Sustainability requires radical innovations, but their introduction and diffusion usually encounter the opposition of existing socio-technical regimes. An important challenge is, therefore, to understand how to catalyse and support the process of transitioning towards these innovations. Building upon insights from transition studies (in particular the concepts of Strategic Niche Management and Transition Management), and through an action research project (aimed at designing, introducing and diffusing a sustainable mobility system in the suburban areas of Cape Town), the paper investigates the role of design in triggering and orienting societal transformations. A key role is given to the implementation of socio-technical experiments. A new socio-technical system design role emerges: a role in which the ideation and development of sustainable innovation concepts is coupled with the designing of appropriate transition paths to gradually incubate, introduce and diffuse these concepts.