The road over Tronåsen was built between
1840 – 1844 as a part of the main Western route.
This was the first purpose built road for horse carriages
between Oslo and Stavanger. Prior to this, there had
only been a horse track. Bakke bridge, built in 1844,
was designated as a National historic monument in
1959 and the remainder of the road in 2009.
The road over Tronåsen is about 5 km long. Along this
stretch there are eleven 180 degree curves or
‘switch backs’ : Eight on the western side, and three on
the eastern side. The eastern side of the hill is steepest
with a gradient of 1:4. The road is 2.5 – 3.5 m wide.