At least where I grew up, bikes were not supposed to ride on roads without sufficient shoulder for them to pull out of the way of vehicles. The flip side is that there's a big lobby that tries to ensure that most roads have such a shoulder. Lots of rural roads don't have them, but I've found that it's better for me as both a driver and a cyclist to have such shoulders in the case I need to pull off or out of the way with either kind of transport mode.