Following the integrated approach for the planning of a (sufficiently) safe tunnel system design and operation of a tunnel has to comply with minimum safety requirements. Further, alternative or additional safety measures may be required for various reasons, like for instance:
The decision whether or not additional safety measures are required, can be based on a prescriptive as well as a risk-based approach
In general, safety measures can be grouped into 4 categories, according to their mode of action and their main effects on risk:
Measures may concern tunnel structure or tunnel equipment, as well as procedures for tunnel operation or emergency response. If the transport of dangerous goods is authorized in a tunnel specific measures may be required. Any additional safety measure needs to be integrated into the existing complex safety system of a tunnel in the best possible way, taking all relevant interaction effects into account.
Prevention is a key approach in the effort to minimise the risk of collisions and other significant incidents in a tunnel and therefore crucial for the development of a safe tunnel system. The preventive measures are those acting ahead of a dangerous scenario to reduce the probability of occurrence of situations which can have a negative impact on the safety of tunnel users.
Preventive measures are developed using activities and practices aimed at anticipating, avoiding, and removing possible causes of a hazardous event.
Safety measures in general are based on a systematic consideration of all aspects of the tunnel system, i.e. infrastructure, operation, users and vehicles. Each tunnel provides its own unique safety challenges, based on where it is located, its layout, length and cross section, as well as traffic volumes and patterns; therefore the safety measures have to fit the tunnel system.
In the specific confined environment of the tunnel, fire and the release of dangerous substances are the most critical incidents that are considered in relation to the catastrophic consequences that they can generate. For this reason, many safety objectives are closely connected to these phenomena.
Fire in a tunnel can be the consequence of a collision, or be triggered by different other causes such as overheating of the engine in traffic congestion or on long stretches of a climb, overheating of the braking system of heavy vehicles over long downhill sections, or shorting of electrical circuits.
If vehicle maintenance is outside the responsibility of a tunnel operator (which is typical), several technical measures can be implemented to avoid collisions in a tunnel.
First of all, a safe road design is fundamental to minimizing collision risk but of course it cannot be implemented as an additional preventive measure in existing tunnels. In any case, creating (and maintaining) an environment for drivers that enables them to assess the traffic situation and the course of the road and to anticipate the necessary actions they need to take as part of their driving task is fundamental to minimizing collision risk.
For tunnels in general, the relevance of tunnel lighting and adequate road signage are highly ranked. The quality of tunnel lighting assures better visibility, higher illumination density, a calming effect and in long tunnels it can be used to break monotony and keep the user focused. Moreover, a good quality lighting system should reduce or avoid the so called “black hole effect” when the eyesight of a driver entering a tunnel has to adapt to the changing light that can limit the sight distance or can cause drivers to slow down. Also optical lane guidance aids, such as reflectors or LED marker lights on the tunnel wall can help users to correctly perceive the road profile. The ventilation system plays an important role too in assuring the air quality and consequently good visibility inside the tunnel.
Dynamic warning signs (activated by detection) are likewise important to inform and alert users about dangerous situations (e.g. presence of an object on the road, stopped vehicles, closed lanes, difficult environmental conditions at tunnel portals), to reduce their speed, increase their level of attention and to avoid risky behaviour or to close the tunnel in order to avoid congestion occurring in the tunnel. From this point of view, all monitoring systems (sensors, video surveillance etc.) can contribute to minimizing the occurrence of critical situations.
An electronic overhead signal system is essential for lane control (such as lane closures) and to guide traffic away from closed lanes ahead.
Speed differences between vehicles is another common cause of collisions. Therefore an overtaking prohibition for heavy goods vehicles or the implementation of speed limits and speed control can be efficient measures especially when the viewing distance is limited. Furthermore, to prevent rear-end collisions ( a quite frequent collision type), it might be useful to control and enforce an adequate distance between vehicles driving in the same direction.
