During COVID-19, many cities built pop-up infrastructure for cyclists and pedestrians. We analyse the experiences of Geneva and Lyon through a qualitative approach based on document analysis and interviews with institutional and societal actors. We explore what contributed to the development of pop-up infrastructure during COVID-19, and how these interventions were shaped by and affected policy-making processes and actors' agency. We found that COVID-19 accelerated social and political trends regarding urban mobility. In both cities, authorities used the crisis to push through existing plans. Authorities' commitment and the existence of ready-to-implement plans proved crucial. The implementation processes constituted a breach from usual procedures. In Geneva, this empowered actors who usually act from the margins. In Lyon, authorities adopted pop-up infrastructure as a way to reduce costs. Our study clarifies the potential of experimentation in a context of crisis for urban climate governance and highlights the democratic implications of such interventions.