Real world engine emissions measurements were carried out from the University of Antwerp in Belgium and more than 600 passenger cars were measured when entering and leaving two different University campuses. All measurements were done according to the European Commission Directive 2010/48/EU on roadworthiness tests for motor vehicles and their trailers. A database, including a wide variety of vehicles with completely different engine specifications and technological characteristics (engine size, emissions standards exhaust after-treatment devices etc.) has been created and various parameters influencing emissions will be examined.The influence of various parameters on NOx emissions was considered and discussed in this paper. Important conclusions have been made for diesel vehicles and presented in this work. Cold and hot start engine emissions were taken and analyzed in order to determine the percentage that NOx emission increased over the years. A comparison of different generation of cars is also included in the research. Cars' mileage has been considered and divided into two big categories, those with less than 100000 km and the rest with higher mileage.Finally, a comparison between drivers' habits while driving from home to work and different driving styles has been achieved and included in the paper. Some statistical analysis has been also carried out and useful information for local authorities and policy makers can be found in this work. Drivers' age, sex and travelling habits were put together in order to create a database and evaluate them. At the end of this study several suggestions regarding traffic flow and traffic jams are made and should be taken into account by various traffic authorities.