This research analyzes the importance of population, economic activity, transport volume and structural characteristics of transport activity-in terms of transport energy intensity, of transport modes' share and of energy sources' mix-as driving factors of greenhouse gas emissions in transport activity in the EU-28 during the period 1990-2014. The analysis is based on the STIRPAT model, which is broadened to investigate in depth the impact on transport emissions of changes in the transport activity and in the whole economy. Using panel data econometric techniques, the significance of each factor and the impact of its change on emissions are identified. A better knowledge of the key driving forces is crucial for implementing policies focused on successfully reducing emissions in transport activity. The results allow a preliminary assessment of the potential effectiveness of the 2011 Transport White Paper measures aimed at cutting transport emissions.