Passenger travel CO 2 emissions in US urbanized areas: Multi-sourced data, impacts of influencing factors, and policy implications

Policies that encourage reduced vehicle-miles traveled and the use of more efficient transportation modes are typically considered as means to reduce greenhouse gas (GHG) emissions. In support of motivating, developing, and evaluating such policies, the impacts various transportation infrastructure and use, population density, and policy variables have on passenger travel related carbon dioxide (CO2) – the primary GHG – emissions are assessed and resulting policy implications are discussed. A methodology for integrating data from multiple sources in a consistent manner is developed and implemented, producing a rich dataset consisting of 146 of the largest urbanized areas in the US. The magnitudes of the impacts that changes in certain variables have on CO2 emissions in select urbanized areas are quantified. The results indicate that the variable used as a proxy for the presence of policies aimed at addressing environmental concerns and travelers’ attitudes and behaviors towards such concerns influences the impacts changes in transportation characteristics and population density have on CO2 emissions. Depending on these effects, the impacts of changes in average private vehicle occupancy and freeway lane-miles per capita are found to be the largest. In addition, changes in average travel time have a substantial impact on CO2 emissions. While the explanatory effect of transit share is found to be statistically significant, the magnitudes of the impacts of changes in this variable are less appreciable in comparison to those of the above variables, which is understandable in light of the fairly low values of transit share and transit service utilization across most US urbanized areas. Furthermore, the impacts of changes in population density are the smallest among all the variables that are found to have statistically significant explanatory effects. However, this finding does not undermine the role land-use policies could play as increased density could have a direct or an indirect effect on reduced travel times and increased transit use, which in turn contribute to reduced CO2 emissions. In addition to quantifying the impacts, several policy implications stemming from the findings are identified and discussed. Notably, the relative magnitudes of the impacts corresponding to the different variables are found to vary appreciably across urbanized areas, implying that policies aimed at reducing CO2 emissions should focus on different sets of variables depending on the overall characteristics of the specific urbanized area and any existing policies aimed at reducing CO2 emissions.