- Why do we need to classify highways?
If roads aren't classified, the map will be a sea of unclassified and residential roads, with no way to identify the major roads. This is not an issue of tagging for the renderer; rather it's an issue of tagging so the data is useful.
- Why can't we rely on physical characteristics, which the renderer can then process to determine how to draw the road?
Characteristics differ greatly based on population density. A minor city street may be built to higher standards than a major rural highway. In cities, major through routes may look identical to other streets. We need to show which roads function as through routes. Physical characteristics may help us determine which roads are major highways, but cannot be used on their own.
Why can't route classifications be used?
- Why can't we mark only Interstate Highways as motorway?
In the UK, one government agency builds and maintains the most important highways, including all motorways. Although some non-motorways are built to or near motorway standards, the public is used to seeing motorways as a separate classification.
On the other hand, Interstates are built and maintained by the states (or in a few cases cities). Different states have different ideas of what should be an Interstate, especially when dealing with already-built freeways that are not eligible for federal funding, and with cities that have grown significantly since the initial planning of the Interstate system (such as Phoenix and Orlando). Additionally, the Interstate system is a connected system, as opposed to the UK motorway system, which has a number of isolated pieces, usually bypassing a city on a major surface road. Almost any map of the US will not distinguish between Interstates and other freeways, since the quality of road is comparable (or if it does, will still use a separate symbol for non-Interstate freeways); a US motorist will expect to see non-Interstate freeways shown as freeways.
- Why can't we use U.S. Highways as a classification?
Short answer: inconsistency to a much greater extent than the Interstates. Even from the beginning, states had different ideas of the intent of the system. This is obvious when looking at the "final" 1926 plan, in which eight routes leave Pennsylvania to the north, but only two continue into New York.
More recently, the Interstates have replaced many U.S. Highways as the primary network, and again different states have handled this differently. Some have moved the U.S. Highways onto parallel Interstates and eliminated the U.S. Highway designation entirely where possible, while others keep them on the pre-Interstate surface road.
Even the American Association of State Highway and Transportation Officials, which coordinates the numbering, recommends against specially marking U.S. Highways on maps:
A suitable highway legend, which may be copyrighted, shall be adopted by the Standing Committee on Highways on behalf of AASHTO. Such legend will be recommended for use to all travel map makers, also for use by the State Highway Departments. This legend is to show, in a uniform manner, the suitability for travel not only of the U.S. numbered routes but also of State routes.
That said, the U.S. Highways may be a good starting point, in that they are usually the best (or best non-freeway) routes between the cities they serve.
- Why can't we use state highways as a classification?
Inconsistency is even worse than with U.S. Highways. Indiana, for example, has a hard limit of 12,000 miles of state highways. The effect of this is that, except for I-65 and I-70, there are zero state highways inside I-465, the beltway around Indianapolis. At the other extreme, many state highways in Kentucky are rural roads that serve either tiny communities or even unnamed clusters of residences. (Some states maintain even more highways than Kentucky, but separate them into "primary" and "secondary" systems.)
As with U.S. Highways, the state highway system, or a suitable subset depending on the state, may be a good starting point.
How about other classifications?
- Why can't we use functional classification?
At first glance, the Highway Functional Classification System looks perfect for our needs. But, since it's not used by the public or for most functions of the state transportation departments, in many areas the system is unpolished, to say the least. For example, in Orlando, some roads inexplicably change classification several times, some roads of higher classification are only connected by roads of lower classification, some one-way pairs do not have matching classifications, and some classifications are outdated by over 10 years.
Nevertheless, functional classification may be a useful starting point.
- How about the National Highway System?
The National Highway System only covers the most important highways in the country, so it will not help for lower classifications. But it may be a great start for the higher classifications.
- How about state-defined systems?
Some states have networks similar to the National Highway System, such as the Florida Intrastate Highway System. As with the NHS, these cover only higher classifications, but may be a useful starting point.