This paper explores how applying psychological and cognitive theories in nudge design can increase the effectiveness of nudging in public space. Nudges are those policy instruments that alter human behaviour by exploiting cognitive biases/heuristics, without limiting the choice set. Based on interviews with Dutch urban planners, barriers in applying such behavioural theories are identified. These barriers relate both to urban planners’ inexperience with nudging and to the organisational and societal context in which nudges are designed. A design framework is presented to optimise the design of nudges by helping planners to identify when and where nudging is feasible.