There is a concept in real estate law called adverse possession. Each state has different ideas about how it works, but generally one must make it clear that one is taking control over a certain portion of land, making it clear to all observers of that intent, and keeping everyone else out. There is a certain period of time during which this must be maintained before one can then file a lawsuit to obtain title over that portion of land, and take it legally from another private party. The reason behind this law is that it allows people who want to use property and gain value from it, to actually take the property and then put it to good use. This maximizes the value of land. But this is normally done to acquire private property land, not publicly owned property. Another issue here is whether or not you have a right to use the alley based on an easement. There are various legal issues here that should be reviewed by an attorney. Pay for a consultation or two to get the answers you need, unless you are willing to give up your rights to using this alley to access the back of your property. Before you spend any money on this, be sure to analyze whether this is worth the thousands of dollars it will take to litigate this in the courts. If it is not worth it, then don't bother wasting your time and money. If it is worth it, then do not delay, because delay will only damage your rights to do anything.
This answer is designed to provide general information only, does not constitute legal advice and does not establish an attorney/client relationship. I am an attorney licensed in Maryland and California. A consultation and retainer will be required if you would like to obtain my representation. Office: (410) 381-1656. David Mahood, Esq.
You need to gather a few more facts, such as have a lawyer who practices real estate and property law research the land records to establish who owns the alleyway, and whether there are any easements or right-of-ways associated with its use which appear as covenants against the corner lot owner's land (if in fact the alleyway crosses any part of his property). You can file an action against the corner lot owner to enforce your access rights and to have a judgment declared regarding your legal rights. Because you and others have used the alley way within the past 20 years, it seems unlikely that the corner lot owner could succeed in an adverse possession lawsuit to claim the property, as that would require exclusive and openly notorious claim to the alley way for 20 straight years (exclusive as in, nobody else used it). Hiring a lawyer to file a court action may be expensive, but abandoning the alley way access may also diminish the value of your property.