Spatial Allocation of Transportation Greenhouse Gas Emissions at the City Scale

Greenhouse gas (GHG) accounting for individual cities in large metropolitan areas is confounded by spatial scale and boundary effects that impact the allocation of transportation fuels used for surface transport and airline travel. This paper expands on a demand-based methodology to spatially allocate transportation fuel use (surface and airline) among colocated cities in the United States that are part of a larger metropolitan area commutershed on the basis of demand for surface vehicle miles traveled (VMT) exerted by individual cities. By using travel demand models for metropolitan planning organizations, the demand method was first applied as part of the city of Denver's GHG inventory in 2005 to develop a material flow analysis (MFA) of gasoline, diesel, and jet fuel consumption at the city scale. This paper reports on the application of the same method to six major metropolitan regions across the United States and a detailed analysis of all 27 cities within the larger Denver metro region. The analysis of six metropolitan areas in the United States demonstrated (1) the demand method produces VMT estimates allocated to cities across the commutershed that are similar (within 6%) to the boundary-limited polygon approach (linear correlation coefficient R(2) = 0.82; slope m = 0.98); (2) airline travel allocation by vehicular trip count ratios to the regional airport from colocated cities produces results similar to regional airline travel allocation by population ratios (linear correlation coefficient R(2) = 0.96; slope m = 0.99); and (3) the method is replicable with necessary data available from all cities in this study through their corresponding metropolitan planning organizations. In addition, the demand-VMT allocation method is found to be more responsive to future simulated mass transit growth. Further, the method is sensitive to local travel demand features of the individual cities, of which employment intensity was found to have the largest impact on per capita VMT allocated to a city. Within the Denver commutershed, daily VMT per capita estimates ranged from 13-129 km/capita/day (8-80 mi/capita/day) among the 27 communities, with a strong positive correlation with employment intensity (employment/capita; R(2) = 0.97) and employment density (employment/kilometer; R(2) = 0.59). DOI: 10.1061/(ASCE)TE.19435436.0000136. (c) 2011 American Society of Civil Engineers.