Development of sustainable transportation metrics for vehicular pollutants

A major challenge for transportation policy makers is to identify policies that will result in a sustainable transportation system. However, given the wide range of definitions of sustainable transportation it is not always clear how a given policy will affect this overall goal. In addition, there are very few quantifiable metrics that can be used to assess sustainability at a disaggregate or individual vehicle level. The amount of air pollution is often identified as a critical element in sustainability concepts and thus is the focus of this paper. A method is developed for assessing the responsibility of different vehicle classes for air pollution as a function of operating conditions. The responsibility measures are referred to as pollutant equivalency factors (EF_p's). MOBILE 5a, which is the standard mobile source emission model in North America, was used for the analysis although the methodology can be applied to any emission model. The EF_p allows policy makers to make a quantitative comparison between vehicle types under stated traffic conditions, which can then be used to assess directly the effects of different transportation policies. It was found that, in general, responsibility increases with increasing vehicle age, increasing vehicle size, increasing mileage, and decreasing speed. It is shown subsequently how one element of sustainability, namely equity, may be obtained by using EF_p to allocate costs associated with air pollution. The allocated costs then can be used to pay for externalities associated with the transportation system. A hypothetical test case, which involved the upgrading of an arterial roadway section to a freeway, was used to demonstrate this approach. It was found that EF_p's can be used both as metrics for sustainability issues and for the implementation of sustainability policies.