The current state - of - practice in the U.S. for estimating vehicle emissions involves a two - step procedure. The first step estimates average vehicle speeds and total vehicle miles of travel using a macroscopic transportation - planning model. Vehicle emissions are computed in the second step based on the average speeds that were estimated in the first step. Network - wide emissions are finally computed as the product of the estimated vehicle emissions and vehicle miles of travel. This state - of - practice methodology would produce identical emission estimates for all drive cycles exhibiting identical average speeds, regardless of the specific speed profile associated with each drive cycle. In an attempt to overcome this limitation of current state - of - the - art procedures, Dion and Rakha (2001 and 2003) developed a mesoscopic model that estimates average light - duty vehicle fuel consumption and emission rates on a link - by - link basis based on up to three independent variables, namely: average travel speed, average number of stops per unit distance, and average stop duration. The model performs these estimations by separately analyzing the fuel consumption and emissions during each mode of operation of a vehicle (decelerating, idling, accelerating, and cruising) using relationships derived from instantaneous microscopic fuel consumption and emission models. Consequently, an accurate characterization of typical vehicle deceleration behavior is critical to the accurate modeling of vehicle emissions. The scope of this research effort is to characterize vehicle deceleration behavior and apply VT - Meso model for deceleration mode. Future research will be conducted to apply VT - Meso model for idling, accelerating, and cruising modes. The study demonstrates that while the deceleration rate typically increases as the vehicle approaches its desired final speed, the use of a constant deceleration rate over the entire deceleration maneuver is adequate for environmental modeling purposes. Also, this study illustrates that the application of the mesoscopic model is both feasible and practical and that it produces results that are reasonable in terms of both their absolute magnitudes and their relative trends for the decelerating mode.