Hello Stumpfield,
I am a licensed real estate agent licensed to practice in California. Please take everything seriously that the folks here have advised for you. While your neighbor can be very nice and sweet, allowing use of the property can only open a pandora's box for the future. Numerous issues are involved not the least of which are opening the door for a future prescriptive easement as well as liability concerns. A written agreement granting permission might protect you from future easement claims, or it might not. What shall happen 20 years from now when your memory fades a bit, you are unable to recall or find this document and/or heaven forbid the unexpected happens to you? The only thing for certain is that the use has been ongoing and open. Furthermore, accomodating traffic ensures that you now have agreed to take on the liability of additional traffic on your property. So someone she knows (or will know in the future) begins to utilize the drive for trucks, tractor, who knows what else, will you put a stop to their access after say 5 years? Too many factors, variables and unknowns for the future can only lead to trouble. Besides, I guarantee that as difficult as it may be to say no, or to put a stop to it now, it will be infinitely more difficult 5 years from now when you have a change of heart.
Do the wise thing and DO NOT GIVE PERMISSION. If you have already given permission, REVOKE it immediately. Heck, give an excuse and say that your attorney friend, or brother in law, or someone close to you, TOLD you to put a stop to it. In any event, the following link might be valuable for you to read up on involving adverse possession. While adverse possession is much more difficult (someone taking ownership of all, or a part of anothers property) through open use, an easement by prescription claim is quite common. Once it is in place, it is almost impossible to revoke and requires you to maintain all of the costs and burdens of ownership but yet continue to provide access for another's benefit. Heck, your future neighbor might not look like Christie Brinkley but rather look like Oscar Madison. Doesn't matter, he has access. And of course, it limits yours, and your buyers options and ability to freely develop and use your property as you see fit. Know also that you reside in one of, if not the most liberal states in the nation with courts to match. Therefore, should you find yourself defending a future claim of ownership or possession or bodily injury or property damage, know now that the outcome is far from a certainty given our courts history and record. The one certainty that you can derive is that it will be a costly legal battle.
Just my 2 cents.
Disclaimer (standard stuff to protect my butt): I may be right, but then again I may be wrong. This is not legal advice, or even Real Estate advice. Everyones situation is unique and sound advice can only be obtained when all facts and factors are considered. Certainly, you are warned not to rely on this users forum post (or any post responding to a one paragraph question by someone not 100% familiar with your situation) to manage your legal affairs. You should seek the advice of a Real Estate attorney to discuss your situation fully and in depth PRIOR to making any decision, without which you should proceed at your own risk.
Link below (even includes sample consent form, as requested):
Trespass, Adverse Possession & Easements