User:Vid the Kid/Expressway
It seems that there is some confusion among some OSM mappers about what an Expressway is, and how to tag it. Additionally, not many mappers seem to understand why it's important to make Expressways distinct from conventional roads on maps, be they paper or OSM. That's why I'm writing this essay.
Access control terminology
Full access control means a roadway is only accessed via entrance and exit ramps, which allow traffic to enter and exit the main lanes of the roadway at (nearly) full speed. All cross-traffic is grade-separated from the access-controlled roadway.
Partial access control means that direct access to and from adjacent private properties is prohibited or extremely limited. A partially-controlled-access highway can (but does not always) have at-grade intersections with cross-roads, but the frequency of such intersections is usually limited. Often, a partially-controlled-access highway has grade-separated interchanges, rather than at-grade intersections, with major crossing highways.
What's a Freeway?
Most people are already familiar with the concept of a Freeway. A Freeway is a high-speed, divided, multi-lane, grade-separated, and fully-access-controlled roadway. Nearly all Interstates are Freeways. Certainly not all Freeways are Interstates; some are US routes, some are State routes, and a few are even county or city roads. Some Freeways charge a toll to drive on them, and therefore aren't "free" as in free of charge, but they're still Freeways by the roadgeek definition. Freeways (and other roads) are often given names that end in "freeway", "tollway", "expressway", "thruway", "turnpike", "parkway", or "highway", but these names do not have any impact on whether or not the road is actually a Freeway.
There is near-universal agreement that, in OSM, the tag highway=motorway should be used exclusively for Freeways.
What's an Expressway?
The unifying property of Expressways is access control. All expressways have either full or partial access control. Expressways (and other roads) are often given names that end in "freeway", "tollway", "expressway", "thruway", "turnpike", "parkway", or "highway", but these names do not have any impact on whether or not the road is actually an Expressway.
Classic Expressways
The most common Expressway is a high-speed, divided, multi-lane roadway with partial access control and grade separation from some or most cross roads. When driving on an Expressway, the casual driver might think he is actually driving on a Freeway; indeed, the design and signage standards are extremely similar. Generally, the only difference between a Freeway and this type of Expressway is that the Expressway can have at-grade intersections with cross-roads.
Undivided Expressways and Super-Twos
While not always considered expressways by everyone, there is a lesser class of Expressway that does not have a separation between the two directions of traffic. There are some instances where the 4-lane scheme is retained, but most undivided expressways are only two lanes, one for each direction. Some two-lane expressways have full access control. This makes them more like a Freeway, except for the fact that they are only two lanes. Roadgeeks tend to call such a road a Super-Two. Many other two-lane expressways have at-grade intersections with some or all crossroads. Some roadgeeks call this type of road an "Ohio super-two", though it exists in other places. To the casual driver, such a roadway is often indistinguishable from an ordinary two-lane road.
Identifying Expressways
Access control
Expressways do not allow direct access to and from adjacent private properties. Therefore, an Expressway will not have houses or businesses with driveways attached to it. Sometimes, an Expressway is adjacent to houses, businesses, or other developed land, but all access to these properties is via side roads. This is usually spotted rather easily. Sometimes, a fence is erected to enforce the controlled-access status. An expressway might have frontage roads to provide access to adjacent properties.
High speed
Expressways are designed to allow high-speed travel. This means that curves and hills along an expressway will be very gentle, just like on Freeways. Expressways usually also have a Clear Zone resembling that of a Freeway. Many states allow speed limits greater than their general rural speed limit on Expressways.
Rights-of-way
The right-of-way of an Expressway is usually wide enough to allow a safe Clear Zone. In the case of undivided Expressways, there is often sufficient right-of-way on one side of the road to allow for future expansion of the road to a divided Expressway or Freeway, though this excess right-of-way is not always cleared of trees. Some expressways also feature additional right-of-way at crossroads, to allow for future construction of a bridge or even a full interchange.
Old alignments
Many Expressways are built on new alignments, where no road existed before. This is done to reduce the need to relocate residents or construct frontage roads. This is also done when the Expressway is built to bypass a town. The old alignment is typically retained for use as a local road. Therefore, the presence of an apparent old path of a roadway is a clue that the new road might be an Expressway.
Signage
In the United States, we have the Manual on Uniform Traffic Control Devices which describes, among other things, road signs and where to use them. It has two chapters about Guide Signs: Chapter 2D describes Guide Signs on conventional roads, while Chapter 2E describes Guide Signs on Freeways and Expressways. It's not hard to learn the difference, and then a mapper can tell the difference between an Expressway and roads without access control by looking at what type of signs are used. Of course, field observation is required, as signs aren't usually visible on aerial photography.
Tagging an Expressway
Trunk
Currently, most US-based guidelines for using the highway=* tag define highway=trunk to be used for roads that have properties similar to Freeways, but are "deficient", not fitting all the requirements for highway=motorway. Specifically, many state-level mapping coordination pages of this Wiki say that highway=trunk should be used for roads that lack exactly one requirement for highway=motorway. Of course, the specific wording and redundancy of the requirements for highway=motorway, which varies slightly with each Wiki page on the subject, has significant impact on how "deficient" a highway can be and still be highway=trunk.
I've been using highway=trunk for any divided Expressway as defined above, and for some undivided Expressways – particularly those which are extensions of divided Expressways or Freeways. Generally, this policy is in line with other mappers' definitions of highway=trunk. Some mappers want to restrict its usage to a very narrow class of "deficient" Freeways. Some mappers want to extend the usage of highway=trunk to important roads without any access control, such as US routes and even some State routes. Perhaps that's more in line with its original use in Britain...
New tag?
Maybe it would be a good idea to use highway=trunk more like its use in Britain. But then we would not have a way to distinguish Expressways from roads without access control. I personally suggest highway=expressway for this purpose. But until such a proposal gains momentum, I will continue to tag Expressways with highway=trunk.
Update: See Proposed features/Expressway indication. Rather than a new highway=* value, this proposal treats Expressway-ness as a physical characteristic, creating a new key and allowing highway=* to express functional classification. I now believe this is the best practice regarding Expressways.
Importance of distinguishing Expressways
When driving on an Expressway, the casual driver might think he is actually driving on a Freeway. A divided Expressway functions very similarly to a Freeway, and provides most of the speed and safety benefits a Freeway does. An undivided Expressway also provides some speed and safety benefits over roads without access control. Just as Freeways are featured very prominently on maps, so should be Expressways. This directs map users to choose safer, faster, more fuel-efficient routes that might not be obvious if Expressways are presented in the same style as conventional roads. If a computer program is choosing the route, assigning Expressways with a lower route cost than conventional roads (but still slightly higher than Freeways) will also direct the end-user onto safer, faster, more fuel-efficient routes. It's just good cartography.