Three classes of fire suppression system can be installed in tunnels.
Deluge or sprinkler systems are well developed and reliable. Most applications use three rows of sprinkler heads and sensors, one in the crown of the tunnel, and one part way down each side. A Japanese variant relies on a single row offset from the crown.
Sprinklers may not extinguish some types of vehicle fires, but can control them, enabling drivers and passengers to escape while allowing emergency services to get close to the seat of the fire. Their main drawback is the need for water storage.
Fog or mist systems use much lower volumes of high pressure water forced through small jets at up to 200 bar. Their main purpose is to stop the fire spreading, while allowing people to escape and fi refighter access.
Installation costs are higher.
Compressed air foam systems are the most effective at extinguishing fires. Installation costs are said to be competitive, but there have been fears that the foam is slippery.