[Talk-us] US Highway Classification (Was: directions of ways in MassGIS data)

Alan Brown adbrown1967 at yahoo.com
Tue Feb 3 17:51:47 GMT 2009

There was a discussion on this list ("Road classification") a short while ago about the "Highway" tag that should be revisited. Someone posted a classification system back in December that made a lot of sense, and is more in sync with what I know commercial data providers.  Basically, it would be treating the "highway" tag as providing a functional road classification, rather than (primarily) a legal classification or a physical description.  In a nutshell, "Primary" = Road to take you between cities, "Secondary" = Road to take you across cities, "Tertiary" = Road to take you through neighborhoods.  It ends up roughly correlating with physical or legal designations - but not exactly.  It's marking the relative importance of a particular road.  (Motorways and Trunk Roads are exceptions to this, as they do have specific physical descriptions.) Because of this, I quite disagree with the statement that the tag should be used as a general description of the
 physical attributes of a road; there are physical description tags to serve that purpose.  There are many cases where a two lane highway is by far the most important road in a region (Transcanadian Highway, US 50 in Nevada), while in urban areas like San Jose, you have relatively unimportant roads with two or three lanes in each direction for short distances.  Yes, there is definitely subjective judgement here, but it's the sort of subjective cartographers make on a regular basis, and it is very useful.  You want a good balance between primary, secondary, and tertiary roads.  


Here is mapboy at att.net's old post:

In OSM language the Highway feature is used to designate what we call roads. 
A motorway is a four+ lane, limited access, grade separated freeway. These can include Interstates, US Highways, State Highways, County Highways or even Farm to Market Roads if they meet certain criteria. These
criteria are limited access,the use entrance/exit ramps to access the
freeway. Intersections with other roads are at grade seperated
crossings or ramps. A grade separated crossing means one road goes over
or under the other. (ie. over/underpass) When Motorways meet other
motorways they generally use ramps that are classified as Motorway
Link. These motorways usually connect to other cities or move the
traffic around and through a city. Limited access ring roads usually
fall in this feature class also. 
A trunk is what a motorway becomes when it loses one of it's
criteria. This usually occurs to US, State, County highways as they
move outside the urban areas. Intersections with other roads can occur
at grade and/or when ramps are no longer needed to access the road.
Usually they remain 4+ lanes and may or may not be divided by a
physical median. 
A primary road can be a US, State, County Highway or other road that connects two cities or moves traffic from one part of the city to
the other. These are the highways that become Main St when they go
through a small rural town. They will have traffic signals when they reach more densely populated areas. These are the roads you
jump on when the freeway has an accident and you don't want to sit and
wait it out. 
A secondary road moves traffic within a city. It would service only a certain area within a city. 
A tertiary road connects the residential roads to the higher classes: motorway, trunk, primary or secondary. 
I hope this clarifys things for some users. I know it's not going to please those who have already used other classification schemes.

From: David Lynch <djlynch at gmail.com>
To: Talk-us <talk-us at openstreetmap.org>
Sent: Tuesday, February 3, 2009 8:49:26 AM
Subject: Re: [Talk-us] US Highway Classification (Was: directions of ways in MassGIS data)

What I've tended to do in my part of Texas is:

Motorway - two or more consecutive intersections with grade separation and no driveways, or any interstate (some very rural locations do have the occasional turn off directly from the main travel lanes)
Trunk - US highways without any other reason to classify them up or down and high speed divided roads with relatively few crossings and an occasional grade separation
Primary - State highways, US highways running near and parallel to a motorway, and wide, heavily-traveled urban/suburban streets
Secondary - Other state highways (farm-to-market roads, loops, spurs) and major city streets
Tertiary - Residential collector streets, some rural roads that are in good condition and connect to classified highways, and service roads running parallel to a motorway.

On Tue, Feb 3, 2009 at 09:44, Zeke Farwell <ezekielf at gmail.com> wrote:

We need a detailed road classification chart on the US wiki page to straighten all of this out !  Maybe I'll find the time to get working on that one of these days...  I think I'm also having trouble making the OSM road classification system do everything I want it to.  

"the consensus is that the highway tag is for making a general description of the physical attributes of a highway. This gives the user of a highway more useful information than a legal classification."  -  Key: Highway (OSM Wiki)  

This is true for high zoom levels, but at low zoom levels this can be too much information.  When I am looking at a map of an entire state, I want to see what the main routes are through that state, not every important road.

In Vermont some important 2 lane north/south routes are VT 100, VT 116, US 7, and VT 22A.  Physically they are pretty similar, and so by OSM standards they should all be primary.  In my opinion though, at low zoom levels US 7 should stand out from the others because it connects larger towns, has a higher legal designation, and it stands out on other maps.

I suppose this is really an issue for renderers, but we do need some sort of tagging for the renderers to use.  Maybe once route relations become more prevalent renderers could emphasise different higher level route networks more, regardless of the physical highway tag.  Any thoughts?

Zeke Farwell
Burlington, VT

On Mon, Feb 2, 2009 at 10:20 PM, Greg Troxel <gdt at ir.bbn.com> wrote:

(replying to Zeke and Chris both)

I agree that if there is only 1 mile of motorway class road among
trunk-class road that tagging it motorway isn't useful.

The parts of Route 2 that I was thinking of tagging as motorway are
physically indistinguishable from an interstate, and at least 10 miles
long.  I am not 100% clear that pedestrians/bicycles are banned, but I
think someone would call the police if either were on the road - I never
see them when driving on 2.`

As for the trunk designation, I find that a bit ocnfusing, but my
impression is that it is a road that is somewhat more than a regular US
highway physically, but not all the way to interstate.  An example would
be a road with 2 lanes in each direction and jersey barriers, but
same-grade junctions with lights every few miles.  Rt 2 is like this
between 128 and Tracey's corner (1st intersection west) and really all
the way to South Acton where it is motorway again until about 7 miles E
of Orange and then it's back to trunk (1 lane each way only, but with
exits).  Further west it is just a primary road, but gradually being
made more like trunk.

I don't think being the only main road should qualify a way as trunk; it
seems being important is only enough to get a road to primary.  But some
degree of limited access and being divided would be enough for trunk.
So I'd leave rt 2 as motorway/trunk mixed.  I've driven rt 7 from
Bennington to Burlington, and it didn't in general feel like "almost an
interstate but not quite" - it felt on balance more like primary.

Talk-us mailing list
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David J. Lynch
djlynch at gmail.com
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