I think it doesn't really matter how far away you grow as long as you are smart about it. Don't let people see you entering the area all the time, and when you do go in there take some binoculars or something so if someone does approach you, you have a story already made up of what you were doing there. Everyone says on here to take a dollar or some half dollar coins and put them around your grow site, and check to see if they're still there, or if someone picked em up. Good way to tell if your site is safe or not. 31 miles is quite far in my opinion, though. If it's a small grow, move it closer, and save yourself some gas money. Remember, stupidity is the reason most people get caught. Don't tell anyone about your spot, and go early in the morning, just as the sun is coming up. Good luck with your grow.