If there is nothing in the easement addressing this (which there usually isn't) and there is no joint maintenance agreement, what you are left with are general principles of easement and real estate law that are not going to give a very clear answer. In general, the beneficiaries of the easement have a right to "maintain" the easement, but not to "improve" it in the sense of changing it from gravel to pavement, for example. And if the proposed "improvement" means widening the traveled roadway, that can be done, but only to the extent that the user actually intends to use it for ingress and egress consistent with the scope of the use contemplated when it was granted.
Unfortunately, there is no case or statute that I am aware of that will delineate who is responsible for what in the absence of a maintenance agreement for a multi-user right of way.
As a general proposition, all owners of the easement have an obligation to contribute to maintenance.
However, determining who is responsible for what amount is decided on a case by case basis and is dependent upon the facts of the situation. For example, a user who regularly utilizes the first 50 feet of a mile long right of way may not be responsible for maintaining the entire length of the right of way.
I am not a WA attorney, laws vary from state to state, therefore you should always consult a local attorney.
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