Is it a violation in and of itself or would it fall under a general unsafe driving statute?
What if there is a slow moving farm vehicle in front of you moving so slow that you can pass it very safely?
What if there is a car stopped on the side of the road and you need to pull into the other lane for a few seconds to more safely get around it?
Usually, no passing zones are there for a reason, but common sense ought to prevail in not-normal scenarios, such as a stopped car. I mean, what are you suppose to do, sit there and wait until a tow-truck comes along? Slow-moving equipment…a bit fuzzier. I think you could get a ticket for the former, but not the latter.
Around here people driving tractors and other farming equipment on public (rural) roads tend to be very courteous about driving along the shoulder and giving motorists ample space to pass. That’s common and accepted custom - technically vehicles aren’t supposed to drive on the shoulder either, but in rural areas I very much doubt a passing cop would ticket either driver as long as nobody’s acting recklessly.
In California, passing in areas where the lines on the road prohibit passing are specifically mentioned in the Vehicle Code.
Just recently (technically not effective yet) it is now legal in Wisconsin to pass a bicycle (and other vehicles travelling “significantly less than the posed speed limit”) in a no passing zone.
http://legis.wisconsin.gov/2011/data/AB-265.pdf
Brian
In Ohio, it’s a violation in and of itself, but not if the vehicle you’re passing is going slower than half of the speed limit (cite).
I remember from doing an online kill-your-ticket course that stopped vehicles forfeit their right-of-way.
[emphasis added—DHMO]
In my four decades’ driving experience, common sense has absolutely no bearing on the behavior of traffic cops. 
I have been ticketed for the most stupid-ass trivial “violations”, while the horribly dangerous behavior of other drivers was given a “free pass” by the law.
In Québec, we’re allowed to cross a double line when passing bicycles and any vehicle bearing a slow-moving vehicle triangle (which includes all farm equipment).
But remember-- there is no speed limit on the shoulder.
Attentive drivers will not do these things if an LEO is around.
See how easy that was…
In the forest but did I see … er …hear anything? 
