Underground tunnels form an important part of modern road transportation systems. As the development of hydrogen cars advances into the market place, it is unavoidable that in the near future that hydrogen cars will become users of ordinary road tunnels. This paper discusses potential fire scenarios and fire hazards of hydrogen cars in road tunnels and implications on fire safety measures and ventilation systems in existing tunnels. Hydrogen has a low ignition energy, and wide flammable range suggesting that leaks have a high probability of ignition and result in hydrogen flame. Computational fluid dynamics (CFD) simulations of hydrogen jet flame in a full scale 5 in by 5 in square cross-section tunnel were carried out. The effect of the ventilation on controlling the upstream backlayering and the downstream flame were discussed. The results showed that the impact of a hydrogen flame in the tunnel depended oil the hydrogen release rate. The tunnel ventilation system could eliminate the upstream backlayering and fully control downstream flame with a smaller hydrogen release rate. For a larger hydrogen release, the tunnel ventilation system could not provide sufficient air flow into the fire and there is an oxygen deficit hydrogen layer accumulating under the ceiling downstream of the fire. The accumulating hydrogen layer could pose a more serious hazard inside the tunnel. (C) 2007 Elsevier Ltd. All rights reserved.