## 2 Literature Review  
Several studies have applied Computational Fluid Dynamics (CFD) models for assessing the risk level of unidirectional and/or bi-directional road tunnels in the event of fire. [PERSON] [5] carried out fire simulations in a tunnel with unidirectional traffic (one way-one tunnel), under the hypothesis that one of the two parallel tunnels presents an incident and the other one is without any incident. The two tunnels are linked by cross-connections every 400 m for escape. Due to the ventilation direction (i.e., natural caused by the traffic-induced piston effect and/or longitudinal mechanical), they found that the worst conditions occurred downstream of the fire. [PERSON] and [PERSON] [6] simulated tunnel fires by using CFD in order to investigate the effects of different ventilation systems. The authors found that under unidirectional traffic the airflow velocity is able to guarantee safety conditions upstream of the fire; however, it might not be true in bi-directional traffic operations since people evacuate in both directions. [PERSON] et al. [7] performed full-scale tunnel fire tests in order to validate FDS simulations. The tunnel investigated is equipped by mechanical ventilation, and a cross-passage in the middle of the tunnel connecting the two adjacent tubes. The temperatures predicted by FDS matched the experimental results and more hazardous conditions were measured downstream of the fire. [PERSON] and [PERSON] [8], by using the CFD software FLUENT 6.0, simulated a fire in a unidirectional tunnel with axial jet fans. They found that most of the smoke was pushed downstream of the fire, without any significant consequence since the vehicles downstream of the fire escaped from the exit portal, while the smoke dispersed upstream of the fire within a certain distance from the burning vehicle could put users at great risk. [PERSON] et al. [9], by using CFD modeling combined with an evacuation model, evaluated the safety level in one of the longest road tunnels in China equipped with cross-passages between the tubes. They found that it is unsafe to evacuate downstream of the fire due to the direction and more especially the high velocity of the ventilation, therefore measures should be in place to direct user evacuation process upstream of the fire. [PERSON] et al. [10] applied a CFD model for investigating the efficiency of an alternative jet fan in a one-way road tunnel. Two scenarios without traffic and with traffic jam were considered in the fire simulations. They showed that the proposed alternative solution was equivalent to the traditional one in terms of safety level, and that the worst environmental conditions occurred with the presence of vehicular traffic. [PERSON] et al. [11], by using the FDS code, investigated the effectiveness of two ventilation systems (natural or forced). They found that only the natural ventilation was not able to guarantee safe evacuation, while the forced ventilation ensured a safe route for escaping upstream of the fire. [PERSON] et al. [12; 13; 14], by using the ANSYS-CFX code, assessed the effects of a fire located in different positions along a bi-directional road tunnel equipped with longitudinal mechanical ventilation both with and without traffic. More severe fire-induced threats were found (due to the direction of ventilation) downstream of the fire and with the presence of queued vehicle, which acted as obstacles, and when the fire was located in the middle of the tunnel length. Moreover, other fire sources were investigated: safety conditions, along the evacuation path downstream of the fire, were found up to 5 min, 6.6 min, and 8 min after the fire started in the event of petrol tanker, HGV, and bus fire, respectively. This meant that the tunnel users to be safe should leave the tunnel within 5 min from the fire start.