Merge Divided Roads

‘Merge Divided Roads’ question–how does it determine if roads are sufficiently parallel to be merged? I have a couple of road couplets (2 parallel roads, a block apart), within the merge distance (500′), but won’t seem to merge. (Using ArcGIS 10.4)

I’ve dissolved the roads, and done the multipart to single-part thing. I’ve selected out the roads by functional class, so they all have the same functional class (and same field name in my ‘FC’ field.

Do I need to start playing with Integrate?

Relevant, but quite old (8+ years):

Using "Merge Divided Roads" tool with lesser parallel lines
Why does collapse dualline to centreline only create an outline of the dataset?