Traffic can reach up to 200 000 vehicles a day in a road tunnel and the safety of users is a major issue. A fire taking place in a tunnel can have awful consequences.
The same risks with the same consequences can occur also in a metro carrying passengers.
Acoustic in tunnels is the other main issue to handle in regards to both users and neighbours.
The noise level inside must be low to allow users to hear to the fire alarm and people to work properly. The surrounding outside noise must be limited not to bother neighbourhood.