I've just noticed that a bunch of road segments on bridges in Auckland (and possibly a wider area) have been unmarked as bridges. For Hopetoun Street over SH1 at least (1) myfanwy has added bridge polygons under the roads and added comments on the no-longer-bridge road segments that say "please don't double bridge this, it's wrong" (2).
Can anyone (myfanwy?) explain the logic behind this? (The relevant OSM wiki page is here (3). It does not mention "double bridge")
I run a verifier on the road network and try to correct faults it finds from time to time. One of the things it looks for is roads that cross without a common waypoint - for example (3). However, if one of the roads is a bridge or a tunnel, and the other is not, then the verifier is smart enough to work out that roads aren't supposed to have a common waypoint and doesn't complain. Not getting swamped in spurious error messages makes the job a lot more manageable.
This is really easy to work out when bridge road segments themselves are marked as bridges, but if the bridge is a separate polygon, then working out which road segments incident on the bridge polygon are most likely to be *on* the bridge polygon and then guessing those segments are bridges is, shall we say, a bridge too far.
(2) Sorry, I double-bridged a few of these before I found the comment in the history - it isn't present on all un-bridged ways, but I'll undo my changes if there's a good reason.