I looked up the forum topic "Increased." After reading Cliff Goodall's reply, I agree. Our road association includes a couple of subdivisions that have in the deeds to all their properties a clause prohibiting commercial uses except for cottage industries. But if you don't have that in the deeds, I don't think you can restrict what people do with their property. However, you certainly can point out to the owner who is planning the retreat center that the road was not designed for such use. I would expect a retreat center would involve large groups of people arriving all at once and leaving all at once. That could cause problems with opposing traffic on a single lane road, and it will certainly place an increased burden on the road beyond what would be expected from use of a single residence. At the very least, a "fair and equitable" distribution of the cost of maintenance is going to require a different fee for the retreat center due to the multiple vehicles that will be going in and out to that one property.
Some other considerations:
Has the retreat center already been constructed, or will there be a lot of construction vehicles going in and out? Do your bylaws have a clause that allows you to charge extra to cover damage to the road caused by a landowner? (There is a forum topic on that subject.)
There is also the question of HOW the road will be used by people who have no particular interest in the condition of the road in the long term, as was mentioned in the topic on increased use. People coming to a retreat center probably have no idea how to drive on an unpaved road. Especially since it's only one lane wide, I would be willing to bet that nearly all of them will drive right down the same two strips down the middle of the road, instead of avoiding other people's tracks and keeping their tires on the high spots so as to wear the road evenly.
If there happens to be a retreat scheduled after a rain or when the road is soft, the impact on the road could be extremely detrimental. Is the manager of the retreat center going to accept reservations at all times of the year, including mud season? Or if there is an unexpected thaw or heavy rain, is there any plan for saving the road?
My husband suggests that if the owner of the retreat center is willing to bring the road up to a standard that will support that sort of use, (short of paving, which runs into statutory prohibition under section 3101,) then the road association could continue to take care of the road from there. But keeping the road to that higher standard could potentially increase the cost to everyone. The road association should not have to take on the expense of upgrading the road for the benefit of the retreat center, nor of upgrading it to prevent the retreat center's use from damaging everyone else's access.
All in all, there are a number of factors that would justify charging the retreat center at a higher rate than single family residences. If you make that clear to the owner before they get too far along with plans, perhaps they will think twice about whether their plan is reasonable or prudent. And if your bylaws do not already have a clause regarding charging for damage by an owner, I would urge you to amend your bylaws to allow that. I'd also make sure that the bylaws specify your "formula" for determining each member's dues, so it's clear that all properties that place a higher burden on the road will pay at a proportionally higher rate than single family residences.