The most important transport emissions are related to carbon monoxide (CO), Volatile Organic Compounds (VOC), and nitrogen oxides (NOx), but transportation plays a marginal role for particulates and sulfur oxides emissions. Due to better engine technology and more stringent standards, the amount of pollutants released on highways has declined substantially. Contributions patterns are fairly consistent among advanced economies, but differences exist. They are attributed to modal choice, the composition of industrial activities, commercial energy sources, and general climatic conditions (colder temperatures are linked with more NOx emissions). The environmental profile of transportation, particularly in terms of the emission of harmful pollutants, is improving substantially.