[Talk-us] Road classification

[Alan Brown]

This is a good scheme - although it's important to remember that some of these judgments are partially subjective rather than purely objective.  It's not always clear to know whether to judge the difference between a primary and secondary road, or a secondary and tertiary road.

This is the way I like to think about it - if you're zoomed way out, a map of motorways and trunk roads alone is best: plenty of useful information, but not cluttered.  Zoom a little more in, motorways + trunk roads + primary roads are what you want; zoom a little more in, motorways + trunk roads + primary roads + secondary is best; and so forth.   The further away from your origin or destination you are, the less you should care about roads lower in the hierarchy.  A typical trip should start at a residential road, and work its way up and then back down the hierarchy once on its way to its destination.

There are some tricky judgements to make.  There are some roads that I travel on all the time because they are the best way to get to important destinations; yet they move slowly.  I know of roads with high speed limits and multiple lanes; yet they aren't the most important, because they're intended to handle rush-hour traffic for a few major companies; the rest of the time, they're empty.

Aesthetics matter; you want a proper balance between roads of different priorities on your map; this is a case where considering aesthetics improves utility and vice-versa.

-Alan





________________________________
From: "mapboy at att.net" <mapboy at att.net>
To: talk-us at openstreetmap.org
Sent: Tuesday, December 16, 2008 8:45:28 AM
Subject: [Talk-us] Road classification


Hi,

I hate to reopen an old subject but I'm not in agreement with the US classification system that exists.
From reading past archive, it looks like this has been a contentious subject in the past.
I propose the following. I have already added it to some web pages as in my experience it's the best/most logical heirarchy that works. I apologize in advance if I'm stepping on any toes.

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.
 
thanks,
pete
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