Vehicle emissions are the primary cause of air quality standard exceedances in the majority of UK Air Quality Management Areas. Increasing pressure to reduce the impact of pollution from vehicles has led to growing interest in the introduction of ‘Clean Air Zones’ and ‘Low Emission Zones’. However, such mitigation measures are expensive to design and implement and, while street level air quality monitoring can highlight the problem in the form of the local hot spots at which exceedances occur, it does not provide information on which of the passing vehicles are the most polluting.