This tunnel is actually 20kms in length if you don't include a 200 metre open air section on the island of Hundvåg. It then joins with another tunnel which is nearly 6kms length from the island of Hundvåg into Stavanger city centre. So, except the 200 metres where it surfaces on the island of Hundvåg, it's basically a 20km subsea tunnel. But all of those technicalities don't matter when you take into account they are currently busy constructing a 27km (17 mile) and 392 metre-deep tunnel from Stavanger to Bokn. The new tunnel under construction will be the new world's longest and will remove the need for ferry connections on the E39 road from Stavanger to the city of Haugesand - therefore hugely reducing travel time between the two cities. It will be called the Boknafjord Tunnel and is part of the Rogfast project.
Cleaning machines just like with any road. In a tunnel it's even easier because there are no ditches. Traffic accidents by themselves ain't a problem, but fire. This is of course a huge problem, but met by surveillance 7/24 and firing squad on constant alert. There are also security rooms every 250 meters. The biggest problem is probably people not taking fire seriously enough and are too slow to get to one of the secure rooms. You can also choose to get out through the other tube and hitchhike. There are too few people to kill in a tunnel, thus a terrorist would prefer a place with higher concentration. This is why airplanes and tall buildings have been popular.
@@RU-vid_Stole_My_Handle_Too Also any fire alarm activation AND/OR any instance of someone even touching one of the SOS telephones, activates an emergency closure of the tunnel and it is closed by barriers at each end of the tunnel not allowing anyone else to enter.