Computational fluid dynamics (CFD) calculations were performed simulating tunnel accidents with a hydrogen powered vehicle. The investigated scenarios assume damage of the LH2 system, release of gaseous H-2, mixing with air, ignition and finally combustion. Gaseous H-2 rises to the tunnel ceiling forming a strongly stratified mixture. Shape, size, inner structure and temperature of the evolving H-2-air clouds were calculated. Using new developed criteria, the time and space regions with potential for fast combustion modes were identified. For the given H-2 sources the combustion regime is governed by the ignition time. For late ignition a slow and incomplete combustion of the partly premixed H-2-air cloud along the tunnel ceiling is predicted. For early ignition a standing diffusion flame develops with dimensions and heat fluxes determined by the H-2 release rate. Temperature, oxygen and flow velocity fields during the combustion were computed. In both cases only minor pressures are generated. The highest damage potential appears to exist for intermediate ignition times. Design measures can be used to limit the risk of hydrogen driven vehicles to the level of gasoline cars.