Political instability and armed conflicts require that the Polish state protect the civilian population, including through large-scale evacuation. Public transport fleets may support evacuation. However, the growing share of zero-emission busses introduces constraints related to charging and hydrogen refueling infrastructure. The main research problem is whether increasing adoption of zero emission buses affects the flexibility of public transport fleets in evacuation scenarios and how this risk evolves over time. The research niche of this article is the impact of zero-emission propulsion on the feasibility of using public transport vehicles for evacuation. The study is based on a literature review and desk research (analysis of legal regulations, market trends and statistical data for selected Polish cities), complemented by a comparative case study of selected urban bus operators. Two related hypotheses are tested: H1 - propulsion source is related to flexibility of the public transport system in evacuation; H2 - the importance of this relationship increases as the share of zero emission vehicles grows. The results indicate that dependence on electricity/hydrogen supply and specialised infrastructure may reduce fleet flexibility during prolonged crises with disrupted energy supply. The article concludes with recommendations (e.g., maintaining a minimum reserve of conventional fuel vehicles, diversifying energy sources, and considering mobile charging/refuelling solutions) to strengthen evacuation readiness.