The city council of Bilbao commissioned Opus to carry out a study to characterize the emissions of the fleet of vehicles in the metropolitan area of Bilbao, in order to have real-world data that will allow modeling the impact of a future Low Emission Zone, and therefore efficiently design the restrictions to one or another vehicle. This work is being one of the fundamental pieces for the city council team to correctly define its LEZ, and it will also allow it to update its inventories and improve its air quality model.