I map in a region where some kinds of boundaries are normally defined as following roadways and waterways – more specifically, road centerlines and river centerlines or thalwegs. Other kinds of boundaries are defined based on a land survey, but the roads are laid out so that the road centerline follows the boundary precisely, giving the government on either side jurisdiction over half the roadway, resulting in differences in addresses, road signage, and more. Whatever the cause, the boundaries align with these other features; there’s value in explicitly indicating that in the database.
Originally, mappers in this region used to make the roadways and waterways members of the boundary relation. But this made the boundary relations brittle and prone to breakage, as we would routinely have to split the roadway into dual carriageways or transform an intersection into a roundabout. (The boundary hadn’t moved, because the metaphorical “centerline” hadn’t moved.)
Sometime about 12 years ago, we switched to drawing distinct boundary ways that share nodes with roadways and waterways. This made the previous kinds of breakage somewhat more difficult to carry out by mistake, but it introduced other kinds of mistakes, like connecting a driveway to just the boundary but not the roadway, something that is very easy to do in some editors.
There’s also the option to leave the boundary disconnected from the other features. It’s a good idea whenever we’re unsure whether the alignment is intentional or a mere coincidence. It’s also a good idea if the boundary follows something other than the centerline, strictly speaking. But if we know it’s intentional and correct, then keeping them disconnected means the features can diverge over time for reasons that have nothing to do with the boundary, just because mappers weren’t paying attention to the boundary when refining the roadway.
It’s useful to consider whether the boundary automatically changes in response to physical changes to the roadway or waterway, but there are other considerations too. Some kinds of boundaries, like a city’s formal neighborhood boundaries, are defined by the roadway as a whole and don’t need to be clarified beyond that. Some waterways aren’t good candidates for sharing nodes because the “centerline” is unclear or subject to interpretation.