Abstract
We have developed a traffic simulation model to quantitatively assess the resilience of a twin-tube motorway tunnel in the event of traffic accident or fire occurring within a tube. The motorway section containing the tunnel was investigated for different possible scenarios including its partial or complete closure. The functionality of the road infrastructure, in the case of an accident in one of the two tubes (each tube presents two lanes with unidirectional traffic under ordinary conditions), was assumed to be recovered both by using the remaining undisrupted lane of the tube interested by the disruptive event (only one lane is closed) and reorganizing the traffic flow by utilizing the adjacent tube for bi-directional traffic (both lanes are closed). The effects of an alternative itinerary individualized in the corresponding open road network were also examined. The level of functionality of the system during the period in which the tube is partially or completely closed was computed as the ratio between the average travel time required to reach a given destination from a specific origin before and after the occurrence of the disruptive event. The resilience metrics were assumed to be resilience loss, recovery speed, and resilience index. The best scenario was found to be the partial closure of the tube in contrast to the complete one. However, in order to contain the negative effects on the functionality of the motorway section due to the complete closure of the tube, it is worth highlighting how the traffic by-pass before the entrance portal of the closed tube should be open in a very short time by the tunnel management team to allow for the quick use of the adjacent tube for bi-directional traffic. An additional improvement, with reference exclusively to passenger cars traveling through the adjacent unblocked tube, might be obtained by activating the variable message signs, located at a sufficient distance from the motorway junction before the entrance portal of the closed tube, in order to suggest an alternative route to heavy good vehicles (HGVs) only. Whereas, when the alternative itinerary is used by all vehicles traveling towards the blocked tube (i.e., both passenger cars and HGVs), this redirectioning of the motorway traffic flow was found to be characterized by an excessive travel time, with it therefore not being advisable. The results obtained might be useful as a decision-making support tool aimed at improving the resilience of twin-tube tunnels.
Highlights
Given that the partial or complete closure of the tunnel tube affected by an incident was assumed to happen in the morning between the 7:00 a.m. and 10:00 a.m., it is to be recorded that we investigated in the simulations the effects on traffic flow due to the duration of a disruptive event occurring in the north tube of 1, 2, and 3 h, respectively (i.e., from 7:00 to 8:00 a.m. (1 h), from 7:00 to 9:00 a.m. (2 h), and from 7:00 to 10:00 a.m
In the case of the complete closure of the tube, the delay time on the motorway is computed to be smaller when the traffic by-pass is opened—by the emergency service team in order to allow for the use of the adjacent tube for bi-directional traffic—within a time of 10 min from the start of the incident (i.e., Scenario 2a in contrast with Scenarios 2b, and 2c)
The occurrence of the disruptive event was assumed to lead to different possible scenarios including the partial or complete closure of a tube, with consequences affecting the vehicular transit on both the motorway and the corresponding road network connected by means of the motorway junctions
Summary
To provide the social and economic benefits for which a tunnel is designed and built, it should be kept open to traffic as much as possible. The functionality of a road tunnel might be temporarily compromised by a traffic accident or fire occurring within the structure. The consequent non-availability for the passage of traffic flow through the tunnel, which can be caused by the aforementioned incidental event, might require a great deal of time and effort to normalize the situation. The recovery process of the operating conditions of the road tunnel after a disruption of traffic flow, as well as the possible presence of alternative itineraries in the nearby road network, is nowadays becoming a crucial issue for community interests at different levels
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