Ventilation is an efficient method to keep the fire induced smoke dispersion under control in tunnel fires. Critical ventilation velocity (Uc), defined as the minimum longitudinal velocity to prevent smoke back flow from the fire source in tunnels, plays a vital role to provide safe conditions in the event of a tunnel fire. Although, it is one of the most remarkable and well-studied parameters for tunnel fire ventilation system design, there are some conflicts about the changing pattern of Uc and the ventilation flow characteristics in the presence of a vehicular blockage in tunnel fires between researchers. The main purposes of this study are to investigate numerically the effects of vehicular blockages, located in the upstream and downstream of the fire source, on the smoke spread and the back layering length and also to estimate the critical ventilation velocity in the presence of a vehicular blockage in tunnel fires. In addition, the review of the related studies and the elimination of the conflicts between the researchers on this issue are some important objectives of this study. Simulations of this study show that the presence of a vehicular blockage in tunnel fire cases make considerable changes on smoke flow pattern and critical ventilation velocity. Current knowledge in the related literature and this study have been assessed and some estimates have been made about the parameters leading these changes on the ventilation conditions.