In this situation, can you go straight on to any lane if there are no other vehicles in the tunnel?

In accordance with the principle of driving on the right, the driver is obliged to travel in the right-hand lane, as close as possible to the right-hand edge of the carriageway. The absence of other vehicles in the tunnel does not exempt you from this obligation. The left lane is for overtaking or passing, not for continuous driving, as specified in Art. 16 para. 4 of the Road Traffic Act.