I've had discussions with colleagues about adding topology for culverts.

The consensus here is that adding topology for culverts is a bad idea and brings no benefit to the road network. In fact it hinders editing of the road above the culvert.

Culverts should be treated as tunnels (the tag is tunnel=culvert after all) and the two ways should cross each other without connecting at the intersection point. Other assets like power and telecommunications conduits under the roadway do not have topological links to the surface either.

There are 4 culverts in all of Victoria that have topology with the trunk/motorway roads above it.