The detailed light-duty vehicle driving pattern and engine operation information provided by the EPA driving pattern database has been used with recent estimates of the relationships between vehicle operation and emission production to provide a view of the conditions under which fleet emissions evolve. Relative exhaust hydrocarbon, exhaust CO, evaporative diurnal, evaporative hot-soak, and evaporative running loss emissions are described as a function of the vehicle driving parameters that most greatly affect their magnitude. These results provide insight about the contribution of different vehicle operating modes to the emissions inventory for real-world vehicle driving patterns. The study indicates that fleet exhaust hydrocarbon and CO emissions could be dominated by emission production during enrichment events. A new model for running losses is presented and the emissions predictions are applied to the occurrence of relevant real-world vehicle operating parameters.