I could see a Sonnenberg Tunnel scenario in which a carload of Swiss PCs are caught between two overlapping alternate realities: the one they come from, in which the tunnel is functioning as a normal roadway -- with dangerous speeding vehicles everywhere -- and one in which it's sealed as a fallout shelter during a nuclear attack. The PCs have to figure out why the realities are bleeding over into one another, while trying to escape the fallout quarantine. If they mess it up, they do escape the structure - but only into the nuclear apocalypse, not into their normal Swiss road trip.