For those in Europe and the UK, in most of the U.S. there are fairly draconian local laws regarding public lewdness, indecency, etc. - depending on your locale, getting caught by law enforcement can be devastating. I think this is certainly one discouragement against things like dogging (in the traditional sense).
We've had one experience at a truck stop that approaches UK/Euro-style dogging, and a few others that were essentially using our vehicle has an opportunistic venue, to "meet the needs in the moment." i think it would be incredibly hot to have well-known locations to drive to for a quickie random stroke, suck and fuck session.