OpenStreetMap
My initial edits to OSM were to tag highway=tertiary for the roads in Portland, Oregon, USA. I've used a pretty simple formula: if the road has a dividing stripe and it is tagged as highway=residential, change it to highway=tertiary.
This works pretty well in the city. However, it starts to fail out of town, since country roads typically are either gravel or have a center line. Aesthetically, I'm not sure I like seeing remote areas where every road is tertiary. Maybe I'll do some tagging in a rural area and see how it looks.
Discussion
Comment from Baloo Uriza on 27 July 2009 at 03:21
"Don't tag for the renderer."
If it's a county road, it's at minimum tertiary by default (third order of highways after national (primary) and state (secondary), with a little variance depending on design and traffic flow). Easy way to tell who owns a route is to check bridge tags, though some counties make it more obvious than others (such as when you leave city roads and state highways in Salem, you see "Welcome to the Marion County Highway System" signs with no route numbers, and in Lane County they have the generic US-MUTCD county highway indicators with route numbers).
I'm really dissatisfied with the way the TIGER import handled road orders; in my opinion, ideally, the TIGER import should have used highway=road instead of residential, since it's pretty apparent that TIGER didn't know what kind of road...
Comment from Circeus on 27 July 2009 at 03:24
It's about through traffic: if the road is a significant connecting road, but not quite up to secondary level, it's a tertiary. I think a good rule of thumb is to put county and low-use state roads as tertiary (most state road are secondary, though, and the major ones are primary), and leave the rest as residential (or unclassified, if you like that better).
Comment from Circeus on 27 July 2009 at 03:27
@Paul (your post wasn't there when I started): Mapnik actually displays highway=road differently from highway=unclassified/residential. Using tat tag would have resulted in the US being displayed different from any other country (which might have been a good thing: it'd have given US mappers a good excuse to hop on). ~~~~
Comment from Richard on 27 July 2009 at 07:05
Agreed absolutely with Circeus's first post - that's a really good description.
Comment from alv on 27 July 2009 at 10:56
Introducing highway=road was even discussed only after the import was already done.