## 2 Literature Review
This might be done by equipping the tunnel with additional safety measures such as illuminated signs for people evacuation, and the presence of traffic lights at the entrance portal to close the tunnel as soon as possible in the event of fire. [PERSON] et al. [15] carried out CFD simulations in a road tunnel equipped with semi-transversal smoke control system. The aim was to investigate the effects of the obstruction of vehicles queued behind the fire source. They found that with the high vehicular blockages in the tunnel, the proposed smoke control system might not be effective in the event of a real fire. [PERSON] et al. [16], by using the ANSYS CFX+STEPS tools and on the basis of traffic flow expressed in terms of the Annual Average Daily Traffic (AADT), showed the effects of fire-induced threats on users, during their escape process downstream of the fire, for different positions of burning vehicles in a bi-directional road tunnel equipped with longitudinal mechanical ventilation. They also showed the advantages of providing the tunnel with emergency exits, and an optimal distance between the emergency exits was also identified. [PERSON] et al. [17] conducted several CFD simulations to compare two different approaches for assessing the risk level in a one-way road tunnel. While in the first approach the walking speed of tunnel users is based on the smoke opacity, in the second one it depends on the exposure to both heat and toxic gases. However, the comparison showed only minor differences in the case investigated under the aspect of people safety. [PERSON] et al. [18], by applying the FDS+Evac codes for a unidirectional road tunnel tube equipped with natural ventilation only and implementing the peak-hour traffic volume (VHP) during the day, evaluated the effects on escaping users upstream of the fire due to different types of burning vehicles located at different longitudinal positions along the one-way tube. They showed that the mechanical ventilation is not required in this case. However, it is to be stressed that they did not investigate the use of the one-way tube for bi-directional traffic. [PERSON] and [PERSON] [19], after examining the formation of traffic congestion due to an accident by using a traffic simulation tool, simulated the fire development in a one-way tunnel by means of FDS and modelled the evacuation process with the PATHFINDER software. They found that the number of trapped users upstream of the fire varies with traffic (i.e., it is higher for congested traffic because of the higher number of users exposed to risk). The risk increased with the traffic volume, and the factor that most affected the safety of users was smoke rather than temperature. From the above chronological literature review, one may note that the mentioned studies have mainly focused on the evaluation of the effects of fires in unidirectional or bi-directional road tunnels under usual traffic directions. However, the effects of a fire on people evacuation process when a one-way tube is used for bi-directional traffic in fire scenarios, for example if the adjacent tube is temporarily closed for specified activities, do not appear to have hitherto been sufficiently investigated during both day and night. This represents a lack of knowledge that is needed to be investigated in greater depth, which is the intent of the present paper. It is to be stressed that in our previous papers, we did not investigate a one-way tunnel used for bi-directional traffic under particular circumstances, as well as equipped with natural ventilation only. The increase of the risk level in one-way tunnel tube used for bi-directional traffic in the event of fire--when the adjacent tube is closed for maintenance, repair, and rehabilitation activities--has not been hitherto quantified in function of the traffic volume that can vary by hour of the day (e.g., night versus day). This is a gap of knowledge that is needed to be filled, which is the scope of the present paper.