Hiking routes are hiking routes. You will meet very little traffic on a fairly wide road. The main roads may at times carry a lot of traffic. Avoid them if possible, or ride late in the night when trucks are fewer on the road. The sun does not set untill 11.
Tunnels Watch out for tunnels ahead.
As a rule of thumb the tunnels are usually closed to cyclists when more than 4 kilometers long. Try to find a ferry which will take you across the Fjord some time ahead - or ride to the tunnel anyhow. Usually there is a bus-stop on both sides. Sit down, relax and wait for the next bus in a couple of hours. You may also do as I did, when I suddenly discovered an unavoidable tunnel ahead.