Luxembourg is characterised by a rising car park and a high car dependence. The proliferation of new automotive technologies (e.g. electric vehicle) and automobile services (e.g. carsharing) combined to the explosion of the number of daily activities and their complex space-time components renders decision-making for car owners more and more complex. This PhD research addresses the complex decision-making of car owners in Luxembourg by developing a web-based choice architecture that helps them to make more technologically appropriate and environmentally responsible choices. The target population is this the working population form the private sector in Luxembourg (residents and cross-border workers).The research draws on a multidisciplinary team of public and private partners and is divided into three stages. In stage one, we conduct an environmental impact assessment (EIA) for car segments based on existing and future-trend automotive technologies and quantify car use externalities and their societal costs. In stage two, we conduct a field study at Delphi and KPMG and analyse data on the objective and subjective decision aspects of car ownership through a composite approach of time geography, random utility and attitude theory. We apply different regression techniques (e.g. structural equation modelling) in order to identify car ownership profiles and leverage points which are able to address the intuitive mode of car owners and make automobile externalities visible. In stage three, we design the web application and develop algorithms which connect the produced EIA data on car segments with the developed car ownership profiles and environmental nudges. We test the web platform at Delphi and KPMG. The research will produce new knowledge and know-how on the determinants and interrelationships of decision aspects of car ownership and on how to better incorporate the societal costs of automobile externalities into car ownership decision-making.