The tunnel, which cuts through the Lovstakken mountain in Bergen, was built parallel to the new light rail line that opened in November and links the borough of Fyllingsdalen with the city centre.
It doubles as an escape route for train passengers, with dedicated pedestrian and cycle lanes of 2.25 and 3.5 meters wide, respectively, included in its design.
At 2.9km long, the tunnel takes about 10 minutes to cycle and 30 to 45 minutes to walk.
The Fyllingsdal tunnel and the rest of the cycle route to Bergen city centre have been financed through the municipality's state-supported Environmental Promise programme, whose goal is to make it easier for people to choose cycling and walking over driving.
The project aims to help reduce traffic in the city, cut planet-heating emissions as well as unhealthy pollution.
The Snoqualmie Tunnel near Seattle, Washington, in the US is 3.6km long, making the Fyllingsdal tunnel the world's second longest overall — but it is the longest that was built for the purpose of accommodating pedestrians and cyclists.
The tunnel will be open from 5.30am to 11.30pm daily and features well-lit rest stops and security cameras throughout.
Colourful dynamic lighting will create a wave of light when a cyclist or pedestrian enters the tunnel at either end, alerting cyclists to oncoming traffic.
Emergency phones are available every 250 metres, and it will be kept at a constant temperature of 7°C, making it an attractive training route for runners on colder days.
The tunnel project received government investment, with the state covering half of the additional cost of 500 million kroner (approximately £40 million to upgrade the emergency tunnel to a cycle and pedestrian path.
Tunnel inauguration ceremony and events
On Saturday, the tunnel will be opened to the public for the very first time, presenting an opportunity for residents to tour this remarkable feat of engineering.
To mark the occasion, the city of Bergen has organised a weekend of festivities in which the tunnel's opening will be celebrated with various events, including a variety of bike races for young and old.
Sunday marks the official opening of the tunnel to full bicycle traffic. The city of Bergen is set to celebrate this milestone by organising a bicycle parade from Fyllingsdalen into the city centre.
The opening of the tunnel marks a crucial step towards a greener future and a more sustainable transport system, according to organisers.
The weekend of events is set to be a call to action for more cities to prioritise the health and well-being of their citizens by investing in active transport infrastructure.