## 1 Introduction  
Road tunnels, as part of a transport infrastructure, play an essential role in supporting the mobility of goods and people. For providing useful effects in terms of social and economic benefits, it is extremely important to keep tunnels available for traffic as much as possible. Maintenance, repair, and/or rehabilitation works within a tunnel usually make more difficult the transit of motor vehicles, and sometimes the tunnel might also be temporarily closed for safety reasons. In these cases, the consequent functionality loss of road infrastructure might be recovered by using an alternative itinerary or temporary reorganizing--for example if a twin-tube tunnel is present--the traffic flow by using a tube originally characterized by unidirectional traffic for bi-directional traffic, when the adjacent tube is closed. However, a one-way tunnel tube when used for traffic in two driving directions under particular circumstances might present much more dangerous conditions for users than when it is used for one driving direction, since more serious head-on-collisions, in terms of injuries and/or fatalities, might be expected. In addition, worse environmental conditions in the event of fire within the tube (e.g., temperatures, smoke concentration, visibility distance, and toxic substances) might also be expected along the user escape path downstream of the fire. In fact, due to the ventilation direction (mechanical or natural) within the originally unidirectional tube, the hot airflow is generally pushed towards people who are in the zone downstream of the fire, and that having left their own motor vehicles in the queue to run away from the burning vehicle in the direction of a safe place (emergency exits or portal). It is to be stressed, in the case of unidirectional traffic tube, that users downstream of the fire can, instead, escape with their own vehicles towards the exit portal without any significant risk. Indeed, the mentioned problem of additional risks in a one-way tunnel tube when is used for bi-directional traffic in the event of a fire has hitherto been hardly investigated. Moreover, the impacts on increased risk due to the traffic variations are not yet clearly understood. The unit of measure for traffic generally used in a risk analysis is the Annual Average Daily Traffic (AADT), defined as the total traffic volume in one year divided by the number of days in one year. However, the AADT does not reflect the fact that traffic can vary seasonally, by day of the week (e.g., weekdays or weekends), and by hour of the day. This means that the estimation of the number of persons exposed to risk within a tunnel in the event of fire, which is concatenated with the mentioned traffic variations, might not be sufficiently precise. Traffic volumes referring to an interval of time shorter than one day are more appropriate to represent, other than the service level of tunnel, more exactly the number of users who risk their own health due to a tunnel fire. In this respect, peak-hour traffic volumes during the day should be more especially implemented in risk analysis. In addition, hourly traffic volumes at night should be also considered in order to make appropriate comparisons between the different risk levels (i.e., night versus day). The mentioned topic of discussion in the risk analysis of tunnels (i.e., the use of one-way tube for bi-directional traffic during both daily peak-hours and overnight in the event of a fire) indicates a lack of knowledge that is needed to be filled, which is the scope of the present paper. In the light of the above considerations, this study aims to investigate in greater depth the effects of a fire on the people evacuation process from an originally one-way tunnel tube that is used temporarily for bi-directional traffic.