|This document contains proposed tagging guidelines for the United States and the opinion of User:UltimateRiff. They are not recommended or approved by the community as a whole at this time. Viewer discretion is advised.|
The U.S. Federal Highway Administration has a highway classification system based on the function of a roadway, called the Highway Functional Classification system. The Texas Department of Transportation, (and likely others) use this to make their maps, so I feel we should do the same type of thing. I have not yet determined whether we can use the data which should be provided by each state's Department of Transportation for OSM, and that probably varies from state to state, but even if we can't use that data, I still believe a similar classification would do wonders for our map.
Either way, I highly recommend at least reading the first three sections of the guide, as it really helped me figure out what exactly a "collector road" actually is.
Introduction to the System
Basically, every segment of road is classified based on the balance of access to mobility. The more access a road has, the less mobility it has, and vice versa.
Take a typical Interstate Highway. One can travel hundreds or thousands of miles with ease if you take the Interstate, but you generally can't take a turn into one of the properties right next to the Interstate. This is an example of high mobility and low access.
Next, take a short residential road in the suburbs. In this case, there's plenty of access to the properties to the right and to the left, but one would have a hard time getting more than a few miles on one of these roads. In this example, you've got low mobility and high access.
This is the very basics of how this classification system works. Granted, some classifications could be very subjective, but I believe it's better than using only the physical aspects of the road.
Every road should fall in to one of the three main classes, arterial (high mobility, low access), collector (roughly equal mobility and access), and local (low mobility, high access) roads. The HFCS further stratifies these classes, as described below.
These roads are the best roads when traveling long distances. Examples of arterial roads include:
- Interstate Highways
- Other controlled-access or limited-access highways
- Most any highway one can easily travel between medium to large cities on
These roads can be further sub-divided into several sub-categories.
These are by design the highest class of arterial you can get. Interstate highways, therefore, are the easiest to categorize in this system. All designated Interstate highways are considered to be a principal arterial.
Other Freeways & Expressways
These highways aren't designated Interstate highways, but their design is very much like one. This stratum consists of controlled-access highways, and limited-access highways.
Other Principal Arterials
Highways under this classification serve major centers in a metropolitan area, and provide high mobility through rural areas. Unlike the two strata above, this classification does not require being controlled-access, or even limited-access. There can be driveways leading off of roads in this classification and a few at-grade intersections, but these are limited, and even so, driveways rarely break the median, if there is one.
This class of highway would typically be used for shorter trips than the higher classifications, and serve geographically smaller areas as well. They are generally designed to supplement the upper classes of arterials. They can also link major cities to larger towns and tourist destinations.
Collector roads are a major part of the road network, as they collect traffic from local roads and distribute them to the arterial system. Differentiating major collectors from minor collectors is "one of the biggest challenges" in this classification system. Collectors are generally only used for inter-county travel, or to get you to an arterial.
A few of the best criteria for determining if the road in question is a major or minor collector are as follows:
- The length of the road: Major collectors are typically longer than minor collectors, the split is at roughly a mile.
- Traffic signals and speed limits: Major collectors often have more traffic signals and higher speed limits than minor collectors.
- Density: Major collectors are typically found in more densely developed areas than minor collectors.
Local roads make up the majority of the highway system, in terms of mileage, and are designed to discourage through traffic. This class is made up of residential roads in urban environments, and backwoods roads through the countryside in rural areas. By definition, they provide access to all adjacent properties.
Tagging in this System
With all that said, what does that mean to us, a community of mappers? Well, we can use this information whenever we use the highway=* tag. I would like to propose such a tagging scheme as the one given below.
- Interstate Highways: highway=motorway
- Other Freeways & Expressways
- Other Principal Arterials: highway=trunk, even if not limited access and/or divided
- Minor Arterials: highway=primary
I haven't actually gotten any questions, but here are answers to a few of the questions I expect to hear.
A: I don't believe this should be the case. First off, toll roads are designed to be controlled access, otherwise it'd be next to impossible to charge tolls at all. Secondly, we can tag this with toll=yes or other such tags. Lastly, we aren't supposed to be tagging for the renderer. There are very few digital maps that even make a visual distinction between toll roads and highways. Google doesn't, Mapquest doesn't, and Apple doesn't. Bing on the other hand does, but even so, with a tag like toll=yes, renderers can make the distinction if the developer wants.
Q: Is (insert highway name here) a minor arterial, or a major collector?
A: Honestly, it's up to you. If you know the area well enough, you should be able to determine the difference.
- see https://www.fhwa.dot.gov/planning/processes/statewide/related/highway_functional_classifications/ for their documentation on the subject.
- According to HFC: https://www.fhwa.dot.gov/planning/processes/statewide/related/highway_functional_classifications/section03.cfm
- On Talk:Key:highway#Highway tag should be based on functional classification, not cross section