Not to defend them, but here in Germany you are not allowed to build a single two way tunnel. You always need to build two tunnels being single direction only.

Not to defend them, but here in Germany you are not allowed to build a single two way tunnel. You always need to build two tunnels being single direction only. This is so that in case of a fire not both directions are unusable. This does not free you from the obligation to also build emergency tunnels.