The Fv457 runs 2.3 kilometres in tunnel under the Flekkerøysundet strait, from the mainland in Kristiansand out to Flekkerøya. The tunnel opened on 15 August 1989, after just over a year of building.

It reaches 101 metres below sea level, and the steepest gradient is 10 percent. The tunnel replaced the ferry across the strait, and gave Flekkerøya, in practice a district of Kristiansand, a fixed road to the mainland.

In summer Flekkerøya is a destination for outings, and the tunnel is the only way in. When it closes the island is cut off, so check the status before you drive.