Cities across the world attempt to minimise the negative environmental and wellbeing effects of increasing traffic volume and density. To this end, an increasing number of cities have taken to games and gamified applications to motivate mobility behaviours with less adverse effects. Being a novel approach predominantly deployed on online platforms, a major challenge of this approach is designing systems to generate valid in-the-wild mobility behaviour data to assess their effectiveness. Drawing on experiences from an on-going development project of a gamified application targeting tourist behaviour in York (UK) city centre, this paper discusses how a mobile gamified application driving sustainable behaviours can be designed to quantify its impact. It provides recommendations on how gamification design can allow for a measurable output on the levels of modal shift gained through in game promotion of alternative modes of transport
