While it's varies depending on a lot of circumstances, a lack of road frontage does diminish the value. However, on a property that is already developed, the value is mostly based off of appeal to the buyer / renter rather than hard costs of build a structure or perform maintenance.
As Larry said, try to find out about any maintenance agreements.
If you find the property desirable, without the road frontage, because of other merits, then chances are that there will also be others who will see this value.
Regardless, from an investment point of view, unless the road frontage were to somehow change (and depending on the maintenance agreements) the value for the property is already determined with this in place and should not affect future resale value any further.
Hope this helps!