This paper deals with an important pollution problem affecting millions of people, especially in urban areas and large cities: NOx. This dangerous pollutant is emitted by high temperature combustion and therefore its main source is transportation, which in turn is a consequence of an increase in population and better economic conditions. To examine the determinants of NOx emissions in the Community of Madrid, which is one of the most densely populated regions in Europe, we use an augmented STIRPAT model and a massive dataset which includes kriging estimates of pollution records and integrates various sources of information. Unlike previous research, we carry out our analysis at local level, where the relationship between socio-economic factors and environment becomes more complex, by considering the 179 municipalities of the Community over the period 2000-2009. Our results suggest a positive and statistically significant relationship between private transport-related variables and NOR. Interestingly, the coefficients of the variables related to public transportation are all significant with different signs. The overall outcome illustrates that policy makers should encourage a significant modal shift towards public transport and invest in attractive alternative means of transport and particularly railways in order to reduce transport energy use and mitigate pollutant emissions. (C) 2014 Elsevier Ltd. All rights reserved.