During the late 1960s and 1970s, traffic-planners across Europe adopted an ideology of traffic separation–as the optimal way to address traffic hazards and risks. According to this ideology, pedestrian underpasses allowed cars to cross paths with pedestrians without conflict or interruption. Around 1970, the City of Copenhagen made large investments but these investments dwindled towards the end of the 1970s and underpasses diminished. This paper examines the rise and fall of pedestrian underpasses as a solution to perceived and real challenges in urban mobility. Using Actor-Network Theory (ANT) approach, the paper follows the interessement that traffic planners, politicians, and the press engaged in as well as the degree of pre-inscription of children, the elderly, and other imagined users during the construction of pedestrian underpasses. It shows how children were put forward as the main argument for these infrastructural investments. Vulnerable children were framed as the problem that the underpasses were meant to solve. However, as the underpasses became a fixture in the city, the public, politicians, and planners realized that children did not subscribe to this problematization, and Copenhagen-planners gradually abandoned the technology. The article reveals the ambiguous potency-yet-fragility of using children as a rhetorical tool in urban planning. © 2026 The Authors. Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the Historical Associations of Denmark, Finland, Iceland, Norway and Sweden.