United States Roadway Classification Guidelines
This is not an official guideline yet. See United States roads tagging for that. This notice will be removed once consensus is reached.
Roadway classification (primary, secondary, etc) is fairly subjective within the United States. Do not change the classification of any road way unless you are familiar with the area. Although roadway classification is subjective, it is important that it be consistent within a given area. Therefore, unless you are correcting errors or gross inconsistencies, do not change the classification of a single road without considering the entire area, using the process described below.
When adjusting roadway classifications the primary thing to keep in mind is how the classification would be used by the typical map user. The most typical use is to help someone navigate. When searching for a route in an unfamiliar area, travelers likely will choose to travel on roads with higher classifications when practical, as they are likely to be the best routes. It should also be easy for them to locate the major well-known highways on the map. Finally, when zooming out, most map renders will display only roads of a higher class.
What follows is a recommended procedure that should lead to good results. It is a bit biased toward urban areas, but it should work well on all but the most rural settings. It is not important that you follow it exactly. What is important is that, in the final result, you have a coherent network. Unless there is a strong overriding reason to do something different, primary roads should be used sparingly and to emphasize very important routes (for example U.S. highways) or expressway like roads designed primarily to move traffic in and out of the city. Secondary routes should be used for all other important routes, but it still is important that you not overuse the secondary classification. For example, two parallel routes within a few blocks of each other should probably both not be secondary, unless they truly are equal in terms of traffic volume and the speed at which they will move you from one point of town to the other. Finally, tertiary roads should be used for all other routes that one would travel more than a few blocks. It might be acceptable for all grid streets in a downtown core be tertiary or higher, but only if all the tertiary streets really are equal. In many instances this is not the case, and some roads may be better to take than others. For example, traffic lights might be better synchronized on some roads than others.
The area you are working on probably has a lot of inconsistent classification or maybe, in an extreme case, none at all. So for the first iteration aim for the following: 1) All U.S. routes should be primary, 2) Most state routes should be secondary with, possibly, a few very important routes as primary, 3) Everything else should be unclassified or residential (the distinction is unimportant at this point) with possibly a few tertiary routes that you are reasonably sure of. Using this scheme likely will involve demoting many routes. Do not worry about it too much, but check the history to see if you will be stepping on anyone else's work. If the classifications were not in the initial import and, therefore, were created by an OSM member, then it might be wise to email that user explaining what you are doing and why (point to this web page) to avoid a future edit war.
With a clean slate go to http://www.fhwa.dot.gov/planning/nhs/ and find the National Highway System (NHS) for your area. Mark any principal arterials of the NHS system (red on the NHS map) as primary, if that has not been done already.
Next, systematically mark all tertiary routes. Basically, use your own judgment and mark any routes that should be a higher class than unclassified/residential. Here are some guidelines to help:
- In the city lesser roads have stop signs where they meet the tertiary candidate; tertiaries frequently have stop lights where they meet secondary and higher roads, where as residential/unclassified roads merely have stop signs.
- Also within a city, a road should probably be left as unclassified/residential if it is the type of road you would not want to travel more than a few blocks on, for example lots of stop signs or other traffic calming methods, lots of pediatrician traffic (with out stoplights at the cross walks), or, a Salt Lake City specialty, parking in the center of the road.
- In a more suburban or rural setting any road that serves more than one subdivision, commercial area or the like, which are generally county routes of quality (often unsigned, but generally at least have a yellow center line).
Now take a second look at the tertiary roads and consider promoting some of theme to secondary. These would be roads that you consider clearly more important than other tertiary roads in the immediate area. Possible criteria include having an intersection with a freeway, passing under or above a railway where parallel tertiary roads have a level crossing, or simply that it providing a better route through town (wider with a higher speed limit, better traffic light synchronization, etc.).
Now step back and consider the network as whole.
If there are many primary roads within close proximity consider demoting some of them to secondary: If they are roughly parallel, do they both get the same traffic volume? Is one consider a faster way though town? Is one is U.S. highway and the other just a good way through town (This is a tricky one. Generally the U.S. highway should get priority, but not if it is significantly slower than the other). If they intersect, does one highway clearly get priority over the other as far as traffic light timing goes. Or is there a difference in the number of lanes at the intersection (for example one is a two-lane highway and the other has 4 lanes.
If there are a large number of parallel secondary routes, consider promoting or demoting some of them. Consider promoting a road if there are no primary highways in close proximity and if you consider that road a major route through town. Similarly, consider demoting a road, based on roughly the same criteria as for demoting primary highways (giving preference preference to state highways).
If an entire section of the city is either tertiary or higher, consider demoting some of the tertiary roads, again using the criteria above.
If there are large sections of roads that are solely residential or unclassified, consider promoting some of the roads to tertiary. In this case it might help to use aerial photos to see if some roads stand out more than others, for example, if they have a yellow center line.
Finally, tidy up. Do roads change classes at logical points (for example, at the intersection with another road of equal or higher class)? Is a particular route consistent? A route should probably not change from primary to secondary and than back to primary.
The Next Day
Now upload your changes. The next day have another look at the results. Do things look right? If not repeat the process.
If your edits seem right, consult other sources to see if your classifications agree with them. This should include as many maps as practical from independent sources. You will be amazed that none of them precisely agree, but, when they do agree, verify that their view matches yours. If you did your job right, there should be only minor discrepancies. If not, either justify your position or consider making small adjustments to your maps. If your map requires more than small adjustments, then you likely did something wrong. Relying heavily on copyrighted maps will get you in hot water with the OSM community, so it is probably best to abandon your efforts at this point and find someone who is more familiar with the area and will likely do a better job. Further, show your maps to others familiar with the area and see if they agree with your classifications if not make more adjustments. And, you can bring the problem to one of the email lists, where you can get the views of many others in OSM and, hopefully, arrive at a consensus. If you made any changes at all, repeat the refinement process, wait another day or so and have a another look.
When you are finally done, you should have a very useful map, indeed. In fact, because you used mostly local knowledge, it is likely that the map will be more complete and a better aid in navigation than any commercial map.
I did not include trunk in this process, because there doesn't seam to be any real consensus on what it should be used for.
Potlatch is not a good editor to use for this process, because it is very slow in displaying large areas, a function that is vital to this process. JOSM is much better, but the default style makes visualizing roads difficult. I used the layer feature to display only highways (hide "-highway:"), and I modified the default style to avoid displaying the yellow fringe around unreviewed TIGER data.