Highway Directions In The United States
This topic is currently being discussed on talk-us
In the United States, Interstates, U.S. Routes and State Routes usually have signposted cardinal travel directions (North, South, East, West). There is a need to capture the posted directionality of routes along the OSM ways that constitute state highways. Note that Interstates, U.S. Routes and State Routes are all examples of state highways.
Most U.S. states follow American Association of State Highway and Transportation Officials (AASHTO) numbering in that E-W posted routes usually have even numbers, and N-S posted routes usually have odd numbers; but this is not an absolute rule. Some states regularly break the rule for purely in-state routes, while others do so occasionally. Do not change existing tagged directionality solely based on this general rule! As always, local knowledge is king.
In Canada, trans-Canada and provincial routes are similarly posted with cardinal travel directions. Many of this page's provisions will also apply north of the border. Most Canadian provinces are also members of AASHTO, and follow similar policies.
Some of these provisions may also apply in Central and South America, where U.S. signing practices are often followed.
The preferred tagging method sets relations' member role values to north, south east, west.
Posted Directionality of 2-way Single Carriageway Ways
A given 2-way single carriageway way could carry (say) US 53 North (OSM forward along the way) as well as US 53 South (OSM backward along the way). If we post a 2-way single carriageway way member of the route's relation as north, this indicates that the OSM forward travel direction is posted as US 53 North, and by extension, the opposite travel direction of the 2-way single carriageway is posted as US 53 South (in the direction OSM backward).
Physically, numbered routes follow OSM ways. However, a particular one-way half of a divided highway (Brit: 1-way carriageway that is half of a dual carriageway) can carry both US 53 South and MN 194 East (say). Routes are defined by route relations, comprising sets of ways. This is an important reason for tagging cardinal directionality on relations, rather than on the way itself - as one way can be part of many relations.
Partially unsigned routes
We tag these hidden segments with either role=north;unsigned.
Alternate (deprecated) methods
'The tagging methods described in this section are discouraged and are included only because it is still present in the data.'
An alternative method uses direction=north, direction=south, direction=east, direction=west key/value pairs for tagging relations. This method assumes that the whole relation covers only one travel direction, which is not the case for highway routes that traverse mixed single-carriageway and dual-carriageway roadways. It would seem overly complex to create separate relations for each travel direction on mixed single and dual carriageway state roads, just so that we can record the posted directionality of route numbering shields.
Upon review of the existing route relations, it appears that the member role value method described above is used more than setting the direction tag value. There are around 100,000 route relation members (about 570 relations) in the United States tagged this way (the blue ways on the map below). In comparison, there are only 28,000 route relation members that belong to relations that have explicit direction=* tag (about 260 relations - the red ways).
Note that as of December 2013, there are roughly 80 relations that are tagged both ways, meaning they have both explicit direction=* tags and their members are also assigned with cardinal directions as the member_roles.
Some 1-way OSM ways show role=forward on the route number's relation. The role "forward" means the posted route number follows this way only in the direction of the way and "backward" means the route runs only against the direction of the way. For 1-way ways, this is obvious and is therefore redundant. Here we suggest that forward be superseded by north, south, east, and west, which will uniquely indicate the route's posted directionality relative to the way's OSM "forward". There is a very small number of cases only where 2-way single carriageway ways carry a route in only one direction, and it seems to be of no practical significance for posted highway numbering.