Where I live there is a section of highway along the California coast that is the busiest two lane road in the entire country and traffic includes large tractor trailer rigs. The state transportation planners have done nothing over the past 50 years to make it safer and actually did work this past year that made it far more dangerous for cyclists. I used to tour the length of the state a couple times each year but long ago decided this was getting to be much too dangerous with American drivers being as aggressive and inattentive as they are.
The trick with bike routes is dealing with elevation gains and steep grades and not wanting to leave weaker riders behind. In terms of wider shoulders these tend to be only along major highways where they provide a place for motorists to pull over to change a tire (which is much more dangerous than people realize).
There is also the matter of having enough parking area available for riders to leave their cars and start the ride. And ideally it should be an area where the riders can come back and find their vehicles intact. This is more of a problem than one might think but check your own area and look where such areas exist.