In this paper the behavior of the three-way catalyst equipping a 2-liter car is fully analyzed considering exhaust gas and ceramic temperatures trends ( and of related engine parameters) in on-road operations of the car under different traffic conditions. These are classified by means of driving cycles clusters determined by multivariate statistical analysis of car speed and gear usage real profiles, detected on-road by the same car in designed experimental plans. Instantaneous fuel consumption and signals of a linear oxygen sensor, placed up-stream catalyst, have been analyzed to better characterize engine and catalyst performance.Emissions are measured in laboratory performing the most statistically representative driving cycles with car on the dynamometer chassis. The effect of traffic conditions on catalyst behavior and exhaust emissions is analyzed through the study of series of consecutive driving cycles. For each cluster three successive identical driving cycles have been performed.It resulted that a cold catalyst will never reach light off temperature under really congested traffic, while a warm catalyst will maintain its temperature high under the same conditions. This implies that catalyst of cars, starting cold in the center of towns, will not be active unless they will meet more fluent traffic conditions. While cars entering in the town from highways will maintain low levels of emissions. Thus, traffic managers should allow cars originated from the town center to reach lowest acceptable speed (about 50 km/h) in the shortest length.