I am about to have this problem right in my back yard. I know the farm field that is 50 feet from my back door is going to grow sunflowers some time soon, and my driveway is the obvious way for people to access it. I have seen cars parked carelessly on major roads in the area when the same farmer used other fields, and I am really not sure what to do when the problem comes to me. I will be fortunate in that the field is slightly hidden and off the road, plus sunflowers can't be planted in the same place every year, so there is only a single season for people to discover and seek out the site. I am just really not looking forward to dealing with all of the people driving down my driveway to get their shot.