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The value of the critical velocity to create a safe route upstream clear of smoke in the even1 of a tunnel fire is one of the prime important criteria for the design of the emergency ventilation system for road tunnels. For a tunnel with downhill slope, a greater critical velocity than that in a corresponding horizontal tunnel is required to control the smoke movement. The effect of the tunnel slope on the critical velocity has not been studied experimentally in connection with tunnel fires. This work concerns the examination of the effect of tunnel slope. A series of experimental tests has been carried out using a small model tunnel with downhill slopes between 0" and 10". The critical velocities and the ventilation velocities required to control the length of the "back-layering" smoke flow to 1, 3, 5 and 10 times of the tunnel heights were measured systematically. A slope correction factor to modify the effect of the downhill slope on the critical velocity has been proposed from the experimental results. The characteristics of the smoke flow in the upstream of the tunnel are discussed, and there is also a brief discussion on the effect of the bulk temperature of the smoke flow. These results extend the work of Oka and Atkinson  on smoke control in horizontal tunnels.