Rigid pavements tend to distribute the traffic load over a wide area, and flexible pavements, made with pliable material, do not spread loads as widely and thus usually require the additional base layer and greater thickness for optimal traffic-load transmittal. The current road system consists of several types of roads placed into a functional class based on traffic volume and general use.
Roads are classified on the basis of the types of function provided. Functional types of roads that constitute the highway network include 1 interstate highways, 2 arterials, 3 collectors, and 4 local streets or access roads. These four major types of roads can be combined into groupings for administrative and funding purposes.
One major grouping is the NHS, which comprises the interstate highways and a large number of the high-volume arterial roads. Although not exclusive, most of the considerations of ecological impacts that occur in subsequent chapters consider the impacts from the first three functional types listed above. As discussed in Chapters 3 and 4 , ecological considerations such as environmental mitigation and simple physical scale vary by road type.
Recognizing that a road does not serve traffic needs by itself is basic to the development of any logical highway system. Travel involves movement through a network of interrelated roads and streets. The movement channels through an efficient hierarchical system that includes lower-order roads that handle short and local trips to higher-order roads that connect regional and interregional traffic and longer trips.
In addition to movement, access is a fundamental function of roads. Federal law requires functional designations of roads in urban and rural areas for funding purposes. This classification is done by state transportation agencies and is mapped and submitted to Federal Highway Administration FHWA to serve as the official record for the federal. The distinction between rural and urban areas is made using federal census data to create federal-aid and urban-area boundaries WSDOT Urban roads occur in a census area with an urban population of 5, to 49, or in a designated urban area with a population greater than 50, Rural roads are defined as any road not located within the urban-area boundary.
The best known high-speed limited-access highway system is the current interstate highway system. Because of the need to accommodate heavy freight traffic, these roads are the most expensive to build and maintain.
Other types of limited-access highways include some roads within the NHS or some state limited-access roads, such as the New York Thruway. These limited-access state highways are funded and administered under their own sets of legal standards. Despite important administrative differences, the committee determined that the differences in ecological impacts of different types of limited-access highways are minor.
The interstate system accounts for only 1. These statistics also indicate the potential for the interstate system to deliver greater levels of contaminants to air and water than the total miles of interstate roadway would suggest. However, other factors suggest that the contaminant load is not simply proportional to VMT. For example, vehicles traveling at interstate speeds may emit some pollutants at a lower rate than vehicles operating on local streets.
Other federal highway investment, in addition to the interstate highway system, is reflected in components of the NHS. The NHS consists of the many routes with such designations as U. These roads may be high-speed limited-access highways, arterials, or collectors, depending on the location and configuration of the roadway.
The NHS includes urban and rural roads that serve a wide variety of transportation functions. The NHS comprises , miles of roads, 46, of which are interstate highways. The NHS serves major population centers, intermodal transportation facilities, international border crossings, and major travel destinations. It includes the interstate systems, other rural and urban principal arterials, highways that provide access to major intermodal transportation facilities, strategic highway network connectors, and the defense strategic highway network Figure Arterials consist of the interstate highway system, multilane limited-access freeways and expressways, and other road corridors that serve local areas; they also carry substantial statewide or interstate travel volumes.
This system accounts for approximately Collectors connect local streets and roads with arterials. They provide traffic circulation and land access among downtown city centers, industrial and commercial areas, and residential neighborhoods.
Collectors provide lower speeds and less mobility for shorter distances than arterials. Travel represents vehicle miles traveled per year. Source: FHWA The roads and streets functionally classified as local are all those not classified as part of the principal arterial, minor arterial, and collector system.
Local roads and streets primarily provide access to adjacent land and are generally not maintained by a state highway agency. Local functional systems serve only The growth of suburbs, and the spread-out development patterns that come with it, has had a major influence on urban roadway patterns beginning in the s but particularly occurring after World War II.
The layout of the road system is not only a function of history and geography but is also linked to the local and regional development patterns. Layout patterns give roads different types of connectivity.
The layout patterns and spacing among roads as well as their width number of lanes determine road density. There are several types of roadway layouts that are being used for designing roads.
