The anthropogenic origin of environmental impacts has become an increasing subject of interest during the last decades. Especially, the impacts of the transportation sector on Climate Change has been widely demonstrated and studied. Numerous methodologies have been proposed to calculate Greenhouse Gas emissions due to goods and passenger trips. This paper aims at proposing a methodology taken into account both the idea of trip motivation and indirect emissions of transportation. It proposes a case study: the trips of Saint-Etienne Metropole inhabitants during the week. If the results tend to confirm the major contribution of car in total GHG emissions, it also gives prominence to the disparities that occur when travelling one kilometre for one or another reason. These differences can originate from parameters that can vary in function of the motivation (such as the occupancy rates of transport modes) or from modal splits that also are peculiar to it.