After so many hours manually tracing roads, one naturally begins to wonder if there's some software for automatically detecting them. Google turns up only a research project, the "Microsoft Road Detect" at http://magicshop.cloudapp.net/
There's some discussion among OSM people about whether this would be a good thing or not. I think the point's moot because it doesn't work.
First thing I tried was the JOSM experimental plugin "MagicShop"; it hadn't been touched in 2 years which is a bad sign. Current JOSM refused to accept the jar, not a huge surprise.
I'd consider it worthwhile to fix the plugin if it would give useful results, so I went directly to magicshop.cloudapp.net and gave it some test coordinates: a nice clear straight section of road in India I happened to be tracing recently.
And this is what it did:
Yeah. Well, maybe I could write my own algorithm/plugin.