An effective preventive measure is constituted by preventive maintenance, above all for safety-related systems which are not redundant. This includes regular inspections, tests and cleaning of tunnel walls to maintain adequate safe conditions during the whole lifetime of a tunnel.
More information on preventive measures for tunnel collisions is provided in the technical report 2019R03EN “Prevention and mitigation of tunnel related collisions”.
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”.
Self-rescue describes the ability of people involved in a tunnel incident to get away from a source of hazard by their own initiative and appropriate behaviour. Experience from past tunnel fire incidents shows that smoke is the main problem for human safety in a tunnel fire. The opportunity to quickly reach a safe zone, which is not affected by smoke, is essential for mitigating the consequences of the incident. Hence the self-rescue principle is a key pillar of tunnel safety in case of a tunnel fire.
Fire in a modern building will normally give those in the building a reasonable opportunity to escape to a safe zone closely located from the area affected by the fire. In a tunnel, users have to escape to one of the portals to get outside of the tunnel. For long single-tube road tunnels without emergency exits to an escape route independent from the affected tunnel tube, a safe zone may be several kilometres away - often an unreasonable prerequisite for safeguarding the principle of self-rescue. Therefore modern tunnel guidelines include prescriptions for a maximum admissible distance of emergency exits leading to a safe zone, which can be an escape tunnel, an escape shaft, an exit to the open air or a parallel second tunnel tube. For instance, Annex I of the European Directive on minimum safety requirements defines a maximum admissible distance of 500m and national guidelines often define stricter requirements.
Serious incidents that challenge the self-rescue principle are fast-developing vehicle fires involving vehicles with a high fire load (mainly HGVs with combustible loads) which are able to produce a lot of smoke within a short period of time. However, a fire in a small vehicle can also turn into a larger fire, e.g. by involving several vehicles.
In any case, it is important to evacuate early and fast. Therefore, in addition to providing an emergency route to a safe place, measures are required to make sure people react quickly and evacuate early through the emergency exits.
A closer look into the self-rescue process reveals that the evacuation behavior of people can be divided into several phases. At the beginning is the pre-evacuation phase, which includes all events before the start of the evacuation and ends with the decision to escape. In the subsequent evacuation phase, we can distinguish the pre-movement phase and the movement phase. During the pre-movement phase, the tunnel user searches for information and selects an escape route. The movement phase includes all behavior that tunnel users display during the evacuation until they reach an escape target.
Hence, it is important to support this procedure by providing relevant information and giving clear instructions to people - visually or acoustically.
Further, self-rescue may be influenced by tunnel parameters and traffic conditions. For instance, there is a close link between self-rescue and smoke management strategies.
In tunnels with two tubes operating with unidirectional traffic and longitudinal ventilation, the ventilation typically supports smoke propagation in the driving direction (the initial direction of smoke propagation due to traffic-induced airflow). The smoke gradually affects the empty part of the tube in front of the incident. People that are stuck in traffic behind the incident can evacuate to the other tube and are normally not affected by the smoke. The second tube will normally be closed to traffic, and can be considered a safe zone to evacuate to. There is also a safe evacuation passage between the tubes. In unidirectional tunnels with regular congestion or in bidirectional tunnels the situation is more complex, as vehicles are likely to be blocked on both sides of the fire site; hence the interaction between self-recue and the strategy of emergency ventilation gets more complicated. However, modern risk assessment tools are able to investigate this process in detail for particular situations, thus providing valuable information for optimizing the interaction between human behaviour, procedures and technical equipment.
Furthermore, a steep evacuation route may also affect the ability to perform self-rescue as well as smoke propagation behaviour. Therefore, tunnels with high gradients require special attention.
Public education is important in providing knowledge and skills for self-rescue initiatives to road tunnel users. Information campaigns and publicity provided by road tunnel operators or authorities can help road-tunnel users understand the self-rescue measures to adopt when they encounter an emergency. In the context of self-rescue, special issues regarding people with reduced mobility have to be taken into account - see technical report 2008R17 “Human factors and road tunnel safety regarding users “
Many of these topics are also discussed in Section I "Objectives of fire and smoke control" of report 05.05.B, which includes a detailed discussion on tenability criteria under fire situations.
