And yet by definition, a road with shoulders will be a busier road - thus, the justification for the shoulders in the first place. Rural roads with shoulders will be more recently constructed or reconstructed routes. Not only will the road have moderate or heavy traffic in most instances, but speeds will be fairly high since the road is usually a through route and has long sight lines and low grades.

Yes, it is a matter of preference. A road with shoulders is quite safe for cycling although it will not have the magic that many cyclists seek. On roads without shoulders, the most dangerous time is what I call a "scissors" - when two cars in opposite directions meet right where I am cycling. With each doubling in traffic - potential scissors do not double, they quadruple. 2 cars (A & 1) yield A1. 4 cars (A, B, 1 & 2) yield A1, A2, B1 & B2. So, you are right, roads without shoulders get pretty tricky as traffic increases.

Impaired driving is another factor that is extremely difficult to determine. There are two main types - drunk driving and cell phone driving. For drunk driving I have used caution on weekends, especially holiday weekends, and around reservoirs. For cell phone use, there is not much one can do. It is worse in town than on the highway, but I have done a couple of unscientific counts and have found up to half of all drivers talking on the phone. Shoulders aren't much good with cell phone users - thus, traffic count becomes more important. In fact, a winding, back road may discourage cell phone use.

In addition, people are in much more of a hurry nowadays so speeds on all roads have increased - regardless of the posted limits. I'm an old geezer, so remember a time when the mandatory speed limit was 55 mph and going over 60 was risky - Ha-ha. Now 70 mph on two-lane roads is pretty common. Also, with exurban residential development, far more people have long commutes into metro areas. In the olden days, people rarely went over 35 or 40 on farm roads. Now, people are driving 50 on them.

Oh, yes. One more thing about shoulders. There is something that I call "snooze bars". These are the grooves that are cut into the pavement on the edge of highways to alert drivers that they are veering off the road. Depending on the width of the shoulder and the placement of the snooze bars, they can be a real danger for cyclists. If the shoulder is narrow and the snooze bars are right in the middle, you are almost forced to ride in traffic. Yet, drivers assume you should be over on the shoulder. So, it makes it MORE dangerous than if there were no shoulders at all. Anyone with a good deal of experience will confirm this nasty hassle. Even if the snooze bars are on the lane line and there is enough shoulder room, often the shoulder disappears at places like bridges and culverts - forcing you to cross the snooze bars repeatedly into the traffic lane. Also, there are two types of snooze bars - the thin type (1/2 inch deep) which is easy to cycle over and the deep type (2 inches deep) which is impossible to bike over safely without throwing your bike. Of course, with snooze bars, you can hear the person veering off the road behind you as he/she is talking on the phone - so it is a two-edged sword.

Just a few additional thoughts.