Underground or partial underground tunnels form a very important part of modern road transportationsystems. As the development of hydrogen cars advancing into the markets, it is unavoidable in the nearfuture that hydrogen cars would become the users of ordinary road tunnels. This paper discusses potentialfire scenarios and fire hazards of hydrogen cars in road tunnels and implications on the fire safety measuresand ventilation systems in existing tunnels. The information needed for carry out risk assessment ofhydrogen cars in road tunnels are discussed. hydrogen has a low ignition energy, and wide flammable rangesuggesting that leaks have a high probability of ignition and result hydrogen flame. CFD simulations ofhydrogen fires in a full scale 5m by 5m square cross-section tunnel were carried out. The effect of theventilation on controlling the backlayering and the downstream flame are discussed.