My problem with "40MPH" roads is the speed of traffic is usually 55-60. You do indeed have the right (and responsibility) to ride on the road - but then again, you can be dead right, and still dead.

That said, I avoid those roads entirely. You're probably thinking like a driver, who asks "what is the shortest and fastest possible route?" You should think like a cyclist - "what is the safest and most pleasant route?"