Speed- and Facility-Specific Emission Estimates for On-Road Light-Duty Vehicles on the Basis of Real-World Speed Profiles

Abstract

Estimating the emissions consequences of surface transportation operations is a complex process. Decision makers need to quantify the air quality impacts of transportation improvements aimed at reducing congestion on the surface street network. This often requires the coupling of transportation and emissions models in ways that are sometimes incompatible. For example, most macroscopic transportation demand and land use models, such as TransCAD, TranPlan, and TRANUS, produce average link speed and link vehicle miles traveled (VMT) by vehicle and road class. These values are subsequently used to estimate link-based emissions by using standard emissions models such as the U.S. Environmental Protection Agency's MOBILE6 model. In contrast, recent research with portable emissions monitoring systems indicates that emissions are not directly proportional to VMT but are episodic in nature, with high-emissions events coinciding with periods of high acceleration and speed. This research represents an attempt to bridge the gap in transportation and emissions models through the use of the real-world distributions of vehicle-specific power bins that are associated with average link speeds for various road classes. A successful effort in this direction would extend the use of transportation models to improve emissions estimation by using the limited output produced by such models. In addition, the variability of emissions and emissions rates over average speeds for a given facility type is explored, and recommendations are made to extend the methodology to additional facility types.