Collisions

Traffic safety is a key success factor for road tunnel safety. In general, on a yearly basis, most injuries and fatalities in tunnels are related to traffic incidents that could also happen on the open road. However, since a tunnel is an enclosed space, the escalation of a collision, in terms of fire and the release of dangerous goods, could have far more serious consequences than on the open road, because more people than those directly involved in the incident can potentially be exposed to the hazards of heat, smoke, explosions or toxic gases. Moreover, the tunnel itself can contribute to the cause or the effects of a collision, for instance because of changing light conditions (the “black hole effect” when entering the tunnel) or because the tunnel wall is an “unforgiving obstacle” that can worsen the mechanical impact of a collision or impede a successful evasive manoeuvre.

To summarize:

• The prevention and mitigation of collisions in (and nearby) road tunnels needs special attention as compared to the open road;
• Assuring traffic safety in (and nearby) road tunnels is very effective to assure tunnel safety in general (although it must be noted that many, if not most, fire incidents in tunnels are vehicle related and not collision related; therefore, additional measures to assure tunnel safety remain necessary, even in the hypothetical situation that collisions are ruled out entirely).

Compared to the open road, there are several factors that may influence the probability or the effect of a collision in (and nearby) tunnels in a positive or negative way:

Positive:

• Environmental conditions are better controlled within tunnels: absence of rain, snow, ice, wind, fog, avalanches, falling rocks, etc. (but sudden changes may occur at tunnel portals).
• Tunnels can provide a shorter or safer route as compared to alternative routes (like a curvy and steep mountain pass).
• In general, there are fewer (or no) junctions, interchanges, crossroads and intersections (on the other hand, special attention is needed if they are present in or nearby a tunnel).
• Pedestrians and slow moving vehicles, (like mopeds, motorcycles, agricultural tractors or other equipment), are generally prohibited in road tunnels (or at least separated from the faster motor traffic).

Negative:

• Drivers need to perceive, analyse and understand a different driving environment.
• Especially during day-time, drivers are confronted with changing light conditions when entering and exiting the tunnel.
• Tunnels are enclosed structures with confined space that can cause, for some drivers, feelings of anxiety, and a specific behaviour of people in the event of a collision (this can be true for people both directly or indirectly involved in a collision).
• If air quality is not controlled and kept within permissible limits the behaviour of drivers can be affected.
• Proximity of stationary obstacles (like tunnel portals, road signs, ceiling, tunnel wall) may cause major disturbing effects; moreover, these obstacles can contribute to a more severe mechanical impact of a collision.
• Protective measures typical for open road sections (e.g. barriers or other energy absorption systems) are not present in all tunnels.
• Emergency lanes are often not present in road tunnels.
• The monotonous atmosphere of long tunnels may hamper the driver´s awareness.
• The tunnel conditions may cause misjudging of the horizontal and vertical alignment, as well as the safety distance from other vehicles and obstacles; moreover, the tunnel wall and ceiling can limit the viewing distance for the drivers when there are curves or gradients in the alignment.

All in all, the tunnel manager must consider traffic safety in a specific tunnel in terms of risks and measures; he must analyse and evaluate the risks (on the basis of criteria for both traffic safety and tunnel safety) and he must consider, choose and implement measures to control these risks. For new to be built tunnels, this has already to be taken into account in the planning and design phase. For existing tunnels, the tunnel manager has to evaluate the safety situation in practice (feedback from experience, learning from actual incidents or near-incidents) and take measures to improve the situation when necessary.

Of course, not all the causes, like drunk driving, mobile phone use while driving, or the defective technical condition of a vehicle, are within the circle of influence of the tunnel manager. The same goes for the effects of the collisions. However, the tunnel related causes and effects can be effective targets for the measures to reduce the risk of collisions.

More qualitative and quantitative information that is useful for the risk assessment of collisions in road tunnels can be found in the report 2016/R35 “Experience with significant incidents in road tunnels”, notably in chapter 3. In the cycle 2016-2019, WG2 (Safety) of TC D.5 (Road Tunnel Operations) is drawing up a report “Prevention and mitigation of tunnel related collisions” on the (cost) effectiveness of the various measures that are at the disposal of the tunnel manager to control the collision risks.

Other useful reports on the subject include:

• Report 2016/R16 “Lay-bys and protection against lateral obstacles in tunnels, Current Practices in Europe”;
• Report 05.04.BEN “Road safety in tunnels” (1995).