This paper sets out a new approach to the Wicked Problems of obesity and climate change, and the linked and causative Wicked Problem of increasing car ownership and use. Policies to bring about modal shift from car dependence to forms of transport that cause lower emissions, and are less obesogenic, are conventionally based on framing car use as an externality to be addressed by policies such as taxation or regulation. These policies have been hampered by the reluctance of politicians and policymakers to countenance electoral risk by impinging on the individual's perceived right to personal mobility. This paper's approach combines insights and methods from behavioural economics and social marketing: it shifts the focus by considering car ownership and use not as an externality to be addressed in the aggregate, but as the product of individual behaviours and lifestyle choices. Behavioural economics can help to uncover the motivations, heuristics and cognitive biases behind such behaviours. Social marketing builds on the premise that people will only change their behaviour if they are sufficiently motivated to do so. It can be used to design interventions which help sectors of the population make transport choices that are more optimal--both for the individuals concerned and for all of us who are affected by these Wicked Problems.