Implications of Transient Mode Duration for Spatially Disaggregated High-Resolution Emission Inventory Studies

Abstract

High resolution emission inventory studies, such as urban air-shed modeling, often use spatially disaggregated emission parameters in the form of link-specific emissions. A key input variable that influences operating mode mix on highway links is the transient mode duration (TMD) in which start-related emissions occur. The TMD variable also establishes the emission rates to be embedded in emission factor models. The basic emission rates built into the MOBILE model are based on a TMD of 505 seconds, which was based on the federal test procedure (FTP). For newer vehicles, it is indicated that the TMD is much less than 505 seconds. This study examines the effects of variations in TMD on the distribution of cold and hot transient VMT on network links, as well as their effect on overall emissions. A network assignment model developed to track operating modes of vehicles on network links was used on a large California city network. The MOBILE model was used to derive bag-specific emission rates. The experiment includes a series of traffic assignment runs with varying TMD and postprocessing of the assignment output. Link-specific distribution of operating mode mix and total emissions were derived. It is indicated that TMD significantly influences the distribution of operating modes, as well as total emissions. The results indicate that as the TMD decreases, the share of cold transient VMT from freeways diminishes, while that of the local roads increases. It was indicated that total emissions on various link groups may fall within a range of 90 to 300 percent of the total emissions derived from the present FTP duration of 505 seconds. It is recommended that the transient mode duration variable be reexamined for deriving emission rates to be embedded in the future versions of emission factor models.