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”.