[OSM-talk] definition of the main highway-tag

[Martin Koppenhoefer]

2009/7/30 Greg Troxel <gdt at ir.bbn.com>:
>> this is working well for out-of-town situations. Inside urban
> good point; that's what I am used to thinking about.
>
>> agglomerations there should be different criteria though (and not
>> necessarily they are physical, what is my point: let's put the
>> definition according to everyday best-practise tagging).
>
> I think these notions still work in cities, but less clearly.  primary
> roads are those you'd get on to drive to the next big city.  Secondary
> to get to outlying towns.  And tertiary to go all the way across the city.

no, sorry, but I completely disagree with you in this point. Inside
big cities (urban agglomerations / metropols / ...) it's not about
going to another city but about the traffic inside the city itself.
Usually we start with

- residential roads (just in residential areas, no connecting
function, you will not take this if you don't live in the area)
- unclassified roads (not clear, there are voices that they don't
exist in urban areas, I personally use them if there are either no
residents nearby or if they are slightly bigger than residential
streets and are used to access residential streets)
- tertiary (just local significance, streets inside a certain area)
- secondary (connecting streets that connect different areas, lower
importance than primaries)
- primary (main inner city connections, also used to enter and leave the city)
- trunk (streets that are separated from the urban tissue but not
classified as motorways (like elevated roads, overpasses, separated
roads, they have ramps and usually no or very few traffic lights,
necessarily dualcarriageways - in the UK they use administrative
classification)
- motorway (legal classification, in the US I guess you call them
interstates, expressways and freeways but not sure about the
distinction between those, maybe some are trunks)

As in big cities most of the traffic is local traffic (by local I
intend inside the metropolitan area), you can't classify IMHO the
streets according to whether they connect other cities. Think about
NYC. Following your definition there wouldn't be any primary roads in
manhattan.

> The problem is that there is a continuous hierarchy of roads in terms of
> importance, and when you get huge numbers of roads in the city the jump
> From tertiary to residential/unclassified is too big and people tag
> roads that aren't really tertiary as tertiary.  I'm seeing a bit of that
> in Belmont (near Camridge, MA).

well, I personally consider a tertiary road to be quite small, because
it is only on the 4th position (after trunk, primary, secondary), so
it must be of little importance, otherwise it will be at least a
secondary street.

cheers,
Martin