Legal Question in Real Estate Law in California

Easements can be created by "prescription" as well as by express grant. Prescription is equivalent in many ways to ownership by adverse possession. The multi-year (5+) usage you mention was probably sufficient to create a prescriptive easement. There are also other ways (other than express creation by written instrument and by prescriptive use) in which easements can be created, e.g. easements of necessity, implied easements, plat-map references, etc. which are less likely to apply in your situation but which should be considered before assuming that there is no easement

I disagree with Mr. Whipple that the situation you describe necessarily gives rise to a prescriptive easement. The passage of five years is only one element of proof to establish a prescriptive easement. It is far from certain that the other requirements are met. On the other hand, there is no certainty in the limited information you provide, that one was not. In either case, you are all but certain to trigger a lawsuit if you just go and put up a gate. If you really want to prevent use of the driveway to for access to the adjoining parcel, you need to consult with a real estate attorney, preferably one with specific land title and easement experience. I spent the better part of a decade as an in-house litigator for Fidelity National Title and Chicago Title. If I can be of further assistance, feel free to contact me privately.