-In the subsea tunnels a bit further east, in Norway, large fans mounted at regular intervals help maintain a draft through the tunnel.
I don't think much forced ventilation is needed for normal operation - the blades spin very leisurely, if at all - but in case of fire they can direct the smoke and heat away from the bulk of the people trapped inside.
Probably this. And burning batteries may produce less smoke, but it can be more "fun" than regular gasoline or even burning plastics, though even plastics are a sufficient reason for good ventilation.
I drove through the new tunnel today, and this is exactly what the ventilation looks like. I noticed they spun very slowly, which is likely variable based on detected air quality.
I don't think much forced ventilation is needed for normal operation - the blades spin very leisurely, if at all - but in case of fire they can direct the smoke and heat away from the bulk of the people trapped inside.
The fans look like good ol' jet engines.[0]
[0] https://www.systemair.com/fileadmin/_processed_/c/7/csm_Syst...