Wow! A lot of reading about a neighbor, a driveway, sharing, legalities, do's and don'ts, ideas and a whole slew of confusing information. So to add to all this, here's my 2 cents;
Ok, in case anybody hasn't noticed, we are living in a "life is a video" world. So in any decision you make, videotape it!!! Whether it be a visit to the lawyer, a conversation with you neighbor, a security camera on your driveway, or for whatever concerns you have for this problem. I have noticed that a lot of posts in here forget that you mentioned you have given your combination out to other "neighbors" in case of emergency, well, looks like to me you've answered your own question. Even in a case of emergency where someone actually needs to use your driveway for an exit, there's still a chance of an accident happening and the probabilities are most likely increased due to the "panic exit" versus "walk single file to the nearest exit". At least with a security camera you'll actaully know what really happened. Plus you might have yourself a $10,000 video provided it's funny enough. I also like the idea of a tollboth, maybe not a full fledged one, but maybe one like you see at airport/public parking, one of those "mechanical arms" that raise up as soon as you drop .50 in a bucket. Of course, during an emergency, they'll have to do the Dukes of Hazard bust through the gate thing and zip bang boom, ya got yourself a winning video! You can also use the money to go to a local charity or something. Plus, having a "big" security video on property sign will sometimes detur people from entering, apparently they've already had their 15 minutes of fame.
As for our property(25.5 acres), I currently have one semi cheezy camera set up, but have the hardware from a small grocery store that upgraded their system(minus the cameras) to enable us to video 24/7 with 8 cameras. Darn cameras are expensive plus the cable to run 8 cameras who knows where on the property. Having teen agers, we have a lot of teen agers come over from time to time and whe they see the video camera or if our kids tell them we have a video camera, I think it makes them think twice about doing stupid stuff. But again, in a video world, teen agers love to be videotaped!
Anyways, thats my .02 worth, don't know if it helped, but hopefully it didn't hurt.
Steve