In other news, curious: why does the secondary_link appear to be underneath residential ways on the mapnik layer here on OSM ?

That way in question has highway=secondary_link as its only tag and the intersecting residential ways have highway=residential and oneway=no tags...

Check it out on:
osm.org/?lat=14.757354&lon=-17.391758&zoom=18&layers=M

(For those interested in obdl re-mapping, there's a few ways near Dakar, western Senegal, and the Gambia because of a certain decliner.....As a bonus, there's very new bing imagery here, [imagery is from 2011, and just added a few weeks ago].