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In order to assess accurately the influence of acceleration, a study was carried out to determine the most important vehicle parameters that influence the emissions and fuel consumption of passenger cars. Emission measurements have been conducted in three laboratories in France, Germany and UK on the basis of 14 driving cycles specially designed to cover the whole range of vehicle speed and acceleration in urban traffic. 150 vehicles, representing the European 1995 fleet, were tested, and the emissions of CO, HC, NOx and CO2 and the fuel consumption continuously recorded. Average unit emissions are presented as a function of average speed and vehicle type. Subsequently, a model was developed to calculate emissions during an urban trip as a function of the vehicle type and its instantaneous speed and acceleration. The model is given in the form of a two dimensional function with the variables speed and speed times acceleration for all vehicles types.