a function of proximity to the central business district.