The results from the full-scale burning tests were applied to examine the equation. In addition, some scenarios were simulated using Computational Fluid Dynamics (CFD). Critical velocities predicted in this study were similar to those observed in the field tests, CFD simulations and estimation by a simple model by Thomas. As the plume configuration for a bigger tunnel fire is not the same as that for a small fire, other empirical expressions for large tunnel fires gave some lower critical velocity in comparing with the result for small tunnel fire in this paper.