These patterns can vary greatly from city to city but generally involve a rectangular grid, a hub-and-spoke layout, or a combination of the two. The road network in most cities across the United States is an out-growth of the Public Land Surveys and follows a rectangular grid pattern Figure Roads are orthogonal, alignment generally being along ordinal directions north-south or east-west. Grid patterns generally have more total street length, blocks, intersections, and access points than other layouts.
The grid pattern is typically intertwined with a mixed pattern of land use. Mixed land use was the dominant development style in American cities and towns in the early twentieth century and continued to be the primary pattern until the development of suburbs after World War II.
A mixed land-use pattern can ease conges-. Source: MapQuest Reprinted with permission; copyright , MapQuest. The MapQuest. It may also reduce the total VMT if housing and services are intermixed. However, it can also add unwanted through traffic on some residential streets Berkovitz In many locations, the hub-and-spoke pattern developed with the local growth pattern in the s and s.
This system of roads comprises circular roads hubs, belts, and ring roads that go around a city center at various distances and separate roads spokes that go to the center of town Figure The circular roads are often used to route traffic around major urban areas, and the spokes are designed for commuter traffic. Freeways and expressways interstate often form the circular hubs and major spokes in urban and suburban areas.
The hub-and-spoke pattern is correlated with a type of land-use pattern that is designated as conventional land use. This pattern arose, in part, because of central urban planning and the increased importance of. Suburban areas grew in number and size to house families.
Residential areas are often built with cul de sacs and a small number of entry points to reduce the amount of pass-by traffic in residential neighborhoods Berkovitz In the conventional pattern of land use, each type of land use residential, commercial, retail, and industrial is separated from the others. Conventional land-use patterns result in more of a hub-and-spoke or circulatory pattern, with businesses in the center of town and residential areas surrounding the city.
Some of the longest commutes in metropolitan regions are made by residents who live at the metropolitan edge and who work in downtown areas FHWA b. In addition to roads and roadsides, the road system includes many engineering structures. These include concrete barriers, guardrails, noise barriers, bridges, culverts and pipes, and overpasses and underpasses.
Each of these structures has a particular ecological effect. Rigid safety barriers separating lanes on roads are common, especially in urban areas. The most common type of these structures is called a Jersey barrier.
Some Jersey barriers are used for traffic separation on freeways and interstate highways, and many others are used only temporarily. Temporary units are used mainly to enhance safety in construction work zones. The height of Jersey barriers averages around 32 in. Rigid safety barriers are also commonly used longitudinally on major nonlimited-access highways to safely separate the two directions of traffic and preclude left turns and U-turns.
Since the current policy on numbering and designating U. Highways less than miles km in length, "as rapidly as the State Highway Department and the Standing Committee on Highways of the American Association of State Highway and Transportation Officials can reach agreement with reference thereto"; new additions to the system must therefore serve more than one state. Interstate 21 Interstate 23 Interstate U. Route 47 U. Route 0 U. Route 1 U. Route 2 U.
Route 10 U. Route 11 U. Indeed, they are so ubiquitous that if the U. The roads seem to go on forever — but do they, really? Well, no. All told, there are 4. That may seem like a small number, but they contribute to a full one-quarter of all traffic. And all those miles need to be maintained — in popular media, much grief is directed at the federal government for its failures in road upkeep.
Since the early 20th century the nation has devoted significant resources to the creation of a roadway system that connects every major population center. Over , miles of highways in the National Highway System form the backbone of our 4-million-mile public road network. Our highway system comprises three fundamental building blocks: local roads; collectors; and arterials.
Local roads serve homes, businesses, farms, and small communities, and provide access to collector roadways. Collectors channel traffic from local roads to arterials, which provide safe, reliable, and efficient travel between towns and cities. An ideal roadway is one that connects to our driveways access and at the same time leads to interruption-free drives to our destinations mobility. To accomplish this, roadways are planned and designed differently.
Local roads are chiefly to provide access driveways, median openings , while mobility is the primary function of arterials.
The NHS includes all Interstate highways arterials , the Strategic Highway Network defense purpose , intermodal connectors roads connecting to major intermodal facilities , and other principal arterials. Currently, the NHS includes over , miles of highways.
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