If the chance of decay is inversely related to the level of the road, then we'll never see complete low level roads as they'd disappear too quickly. The only way people would be able to build high level roads would be to quickly build them from nothing, lest the lower levels decay away too soon. As a result, we'd have sections of high level road connected by spotty low level roads, and just about nothing else. This is why I suggested having intermediary and primary levels, to create humps in the difficulty of building useful roads, but not requiring people to go "all the way" just to allow their road to exist for any meaningful length of time.
Really, this is a case where simplicity is a bad thing, otherwise we won't get away from the current state of having a single type of ubiquitous developed road.