The purposes of a road designation, usually a number but sometimes a letter or letters, are to a) guide motorists in their travels, and b) to classify roads (often as an internal measure) for administrative purposes. Here we are concerned with the first purpose, which is most visibly apparent to the general public.
In the first case, designations are most highly useful for major roads which transport large numbers of motorists between important destinations such as cities. Single designations can be followed from point A to point B, with designations of various combinations increasing the destination options. Different types of designations serve to further differentiate levels of importance between routes. A road signed as an Interstate, for instance, is understood to be more important than a state or county road, as it serves more important destinations, carries more traffic, and bypasses lesser centers. Often designations are related to the physical typology of the road, with Interstates (usually) being high speed freeways maintained to a high standard, and county roads often being low-quality roadways which are sometimes not even sealed.
Systems of route designation developed gradually over the course of the early 20th century, and by the 1920s it was considered of prime importance to specify certain routes which would serve long-distance interests on a nationwide level. In 1926 the various state highway departments held a series of joint meetings which led to the creation of the US Highway system. For those of you who are geographically illiterate (which seems to comprise the vast majority of Americans) US highways are those roads which are marked with the shield found on the $1 bill. (Of course, residents of San Diego, Fresno, Tucson, etc., not having US highways routed through or near their cities anymore, will have to surf the internet for visual reference, so here's a sample.)
This system of highways was intended to function much like the Interstates do today: connect the various states and major population centers of the continental USA along the most important intercity and interstate (small 'i') roads and highways, using coherent numerical designations which (unlike state highways) could traverse state lines, regions, even the entire country.
Because this system was designated along existing roads, and not necessarily purpose built, it was and continues to be a larger and more comprehensive network than the Interstate system, virtually all of which was purpose built over time.
For thirty-plus years, from 1926 to the middle-late 1960s, the US highway system was the primary means of long distance road travel. By the early 1970s the Interstate system, which was legislated into effect in its present form in 1956, had been largely (around 75%) completed and thus became the principal highway system for intercity and interstate travel.
The vast majority of Interstate routes were constructed in corridors directly adjacent to US routes. In the East, South, and Midwest, the Interstate was often located within a mile or two of the older highway, and almost always replaced the US highway in that corridor as the major travel route. In the Western states the replacement was more dramatic: the Interstate more often than not was constructed directly on top of the US route, physically incorporating the alignment of the older road and essentially eradicating the US highway as a separate routing (except where the freeway was routed around towns and cities, leaving the old road as a business route). Through many Western states today there are long stretches of Interstate without a parallel US route; the US route designation has in this case been decommissioned (eliminated) or truncated (shortened) in favor of the Interstate. This makes sense, as there is no reason for there to be two route designations along a single corridor unless absolutely necessary for reasons of route continuity.
For example, the famous US 66 once extended from Los Angeles to Chicago, but today no longer exists, except as specially bannered 'Historic' routings for benefit of tourists. Highway planners understood that the needs of the traveling public would be equally served by the designations of its Interstate replacements - (parts of) Interstates 10, 15, 40, 44, and 55. The old highway, in many places, is now merely frontage road to the interstate, and is no longer a practical or even traversable through route. In other places, the old road essentially exists in loops off the freeway, and it would not benefit the average traveler (who is interested mainly in saving time and petrol) to use these older bypassed roads when the Interstate provides faster and better service. In still other places, the old route has been directly replaced by the freeway, and a second designation would be superfluous. This is the case along the entire original route of US 66, thereby leading to its elimination in 1985. Other US highways have met this fate, but generally only along a portion, or even the majority (but not the entirety) of their historical routings.
As mentioned above, not all US routes were eliminated when their parallel Interstate replacement was added. Therefore many US routes exist which have been essentially relegated to roadways which serve as low traveled and low standard 'alternate routes' to the freeway mainline, which only see much level of use if the freeway is temporarily blocked in an area for some reason. They are no longer 'primary highways' in function or usage.
Regrettably this has caused confusion as to the level of importance which a US route carries. Ideally US routes should be viewed, in light of this Interstate age, as a secondary 'supplemental national' system serving corridors where full blown Interstates are not necessarily required to serve existing and anticipated future traffic demands. However, the current state of affairs is confusing to motorists, with some US highways serving as major intercity traffic routes to this day (US 101 in California, US 93 in Arizona/Nevada) and others for the most part languishing as essentially local roads or de facto business routes in the shadow of parallel Interstates (US 11, most of remaining US 40, US 31 south of Indianapolis). US route standards must be more clearly defined in the public mind for the US highway system to remain viable in the freeway age. This has been dealt with so far on a piecemeal basis, but never on a full scale level by highway planners.
The Urban Prairie Schooner US Highway Reform program is as follows:
- Eliminate US routes, or truncate those portions of routes, which are routed parallel to Interstates, unless it is absolutely necessary to establish a concurrency for purposes of route continuity. When there is a concurrency, the US highway routing should follow the Interstate freeway for as much of the desired distance as possible.
- Re-numbering of independent portions of existing US routes which mostly shadow the Interstate system should be considered in lieu of long, multi-state concurrencies with Interstates. All new numbers should conform as best as possible to AASHTO US highway numbering rules, and every attempt should be made to preserve the numerical grid system. (In short, no more idiotic designations like 425 and 163.)
- If there must be long concurrencies for any reason, do not sign the US routing along the Interstate, sign it only sparingly, or establish an official 'route break'.
- Consider rerouting existing routes, or if required, designating new routes, along major intercity and small 'i' interstate highways which are currently state highways.
- All US routes should utilize the best available alignment through a region, within reason (as in not bypassing cities which require US highway connections). For example, US 50 in southern Ohio should be routed along Ohio State Highway 32.
- Low traveled US routes and routes which connect lesser centers should be eliminated. (That means you, US 191.)
- Eliminate bannered routes such as 'Alternate', 'Business', and 'Bypass' except for those which are absolutely necessary. For instance, no business routes which pass through towns of 700 in population.
- Eliminate all directional suffixes (e.g. US 70S, US 31W). AASHTO hates those anyway!!
- Intrastate routes (designations only existing within one state) should be discouraged, unless absolutely required, and efforts should be made to eliminate them as circumstances permit. Amalgamate intrastate routes into longer routes where possible.
- Use the fastest and best alignment in urbanized areas. For instance, place a US route on a freeway routing instead of surface streets which are already burdened with local traffic.
- Eliminate or combine several shorter routes into single long routes where possible.