The old Post horse trail, „The King’s Road“, over Tronasen was rebuilt for vehicles in 1839-1844. At the same time the Bakke Bridge over the river Sire was constructed.
In Rogaland county this road has eight hairpin turns and has a maximum gradient of 1 in 4. The steepest part of the road, however, is found in Vest-Agder county where the gradient is 1 in 3. The road was built with width of 2.5-3.5 meters. About 3 km of the old road has been maintained in its original form.

The „West Coast Main Road“ over Tronäsen attained the Status as a national road in 1928 and was open for ordinary traffic until 1946. As late as the 1970s the road was used as a detour route for cars while construction work was being completed on E18 between Tronvig and Sirnes.
The road over Tronasen is one of Norway’s most famous roads. For some years after 1931 it was part of the Rally Monte Carlo. In 1981 the road was improved for the anniversary race of the Monte Carlo rally.

Source: Information board on site