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Spatial and Temporal Investigation of Real World Crosswind Effects on Transient Aerodynamic Drag Losses in Heavy Duty Truck Trailers in the US Bharadwaj Sathiamoorthy Decreasing truck fuel usage and climate change gas production is of national and global importance. This study focuses on large, heavy-duty on-road tractor trailer combinations because of their impact in terms of fuel consumption levels, emissions, and their dominance in freight transportation in the United States, which offers substantial potential to improve efficiency of the transportation sector and reduce emissions. The US Department of Energy completed a study of this topic in 2009, and the EPA and NHTSA are both engaged in regulating truck efficiency. The Energy Information Administration (EIA) reported that more than 50 percent of the total diesel consumed was for transportation and this percentage will increase. With about 65 percent of the total engine-out energy consumed by a typical heavy-duty tractor trailer being spent on overcoming aerodynamic drag at highway speeds (55mph in the USA), improvements to aerodynamic performance offers a substantial avenue for reduction in fuel usage and emissions. Besides being directly related to fuel consumption, emissions, maximum speed and acceleration, aerodynamic phenomena also influence the stability characteristics of road vehicles, and their response to crosswinds. Crosswinds from any directions will affect the drag losses and will cause a significant change in pressure distribution along the truck body. The main objective of this research is to provide a better understanding of the influence of crosswinds on the aerodynamic performance of heavy-duty tractor trailers in the United States. A model to calculate on-road crosswinds for any temporal and spatial conditions from time-varying weather data, vehicle position and road data was developed. This transient model combined with drag data obtained from experimental, steady-state wind tunnel testing and numerical simulations for various tractor trailer configurations, the transient nature of coefficient of drag due to on-road crosswind conditions (from the model) was analyzed. Variations in yaw angle of up to 17 degrees were observed in some cases where the average yaw angle was recorded at only 3 degrees. Relationships between wind speed, yaw angle, drag and overall truck efficiency were clearly established. The research statistically measured the interaction between aerodynamic add-on devices, on-road crosswinds, and drag reduction efficiency. A region-based and time-based analysis was conducted to provide a better understanding of the aerodynamic performance of a baseline tractor-trailer configuration and aerodynamic add on devices. In several cases, the coefficient of drag varied as much as 60% on the routes analyzed and reductions in aerodynamic drag force up to 25% could realized by using the appropriate aerodynamic configurations. The application of these results will improve the estimation accuracy in fuel, emissions prediction models by allowing temporally and spatially disaggregated data input parameters. Finally, the study presented the different methods in which coefficient of drag is estimated and how these differences could play a role in misleading information about the aerodynamic characteristics of a tractor trailer.