Emergency response measures are a group of measures covering the reaction of the tunnel operator, the emergency services and other organisations to a significant tunnel incident requiring intervention beyond normal operational procedures. As there is a wide range of different incidents, the measures required are diverse. For instance, a simple breakdown of a vehicle in a traffic lane might require the closure of this lane or of the whole tunnel tube. This example demonstrates the complexity of the topic “Safety measures”, because this response measure to the “breakdown” incident would also be considered to be a preventive measure against collisions.
Emergency response measures are typically complex because they interact with safety procedures and tunnel safety equipment as well as with human behaviour involving tunnel users, tunnel operators and emergency organisations. Among the possible hazards to be considered in road tunnels, vehicle fires are a particular concern because they can occur quite often, and the consequences can be more severe in a confined space. Therefore it is a fundamental requirement for every tunnel to make rescue and firefighting operations possible, even if the conditions might be quite different from case to case.
Whereas the operators know their tunnels quite well, firefighters need to perform tactically demanding work in a more or less unknown situation, compared with the daily challenges in industry and buildings. It is crucial for safe outcomes that firefighters have basic training in tactical methods for collecting information, search and rescue, and firefighting in tunnels.
Incident command needs knowledge about tunnels and tunnel fires in general and the specific characteristics of an individual tunnel in particular, and also knowledge about tactical dispositions of crews. Incident command needs to make the decisions in a very demanding situation, in terms of information access, efficient cooperation with the tunnel operator and interaction with a complex environment of tunnel infrastructure and technical safety systems. In order to cope with these challenges, it is important to establish close working ties between tunnel operators and local firefighting organisations in order to share knowledge on the specific tunnels, and to keep the fire-fighters updated on any aspects relevant for safety, such as changes in equipment, operational procedures, traffic conditions etc.
Fig. 1 : Tunnel safety exercise with fire brigade
Part of the material included in these reports relates to specific tunnel features and is dealt with in the corresponding chapters of this manual, for instance:
Some tunnels are a crucial part of the traffic infrastructure in a region or a city. If their availability is impaired this might have a major impact on the mobility of a whole area. For such tunnels it is important to assess the consequences of major incidents - accidental or intentional - on tunnel structure and equipment as well as the risk of resulting long-term tunnel closures and consider counter measures that can minimise closure time after a tunnel fire. The emergency response capabilities are often a crucial part of protecting the infrastructure, but other active and passive protection measures should also be considered.
A joint OECD/PIARC research project (ERS2 project: 1997-2001) led to an investigation of measures that could reduce the probability and/or consequences of an incident involving dangerous goods in a tunnel, where such goods are allowed.
Possible measures were identified and in a second step an attempt was made at evaluating the cost-effectiveness of these measures with respect to dangerous goods hazards. The focus was put on the effectiveness of measures as costs are specific to a particular tunnel project and cannot be assessed at a general level.
Some of the possible risk reduction measures are directly taken into account in the Dangerous Goods Quantitative Risk Assessment Model (DG QRAM) developed under the project . The effectiveness of each of these measures, or each combination of measures, can be assessed by running the model with and without the measure(s) and comparing the results. A large number of tests were undertaken, and this showed that no general conclusion could be drawn regarding the effectiveness of measures because the effectiveness very much depends on the specific case. An assessment of effectiveness should thus be made on a project basis.
The effectiveness of other measures (which are not directly included in the DG-QRAM model) was much more difficult to assess and methods were proposed to take a number of them into account. More information can be found in chapter VII of the OECD project report (Risk reduction measures).
Safety procedures are meant to explain how the operational staff should apply safety measures.
A first general level should:
A more precise level explains in more detail how each measure is to be applied:
These safety procedures should be concise and clear, so that the persons involved in applying a safety measure have no doubt about the required actions and can act efficiently.