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Continuity plan for urban systems in case of major risk is a goal recently taken into account by the civil security policy in France. The implementation of operational tools for civil protection raises yet more questions about the definition of major risk and urban planning. Through some examples of Nantes, Lyon and Le Havre, the study of local civil protection plan in case of danger brings the light on technical and concrete modalities, but also on its institutional and territorial issues. The use of civil tool protection, made for crisis management, to prevent any failure in urban systems can lead the image of urban circulatory system where every breakdown of flows of goods, services or information is seen as a major risk.