Heavy duty diesel vehicles account for a substantial – and increasing – fraction of emissions in state and local mobile source inventories, especially particulate matter (PM) and oxides of nitrogen (NOx). The heavy truck activity data on which the current MOVES emissions model defaults are based date from a sample of 120 trucks gathered in the late 1990s, and are both limited and potentially out of date. A recent research project, NCHRP 08-101 (Enhanced Truck Data Collection and Analysis for Emissions Modeling) explored the potential of emerging telematics data sources to update and improve the previous data and inform local MOVES model inputs. Cost, sample size, and content limitations on existing data sources, however, meant that some key questions could not be fully addressed. The project in particular flagged extended idling (truck hoteling) as a large source of emissions that also is subject to considerable uncertainty in local and even national inventories. Uncertainty also persists about emissions related to starts, and about how activity patterns vary in different geographic contexts. The California Truck Survey was noted in the NCHRP study as a potentially valuable data source, but the data were not available at the time of the study. This proposed research would take advantage of this newly available data source with the goal of improving mobile source emissions inventories and better understanding what truck-focused emission reduction strategies are needed to achieve air quality goals.