Mitigation

With respect to the functionality of safety measures, a clear distinction can be made between preventive measures, i.e. safety measures aimed at reducing critical situations, and mitigation (protective) measures, aimed at reducing the consequences in case of an evolving incident.

Mitigation measures are related to softening the mechanical impact (e.g. safety barriers designed to absorb energy to minimise the degree of impact to a vehicle and the vehicle occupants), avoiding secondary collisions (by for example closing one or more lanes or the tunnel) or reducing and controlling the consequences in case of a fire arising within a tunnel.

The issue of fire safety in road tunnels has gained high visibility in the past following a series of dramatic fires which led to human casualties, major structural damages, and lengthy disruptions to the transport system with significant impact on regional economies. In a context where aging tunnels must cope with changed traffic composition and volumes, and where ever-longer new tunnels are built in increasingly challenging urban or geological environments, effective mitigation of the fire risks and their far-reaching consequences is paramount.

After an incident has occurred, the consequences must be faced with effective protection measures so that the users involved can save themselves and, at the same time, all those not directly involved can immediately react to avoid injuries and further damage.

Detection devices provide early warning of traffic incidents or abnormal conditions in the tunnel, which can result in the interruption of normal traffic flow. Verification devices and/or traffic management methods allow tunnel operators to rapidly confirm an incident and implement incident response scenarios to systematically close lanes or redirect traffic to prevent secondary collisions, and in the event of a fire, implement safe and efficient evacuation of users by means of information and communication. Furthermore, automatic fire-fighting devices like fixed fire-fighting systems can significantly reduce the consequences of a fire within a tunnel by controlling the fire development until the intervention of emergency teams.

A number of factors distinguish a tunnel fire from a fire in a conventional building. The escape environment for people is difficult in a tunnel, due to long escape distances and time, the potential for high fire loads and rapidly growing fires with intense smoke production filling up the whole tunnel section, combined with a rapid smoke spread. Tunnel fires may also cause extremely high temperatures, which can not only lead to structural spalling and even collapse, but can also make fire-fighting efforts more dangerous and time-consuming.

Fire prevention is of course the first priority, but it is essential to also integrate appropriate fire detection and response systems that will slow down structural degradation, ensure the safe evacuation of people, prevent fire spread, and facilitate fire-fighting efforts.

In a tunnel, detecting an incident a few seconds earlier can save lives and considerably reduce infrastructure damage and loss. Identifying a developing fire at an early stage (e.g. by video detection, smoke detection systems, linear heat detection systems, etc.) is a significant success factor in effective incident mitigation in tunnels. Indeed, long-term closures of damaged tunnels following a fire have in the past caused long-term economic consequences and extensive impact on other parts of the transport network. The implementation of appropriate fire mitigation measures can therefore be vital even from a merely economical point of view.

Other than the geometric design of a tunnel (length, section, emergency exit spacing etc.), the most important measures for the mitigation of the effects of fires have been demonstrated to comprise a permanent supervision by a control centre, the ventilation system, the monitoring system, the equipment used to close the tunnel and the fire resistance of equipment. In particular, adequate ventilation strategies are fundamental and they need to be correctly defined in order to be able to manage smoke control in different possible situations.

Immediately informing the control centre in the first phase of fire development and following appropriate actions and instructions can prevent worse situations. In this way it is possible to speed up both the evacuation process according to the information the control centre can provide to users and the intervention of the emergency services, contributing to the mitigation of the effects of collisions and fires.

More information on preventive measures for tunnel collisions is provided in the technical report 2019R03EN “Prevention and mitigation of tunnel related collisions”.