This whole debate came up during a Webinar with the FAA last year. It was made very clear that "cars in traffic" while may appear to be stationary they are not considered as such because the potential for movement at any moment. Same thing for a stop sign & red light. Stationary is more than "not moving for a moment".
Now in all reality... if I'm flying a "scene" and I'm going to need to cross the highway and traffic is grid locked I'm going to fly over the cars (with urgency) to get where I need to go. I do this knowing that if there is an incident my tail feathers are going to be in the sling because those vehicles are on a highway and I know how the regs are stated.
