NARSTO has identified that emissions from on-road vehicles are among the highest priority needs for improving emission inventories (EIs). The focus of this research is to bridge the gap between transportation activity and multi-pollutant on-road vehicle emissions models, using state-of-the-art approaches and properly linking them. The objectives are to: (1) develop a robust, multi-scale methodology for estimating EIs at various spatial and temporal scales; (2) evaluate the utility of mesoscopic and microscopic transportation models to predict vehicle activity at sufficient resolution and in analyzing future vehicle, control or network design scenarios; and (3) quantify the relative contribution of vehicle type, vehicle activity and traffic control measures on the magnitude and variability in regional emission estimates.