'Le Passage du Gois' is mostly under water. During spring tide it can be used from 1:45 before to 1:45 after ebb. At neap tide or bad weather, the recommendenation is only to cross exactly at ebb.
IMO, it is more a touristic attraction than a serious road.
How should this be designated? Tertiary road sounds too important and will draw navigation systems towards it (could be dangerous at night, as there is no barrier, just warning signs).